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.\"   see the next note anyway).
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.\" 
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.\"   unless the distinction is absolutely clear from context.
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.\"   descriptive section, or at least in the reference-section description
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.TH TCSH 1 "10 July 2009" "Astron 6.17.00"
.SH NAME
tcsh \- C shell with file name completion and command line editing
.SH SYNOPSIS
.B tcsh \fR[\fB\-bcdefFimnqstvVxX\fR] [\fB\-Dname\fR[\fB=value\fR]] [arg ...]
.br
.B tcsh \-l
.SH DESCRIPTION
\fItcsh\fR is an enhanced but completely compatible version of the Berkeley
UNIX C shell, \fIcsh\fR(1).
It is a command language interpreter usable both as an interactive login
shell and a shell script command processor.
It includes a command-line editor (see \fBThe command-line editor\fR),
programmable word completion (see \fBCompletion and listing\fR),
spelling correction (see \fBSpelling correction\fR),
a history mechanism (see \fBHistory substitution\fR),
job control (see \fBJobs\fR)
and a C-like syntax.
The \fBNEW FEATURES\fR section describes major enhancements of \fItcsh\fR
over \fIcsh\fR(1).
Throughout this manual, features of
\fItcsh\fR not found in most \fIcsh\fR(1) implementations
(specifically, the 4.4BSD \fIcsh\fR)
are labeled with `(+)', and features which are present in \fIcsh\fR(1)
but not usually documented are labeled with `(u)'.
.SS "Argument list processing"
If the first argument (argument 0) to the shell is `\-' then it is a
login shell.  A login shell can be also specified by invoking the shell with
the \fB\-l\fR flag as the only argument.
.PP
The rest of the flag arguments are interpreted as follows:
.TP 4
.B \-b
Forces a ``break'' from option processing, causing any
further shell arguments to be treated as non-option arguments.  The remaining
arguments will not be interpreted as shell options.  This may be used to pass
options to a shell script without confusion or possible subterfuge.
.TP 4
.B \-c
Commands are read from the following argument (which must be present, and
must be a single argument),
stored in the \fBcommand\fR shell variable for reference, and executed.
Any remaining arguments are placed in the \fBargv\fR shell variable.
.TP 4
.B \-d
The shell loads the directory stack from \fI~/.cshdirs\fR as described under
\fBStartup and shutdown\fR, whether or not it is a login shell. (+)
.TP 4
.B \-D\fIname\fR[=\fIvalue\fR]
Sets the environment variable \fIname\fR to \fIvalue\fR. (Domain/OS only) (+)
.TP 4
.B \-e
The shell exits if any invoked command terminates abnormally or
yields a non-zero exit status.
.TP 4
.B \-f
The shell does not load any resource or startup files, or perform any 
command hashing, and thus starts faster.
.TP 4
.B \-F
The shell uses \fIfork\fR(2) instead of \fIvfork\fR(2) to spawn processes. (+)
.TP 4
.B \-i
The shell is interactive and prompts for its top-level input, even if
it appears to not be a terminal.  Shells are interactive without this option if
their inputs and outputs are terminals.
.TP 4
.B \-l
The shell is a login shell.  Applicable only if \fB\-l\fR is the only
flag specified.
.TP 4
.B \-m
The shell loads \fI~/.tcshrc\fR even if it does not belong to the effective
user.  Newer versions of \fIsu\fR(1) can pass \fB\-m\fR to the shell. (+)
.TP 4
.B \-n
The shell parses commands but does not execute them.
This aids in debugging shell scripts.
.TP 4
.B \-q
The shell accepts SIGQUIT (see \fBSignal handling\fR) and behaves when
it is used under a debugger.  Job control is disabled. (u)
.TP 4
.B \-s
Command input is taken from the standard input.
.TP 4
.B \-t
The shell reads and executes a single line of input.  A `\\' may be used to
escape the newline at the end of this line and continue onto another line.
.TP 4
.B \-v
Sets the \fBverbose\fR shell variable, so that
command input is echoed after history substitution.
.TP 4
.B \-x
Sets the \fBecho\fR shell variable, so that commands are echoed
immediately before execution.
.TP 4
.B \-V
Sets the \fBverbose\fR shell variable even before executing \fI~/.tcshrc\fR.
.TP 4
.B \-X
Is to \fB\-x\fR as \fB\-V\fR is to \fB\-v\fR.
.TP 4
.B \-\-help
Print a help message on the standard output and exit. (+)
.TP 4
.B \-\-version
Print the version/platform/compilation options on the standard output and exit.
This information is also contained in the \fBversion\fR shell variable. (+)
.PP
After processing of flag arguments, if arguments remain but none of the
\fB\-c\fR, \fB\-i\fR, \fB\-s\fR, or \fB\-t\fR options were given, the first
argument is taken as the name of a file of commands, or ``script'', to
be executed.  The shell opens this file and saves its name for possible
resubstitution by `$0'.  Because many systems use either the standard
version 6 or version 7 shells whose shell scripts are not compatible
with this shell, the shell uses such a `standard' shell to execute a script
whose first character is not a `#', i.e., that does not start with a
comment.
.PP
Remaining arguments are placed in the \fBargv\fR shell variable.
.SS "Startup and shutdown"
A login shell begins by executing commands from the system files
\fI/etc/csh.cshrc\fR and \fI/etc/csh.login\fR.
It then executes commands from files in the user's \fBhome\fR directory:
first \fI~/.tcshrc\fR (+)
or, if \fI~/.tcshrc\fR is not found, \fI~/.cshrc\fR,
then \fI~/.history\fR (or the value of the \fBhistfile\fR shell variable),
then \fI~/.login\fR,
and finally \fI~/.cshdirs\fR (or the value of the \fBdirsfile\fR shell variable) (+).
The shell may read \fI/etc/csh.login\fR before instead of after
\fI/etc/csh.cshrc\fR, and \fI~/.login\fR before instead of after
\fI~/.tcshrc\fR or \fI~/.cshrc\fR and \fI~/.history\fR, if so compiled;
see the \fBversion\fR shell variable. (+)
.PP
Non-login shells read only \fI/etc/csh.cshrc\fR and \fI~/.tcshrc\fR
or \fI~/.cshrc\fR on startup.
.PP
For examples of startup files, please consult
\fIhttp://tcshrc.sourceforge.net\fR.
.PP
Commands like \fIstty\fR(1) and \fItset\fR(1),
which need be run only once per login, usually go in one's \fI~/.login\fR file.
Users who need to use the same set of files with both \fIcsh\fR(1) and
\fItcsh\fR can have only a \fI~/.cshrc\fR which checks for the existence of the
\fBtcsh\fR shell variable (q.v.) before using \fItcsh\fR-specific commands,
or can have both a \fI~/.cshrc\fR and a \fI~/.tcshrc\fR which \fIsource\fRs
(see the builtin command) \fI~/.cshrc\fR.
The rest of this manual uses `\fI~/.tcshrc\fR' to mean `\fI~/.tcshrc\fR or,
if \fI~/.tcshrc\fR is not found, \fI~/.cshrc\fR'.
.PP
In the normal case, the shell begins reading commands from the terminal,
prompting with `> '.  (Processing of arguments and the use of the shell to
process files containing command scripts are described later.)
The shell repeatedly reads a line of command input, breaks it into words,
places it on the command history list, parses it and executes each command
in the line.
.PP
One can log out by typing `^D' on an empty line, `logout' or `login' or
via the shell's autologout mechanism (see the \fBautologout\fR shell variable).
When a login shell terminates it sets the \fBlogout\fR shell variable to
`normal' or `automatic' as appropriate, then
executes commands from the files
\fI/etc/csh.logout\fR and \fI~/.logout\fR.  The shell may drop DTR on logout
if so compiled; see the \fBversion\fR shell variable.
.PP
The names of the system login and logout files vary from system to system for
compatibility with different \fIcsh\fR(1) variants; see \fBFILES\fR.
.SS Editing
We first describe \fBThe command-line editor\fR.
The \fBCompletion and listing\fR and \fBSpelling correction\fR sections
describe two sets of functionality that are implemented as editor commands
but which deserve their own treatment.
Finally, \fBEditor commands\fR lists and describes
the editor commands specific to the shell and their default bindings.
.SS "The command-line editor (+)"
Command-line input can be edited using key sequences much like those used in
GNU Emacs or \fIvi\fR(1).
The editor is active only when the \fBedit\fR shell variable is set, which
it is by default in interactive shells.
The \fIbindkey\fR builtin can display and change key bindings.
Emacs-style key bindings are used by default
(unless the shell was compiled otherwise; see the \fBversion\fR shell variable),
but \fIbindkey\fR can change the key bindings to \fIvi\fR-style bindings en masse.
.PP
The shell always binds the arrow keys (as defined in the \fBTERMCAP\fR
environment variable) to
.PP
.PD 0
.RS +4
.TP 8
down
\fIdown-history\fR
.TP 8
up
\fIup-history\fR
.TP 8
left
\fIbackward-char\fR
.TP 8
right
\fIforward-char\fR
.PD
.RE
.PP
unless doing so would alter another single-character binding.
One can set the arrow key escape sequences to the empty string with \fIsettc\fR
to prevent these bindings.
The ANSI/VT100 sequences for arrow keys are always bound.
.PP
Other key bindings are, for the most part, what Emacs and \fIvi\fR(1)
users would expect and can easily be displayed by \fIbindkey\fR, so there
is no need to list them here.  Likewise, \fIbindkey\fR can list the editor
commands with a short description of each.
.PP
Note that editor commands do not have the same notion of a ``word'' as does the
shell.  The editor delimits words with any non-alphanumeric characters not in
the shell variable \fBwordchars\fR, while the shell recognizes only whitespace
and some of the characters with special meanings to it, listed under
\fBLexical structure\fR.
.SS "Completion and listing (+)"
The shell is often able to complete words when given a unique abbreviation.
Type part of a word (for example `ls /usr/lost') and hit the tab key to
run the \fIcomplete-word\fR editor command.
The shell completes the filename `/usr/lost' to `/usr/lost+found/',
replacing the incomplete word with the complete word in the input buffer.
(Note the terminal `/'; completion adds a `/' to the
end of completed directories and a space to the end of other completed words,
to speed typing and provide a visual indicator of successful completion.
The \fBaddsuffix\fR shell variable can be unset to prevent this.)
If no match is found (perhaps `/usr/lost+found' doesn't exist),
the terminal bell rings.
If the word is already complete (perhaps there is a `/usr/lost' on your
system, or perhaps you were thinking too far ahead and typed the whole thing)
a `/' or space is added to the end if it isn't already there.
.PP
Completion works anywhere in the line, not at just the end; completed
text pushes the rest of the line to the right.  Completion in the middle of a word
often results in leftover characters to the right of the cursor that need
to be deleted.
.PP
Commands and variables can be completed in much the same way.
For example, typing `em[tab]' would complete `em' to
`emacs' if \fIemacs\fR were the only command on your system beginning with `em'.
Completion can find a command in any directory in \fBpath\fR or if
given a full pathname.
Typing `echo $ar[tab]' would complete `$ar' to `$argv'
if no other variable began with `ar'.
.PP
The shell parses the input buffer to determine whether the word you want to
complete should be completed as a filename, command or variable.
The first word in the buffer and the first word following
`;', `|', `|&', `&&' or `||' is considered to be a command.
A word beginning with `$' is considered to be a variable.
Anything else is a filename.  An empty line is `completed' as a filename.
.PP
You can list the possible completions of a word at any time by typing `^D'
to run the \fIdelete-char-or-list-or-eof\fR editor command.
The shell lists the possible completions using the \fIls\-F\fR builtin (q.v.)
and reprints the prompt and unfinished command line, for example:
.IP "" 4
> ls /usr/l[^D]
.br
lbin/       lib/        local/      lost+found/
.br
> ls /usr/l
.PP
If the \fBautolist\fR shell variable is set, the shell lists the remaining
choices (if any) whenever completion fails:
.IP "" 4
> set autolist
.br
> nm /usr/lib/libt[tab]
.br
libtermcap.a@ libtermlib.a@
.br
> nm /usr/lib/libterm
.PP
If \fBautolist\fR is set to `ambiguous', choices are listed only when
completion fails and adds no new characters to the word being completed.
.PP
A filename to be completed can contain variables, your own or others' home
directories abbreviated with `~' (see \fBFilename substitution\fR) and
directory stack entries abbreviated with `='
(see \fBDirectory stack substitution\fR).  For example,
.IP "" 4
> ls ~k[^D]
.br
kahn    kas     kellogg
.br
> ls ~ke[tab]
.br
> ls ~kellogg/
.PP
or
.IP "" 4
> set local = /usr/local
.br
> ls $lo[tab]
.br
> ls $local/[^D]
.br
bin/ etc/ lib/ man/ src/
.br
> ls $local/
.PP
Note that variables can also be expanded explicitly with the
\fIexpand-variables\fR editor command.
.PP
\fIdelete-char-or-list-or-eof\fR lists at only the end of the line;
in the middle of a line it deletes the character under the cursor and
on an empty line it logs one out or, if \fBignoreeof\fR is set, does nothing.
`M-^D', bound to the editor command \fIlist-choices\fR, lists completion
possibilities anywhere on a line, and \fIlist-choices\fR (or any one of the
related editor commands that do or don't delete, list and/or log out,
listed under \fIdelete-char-or-list-or-eof\fR) can be bound to `^D' with
the \fIbindkey\fR builtin command if so desired.
.PP
The \fIcomplete-word-fwd\fR and \fIcomplete-word-back\fR editor commands
(not bound to any keys by default) can be used to cycle up and down through
the list of possible completions, replacing the current word with the next or
previous word in the list.
.PP
The shell variable \fBfignore\fR can be set to a list of suffixes to be
ignored by completion.  Consider the following:
.IP "" 4
> ls
.br
Makefile        condiments.h~   main.o          side.c
.br
README          main.c          meal            side.o
.br
condiments.h    main.c~
.br
> set fignore = (.o \\~)
.br
> emacs ma[^D]
.br
main.c   main.c~  main.o
.br
> emacs ma[tab]
.br
> emacs main.c
.PP
`main.c~' and `main.o' are ignored by completion (but not listing),
because they end in suffixes in \fBfignore\fR.
Note that a `\\' was needed in front of `~' to prevent it from being
expanded to \fBhome\fR as described under \fBFilename substitution\fR.
\fBfignore\fR is ignored if only one completion is possible.
.PP
If the \fBcomplete\fR shell variable is set to `enhance', completion
1) ignores case and 2) considers periods, hyphens and underscores
(`.', `\-' and `_') to be word separators and hyphens and underscores to
be equivalent.  If you had the following files
.IP "" 4
comp.lang.c      comp.lang.perl   comp.std.c++
.br
comp.lang.c++    comp.std.c
.PP
and typed `mail \-f c.l.c[tab]', it would be completed to
`mail \-f comp.lang.c', and ^D would list `comp.lang.c' and `comp.lang.c++'.
`mail \-f c..c++[^D]' would list `comp.lang.c++' and `comp.std.c++'.  Typing
`rm a\-\-file[^D]' in the following directory
.IP "" 4
A_silly_file    a-hyphenated-file    another_silly_file
.PP
would list all three files, because case is ignored and hyphens and
underscores are equivalent.  Periods, however, are not equivalent to
hyphens or underscores.
.PP
Completion and listing are affected by several other shell variables:
\fBrecexact\fR can be set to complete on the shortest possible unique
match, even if more typing might result in a longer match:
.IP "" 4
> ls
.br
fodder   foo      food     foonly
.br
> set recexact
.br
> rm fo[tab]
.PP
just beeps, because `fo' could expand to `fod' or `foo', but if we type
another `o',
.IP "" 4
> rm foo[tab]
.br
> rm foo
.PP
the completion completes on `foo', even though `food' and `foonly'
also match.
\fBautoexpand\fR can be set to run the \fIexpand-history\fR editor command
before each completion attempt, \fBautocorrect\fR can be set to
spelling-correct the word to be completed (see \fBSpelling correction\fR)
before each completion attempt and \fBcorrect\fR can be set to complete
commands automatically after one hits `return'.
\fBmatchbeep\fR can be set to make completion beep or not beep in a variety
of situations, and \fBnobeep\fR can be set to never beep at all.
\fBnostat\fR can be set to a list of directories and/or patterns that
match directories to prevent the completion mechanism from \fIstat\fR(2)ing
those directories.
\fBlistmax\fR and \fBlistmaxrows\fR can be set to limit the number of items
and rows (respectively) that are listed without asking first.
\fBrecognize_only_executables\fR can be set to make the shell list only
executables when listing commands, but it is quite slow.
.PP
Finally, the \fIcomplete\fR builtin command can be used to tell the shell how
to complete words other than filenames, commands and variables.
Completion and listing do not work on glob-patterns (see \fBFilename substitution\fR),
but the \fIlist-glob\fR and \fIexpand-glob\fR editor commands perform
equivalent functions for glob-patterns.
.SS "Spelling correction (+)"
The shell can sometimes correct the spelling of filenames, commands and variable names
as well as completing and listing them.
.PP
Individual words can be spelling-corrected with the \fIspell-word\fR
editor command (usually bound to M-s and M-S)
and the entire input buffer with \fIspell-line\fR (usually bound to M-$).
The \fBcorrect\fR shell variable can be set to `cmd' to correct the
command name or `all' to correct the entire line each time return is typed,
and \fBautocorrect\fR can be set to correct the word to be completed
before each completion attempt.
.PP
When spelling correction is invoked in any of these ways and
the shell thinks that any part of the command line is misspelled,
it prompts with the corrected line:
.IP "" 4
> set correct = cmd
.br
> lz /usr/bin
.br
CORRECT>ls /usr/bin (y|n|e|a)?
.PP
One can answer `y' or space to execute the corrected line,
`e' to leave the uncorrected command in the input buffer,
`a' to abort the command as if `^C' had been hit, and
anything else to execute the original line unchanged.
.PP
Spelling correction recognizes user-defined completions (see the
\fIcomplete\fR builtin command).  If an input word in a position for
which a completion is defined resembles a word in the completion list,
spelling correction registers a misspelling and suggests the latter
word as a correction.  However, if the input word does not match any of
the possible completions for that position, spelling correction does
not register a misspelling.
.PP
Like completion, spelling correction works anywhere in the line,
pushing the rest of the line to the right and possibly leaving
extra characters to the right of the cursor.
.PP
Beware: spelling correction is not guaranteed to work the way one intends,
and is provided mostly as an experimental feature.
Suggestions and improvements are welcome.
.SS "Editor commands (+)"
`bindkey' lists key bindings and `bindkey \-l' lists and briefly describes
editor commands.
Only new or especially interesting editor commands are described here.
See \fIemacs\fR(1) and \fIvi\fR(1) for descriptions of each editor's
key bindings.
.PP
The character or characters to which each command is bound by default is
given in parentheses.  `^\fIcharacter\fR' means a control character and
`M-\fIcharacter\fR' a meta character, typed as escape-\fIcharacter\fR
on terminals without a meta key.  Case counts, but commands that are bound
to letters by default are bound to both lower- and uppercase letters for
convenience.
.TP 8
.B complete-word \fR(tab)
Completes a word as described under \fBCompletion and listing\fR.
.TP 8
.B complete-word-back \fR(not bound)
Like \fIcomplete-word-fwd\fR, but steps up from the end of the list.
.TP 8
.B complete-word-fwd \fR(not bound)
Replaces the current word with the first word in the list of possible
completions.  May be repeated to step down through the list.
At the end of the list, beeps and reverts to the incomplete word.
.TP 8
.B complete-word-raw \fR(^X-tab)
Like \fIcomplete-word\fR, but ignores user-defined completions.
.TP 8
.B copy-prev-word \fR(M-^_)
Copies the previous word in the current line into the input buffer.
See also \fIinsert-last-word\fR.
.TP 8
.B dabbrev-expand \fR(M-/)
Expands the current word to the most recent preceding one for which
the current is a leading substring, wrapping around the history list
(once) if necessary.
Repeating \fIdabbrev-expand\fR without any intervening typing
changes to the next previous word etc., skipping identical matches
much like \fIhistory-search-backward\fR does.
.TP 8
.B delete-char \fR(not bound)
Deletes the character under the cursor.
See also \fIdelete-char-or-list-or-eof\fR.
.TP 8
.B delete-char-or-eof \fR(not bound)
Does \fIdelete-char\fR if there is a character under the cursor
or \fIend-of-file\fR on an empty line.
See also \fIdelete-char-or-list-or-eof\fR.
.TP 8
.B delete-char-or-list \fR(not bound)
Does \fIdelete-char\fR if there is a character under the cursor
or \fIlist-choices\fR at the end of the line.
See also \fIdelete-char-or-list-or-eof\fR.
.TP 8
.B delete-char-or-list-or-eof \fR(^D)
Does \fIdelete-char\fR if there is a character under the cursor,
\fIlist-choices\fR at the end of the line
or \fIend-of-file\fR on an empty line.
See also those three commands, each of which does only a single action, and
\fIdelete-char-or-eof\fR, \fIdelete-char-or-list\fR and \fIlist-or-eof\fR,
each of which does a different two out of the three.
.TP 8
.B down-history \fR(down-arrow, ^N)
Like \fIup-history\fR, but steps down, stopping at the original input line.
.TP 8
.B end-of-file \fR(not bound)
Signals an end of file, causing the shell to exit unless the \fBignoreeof\fR
shell variable (q.v.) is set to prevent this.
See also \fIdelete-char-or-list-or-eof\fR.
.TP 8
.B expand-history \fR(M-space)
Expands history substitutions in the current word.
See \fBHistory substitution\fR.
See also \fImagic-space\fR, \fItoggle-literal-history\fR and
the \fBautoexpand\fR shell variable.
.TP 8
.B expand-glob \fR(^X-*)
Expands the glob-pattern to the left of the cursor.
See \fBFilename substitution\fR.
.TP 8
.B expand-line \fR(not bound)
Like \fIexpand-history\fR, but
expands history substitutions in each word in the input buffer,
.TP 8
.B expand-variables \fR(^X-$)
Expands the variable to the left of the cursor.
See \fBVariable substitution\fR.
.TP 8
.B history-search-backward \fR(M-p, M-P)
Searches backwards through the history list for a command beginning with
the current contents of the input buffer up to the cursor and copies it
into the input buffer.
The search string may be a glob-pattern (see \fBFilename substitution\fR)
containing `*', `?', `[]' or `{}'.
\fIup-history\fR and \fIdown-history\fR will proceed from the
appropriate point in the history list.
Emacs mode only.
See also \fIhistory-search-forward\fR and \fIi-search-back\fR.
.TP 8
.B history-search-forward \fR(M-n, M-N)
Like \fIhistory-search-backward\fR, but searches forward.
.TP 8
.B i-search-back \fR(not bound)
Searches backward like \fIhistory-search-backward\fR, copies the first match
into the input buffer with the cursor positioned at the end of the pattern,
and prompts with `bck: ' and the first match.  Additional characters may be
typed to extend the search, \fIi-search-back\fR may be typed to continue
searching with the same pattern, wrapping around the history list if
necessary, (\fIi-search-back\fR must be bound to a
single character for this to work) or one of the following special characters
may be typed:
.PP
.RS +8
.RS +4
.PD 0
.TP 8
^W
Appends the rest of the word under the cursor to the search pattern.
.TP 8
delete (or any character bound to \fIbackward-delete-char\fR)
Undoes the effect of the last character typed and deletes a character
from the search pattern if appropriate.
.TP 8
^G
If the previous search was successful, aborts the entire search.
If not, goes back to the last successful search.
.TP 8
escape
Ends the search, leaving the current line in the input buffer.
.RE
.PD
.PP
Any other character not bound to \fIself-insert-command\fR terminates the
search, leaving the current line in the input buffer, and
is then interpreted as normal input.  In particular, a carriage return
causes the current line to be executed.
Emacs mode only.
See also \fIi-search-fwd\fR and \fIhistory-search-backward\fR.
.RE
.TP 8
.B i-search-fwd \fR(not bound)
Like \fIi-search-back\fR, but searches forward.
.TP 8
.B insert-last-word \fR(M-_)
Inserts the last word of the previous input line (`!$') into the input buffer.
See also \fIcopy-prev-word\fR.
.TP 8
.B list-choices \fR(M-^D)
Lists completion possibilities as described under \fBCompletion and listing\fR.
See also \fIdelete-char-or-list-or-eof\fR and \fIlist-choices-raw\fR.
.TP 8
.B list-choices-raw \fR(^X-^D)
Like \fIlist-choices\fR, but ignores user-defined completions.
.TP 8
.B list-glob \fR(^X-g, ^X-G)
Lists (via the \fIls\-F\fR builtin) matches to the glob-pattern
(see \fBFilename substitution\fR) to the left of the cursor.
.TP 8
.B list-or-eof \fR(not bound)
Does \fIlist-choices\fR
or \fIend-of-file\fR on an empty line.
See also \fIdelete-char-or-list-or-eof\fR.
.TP 8
.B magic-space \fR(not bound)
Expands history substitutions in the current line,
like \fIexpand-history\fR, and inserts a space.
\fImagic-space\fR is designed to be bound to the space bar,
but is not bound by default.
.TP 8
.B normalize-command \fR(^X-?)
Searches for the current word in PATH and, if it is found, replaces it with
the full path to the executable.  Special characters are quoted.  Aliases are
expanded and quoted but commands within aliases are not.  This command is
useful with commands that take commands as arguments, e.g., `dbx' and `sh \-x'.
.TP 8
.B normalize-path \fR(^X-n, ^X-N)
Expands the current word as described under the `expand' setting
of the \fBsymlinks\fR shell variable.
.TP 8
.B overwrite-mode \fR(unbound)
Toggles between input and overwrite modes.
.TP 8
.B run-fg-editor \fR(M-^Z)
Saves the current input line and
looks for a stopped job with a name equal to the last component of the
file name part of the \fBEDITOR\fR or \fBVISUAL\fR environment variables,
or, if neither is set, `ed' or `vi'.
If such a job is found, it is restarted as if `fg %\fIjob\fR' had been
typed.  This is used to toggle back and forth between an editor and
the shell easily.  Some people bind this command to `^Z' so they
can do this even more easily.
.TP
.B run-help \fR(M-h, M-H)
Searches for documentation on the current command, using the same notion of
`current command' as the completion routines, and prints it.  There is no way
to use a pager; \fIrun-help\fR is designed for short help files.
If the special alias \fBhelpcommand\fR is defined, it is run with the
command name as a sole argument.  Else,
documentation should be in a file named \fIcommand\fR.help, \fIcommand\fR.1,
\fIcommand\fR.6, \fIcommand\fR.8 or \fIcommand\fR, which should be in one
of the directories listed in the \fBHPATH\fR environment variable.
If there is more than one help file only the first is printed.
.TP 8
.B self-insert-command \fR(text characters)
In insert mode (the default), inserts the typed character into the input line after the character under the cursor.
In overwrite mode, replaces the character under the cursor with the typed character.
The input mode is normally preserved between lines, but the
\fBinputmode\fR shell variable can be set to `insert' or `overwrite' to put the
editor in that mode at the beginning of each line.
See also \fIoverwrite-mode\fR.
.TP 8
.B sequence-lead-in \fR(arrow prefix, meta prefix, ^X)
Indicates that the following characters are part of a
multi-key sequence.  Binding a command to a multi-key sequence really creates
two bindings: the first character to \fIsequence-lead-in\fR and the
whole sequence to the command.  All sequences beginning with a character
bound to \fIsequence-lead-in\fR are effectively bound to \fIundefined-key\fR
unless bound to another command.
.TP 8
.B spell-line \fR(M-$)
Attempts to correct the spelling of each word in the input buffer, like
\fIspell-word\fR, but ignores words whose first character is one of
`\-', `!', `^' or `%', or which contain `\\', `*' or `?', to avoid problems
with switches, substitutions and the like.
See \fBSpelling correction\fR.
.TP 8
.B spell-word \fR(M-s, M-S)
Attempts to correct the spelling of the current word as described
under \fBSpelling correction\fR.
Checks each component of a word which appears to be a pathname.
.TP 8
.B toggle-literal-history \fR(M-r, M-R)
Expands or `unexpands' history substitutions in the input buffer.
See also \fIexpand-history\fR and the \fBautoexpand\fR shell variable.
.TP 8
.B undefined-key \fR(any unbound key)
Beeps.
.TP 8
.B up-history \fR(up-arrow, ^P)
Copies the previous entry in the history list into the input buffer.
If \fBhistlit\fR is set, uses the literal form of the entry.
May be repeated to step up through the history list, stopping at the top.
.TP 8
.B vi-search-back \fR(?)
Prompts with `?' for a search string (which may be a glob-pattern, as with
\fIhistory-search-backward\fR), searches for it and copies it into the
input buffer.  The bell rings if no match is found.
Hitting return ends the search and leaves the last match in the input
buffer.
Hitting escape ends the search and executes the match.
\fIvi\fR mode only.
.TP 8
.B vi-search-fwd \fR(/)
Like \fIvi-search-back\fR, but searches forward.
.TP 8
.B which-command \fR(M-?)
Does a \fIwhich\fR (see the description of the builtin command) on the
first word of the input buffer.
.TP 8
.B yank-pop \fR(M-y)
When executed immediately after a \fIyank\fR or another \fIyank-pop\fR,
replaces the yanked string with the next previous string from the
killring. This also has the effect of rotating the killring, such that
this string will be considered the most recently killed by a later
\fIyank\fR command. Repeating \fIyank-pop\fR will cycle through the
killring any number of times.
.SS "Lexical structure"
The shell splits input lines into words at blanks and tabs.  The special
characters `&', `|', `;', `<', `>', `(', and `)' and the doubled characters
`&&', `||', `<<' and `>>' are always separate words, whether or not they are
surrounded by whitespace.
.PP
When the shell's input is not a terminal, the character `#' is taken to begin a
comment.  Each `#' and the rest of the input line on which it appears is
discarded before further parsing.
.PP
A special character (including a blank or tab) may be prevented from having
its special meaning, and possibly made part of another word, by preceding it
with a backslash (`\\') or enclosing it in single (`''), double (`"') or
backward (``') quotes.  When not otherwise quoted a newline preceded by a `\\'
is equivalent to a blank, but inside quotes this sequence results in a
newline.
.PP
Furthermore, all \fBSubstitutions\fR (see below) except \fBHistory substitution\fR
can be prevented by enclosing the strings (or parts of strings)
in which they appear with single quotes or by quoting the crucial character(s)
(e.g., `$' or ``' for \fBVariable substitution\fR or \fBCommand substitution\fR respectively)
with `\\'.  (\fBAlias substitution\fR is no exception: quoting in any way any
character of a word for which an \fIalias\fR has been defined prevents
substitution of the alias.  The usual way of quoting an alias is to precede it
with a backslash.) \fBHistory substitution\fR is prevented by
backslashes but not by single quotes.  Strings quoted with double or backward
quotes undergo \fBVariable substitution\fR and \fBCommand substitution\fR, but other
substitutions are prevented.
.PP
Text inside single or double quotes becomes a single word (or part of one).
Metacharacters in these strings, including blanks and tabs, do not form
separate words.  Only in one special case (see \fBCommand substitution\fR
below) can a double-quoted string yield parts of more than one word;
single-quoted strings never do.  Backward quotes are special: they signal
\fBCommand substitution\fR (q.v.), which may result in more than one word.
.PP
Quoting complex strings, particularly strings which themselves contain quoting
characters, can be confusing.  Remember that quotes need not be used as they are
in human writing!  It may be easier to quote not an entire string, but only
those parts of the string which need quoting, using different types of quoting
to do so if appropriate.
.PP
The \fBbackslash_quote\fR shell variable (q.v.) can be set to make backslashes
always quote `\\', `'', and `"'.  (+) This may make complex quoting tasks
easier, but it can cause syntax errors in \fIcsh\fR(1) scripts.
.SS Substitutions
We now describe the various transformations the shell performs on the input in
the order in which they occur.  We note in passing the data structures involved
and the commands and variables which affect them.  Remember that substitutions
can be prevented by quoting as described under \fBLexical structure\fR.
.SS "History substitution"
Each command, or ``event'', input from the terminal is saved in the history
list.  The previous command is always saved, and the \fBhistory\fR shell
variable can be set to a number to save that many commands.  The \fBhistdup\fR
shell variable can be set to not save duplicate events or consecutive duplicate
events.
.PP
Saved commands are numbered sequentially from 1 and stamped with the time.
It is not usually necessary to use event numbers, but the current event number
can be made part of the prompt by placing an `!' in the \fBprompt\fR shell variable.
.PP
The shell actually saves history in expanded and literal (unexpanded) forms.
If the \fBhistlit\fR shell variable is set, commands that display and store
history use the literal form.
.PP
The \fIhistory\fR builtin command can print, store in a file, restore
and clear the history list at any time,
and the \fBsavehist\fR and \fBhistfile\fR shell variables can be can be set to
store the history list automatically on logout and restore it on login.
.PP
History substitutions introduce words from the history list into the input
stream, making it easy to repeat commands, repeat arguments of a previous
command in the current command, or fix spelling mistakes in the previous
command with little typing and a high degree of confidence.
.PP
History substitutions begin with the character `!'.  They may begin anywhere in
the input stream, but they do not nest.  The `!' may be preceded by a `\\' to
prevent its special meaning; for convenience, a `!' is passed unchanged when it
is followed by a blank, tab, newline, `=' or `('.  History substitutions also
occur when an input line begins with `^'.  This special abbreviation will be
described later.  The characters used to signal history substitution (`!' and
`^') can be changed by setting the \fBhistchars\fR shell variable.  Any input
line which contains a history substitution is printed before it is executed.
.PP
A history substitution may have an ``event specification'', which indicates
the event from which words are to be taken, a ``word designator'',
which selects particular words from the chosen event, and/or a ``modifier'',
which manipulates the selected words.
.PP
An event specification can be
.PP
.PD 0
.RS +4
.TP 8
.I n
A number, referring to a particular event
.TP 8
\-\fIn\fR
An offset, referring to the event \fIn\fR before the current event
.TP 8
#
The current event.
This should be used carefully in \fIcsh\fR(1), where there is no check for
recursion.  \fItcsh\fR allows 10 levels of recursion.  (+)
.TP 8
!
The previous event (equivalent to `\-1')
.TP 8
.I s
The most recent event whose first word begins with the string \fIs\fR
.TP 8
?\fIs\fR?
The most recent event which contains the string \fIs\fR.
The second `?' can be omitted if it is immediately followed by a newline.
.RE
.PD
.PP
For example, consider this bit of someone's history list:
.IP "" 4
\ 9  8:30    nroff \-man wumpus.man
.br
10  8:31    cp wumpus.man wumpus.man.old
.br
11  8:36    vi wumpus.man
.br
12  8:37    diff wumpus.man.old wumpus.man
.PP
The commands are shown with their event numbers and time stamps.
The current event, which we haven't typed in yet, is event 13.
`!11' and `!\-2' refer to event 11.
`!!' refers to the previous event, 12.  `!!' can be abbreviated `!' if it is
followed by `:' (`:' is described below).
`!n' refers to event 9, which begins with `n'.
`!?old?' also refers to event 12, which contains `old'.
Without word designators or modifiers history references simply expand to the
entire event, so we might type `!cp' to redo the copy command or `!!|more'
if the `diff' output scrolled off the top of the screen.
.PP
History references may be insulated from the surrounding text with braces if
necessary.  For example, `!vdoc' would look for a command beginning with
`vdoc', and, in this example, not find one, but `!{v}doc' would expand
unambiguously to `vi wumpus.mandoc'.
Even in braces, history substitutions do not nest.
.PP
(+) While \fIcsh\fR(1) expands, for example, `!3d' to event 3 with the
letter `d' appended to it, \fItcsh\fR expands it to the last event beginning
with `3d'; only completely numeric arguments are treated as event numbers.
This makes it possible to recall events beginning with numbers.
To expand `!3d' as in \fIcsh\fR(1) say `!{3}d'.
.PP
To select words from an event we can follow the event specification by a `:'
and a designator for the desired words.  The words of an input line are
numbered from 0, the first (usually command) word being 0, the second word
(first argument) being 1, etc.  The basic word designators are:
.PP
.PD 0
.RS +4
.TP 8
0
The first (command) word
.TP 8
.I n
The \fIn\fRth argument
.TP 8
^
The first argument, equivalent to `1'
.TP 8
$
The last argument
.TP 8
%
The word matched by an ?\fIs\fR? search
.TP 8
.I x\-y
A range of words
.TP 8
.I \-y
Equivalent to \fI`0\-y'\fR
.TP 8
*
Equivalent to `^\-$', but returns nothing if the event contains only 1 word
.TP 8
.I x*
Equivalent to \fI`x\-$'\fR
.TP 8
.I x\-
Equivalent to \fI`x*'\fR, but omitting the last word (`$')
.PD
.RE
.PP
Selected words are inserted into the command line separated by single blanks.
For example, the `diff' command in the previous example might have been
typed as `diff !!:1.old !!:1' (using `:1' to select the first argument
from the previous event) or `diff !\-2:2 !\-2:1' to select and swap the
arguments from the `cp' command.  If we didn't care about the order of the
`diff' we might have said `diff !\-2:1\-2' or simply `diff !\-2:*'.
The `cp' command might have been written `cp wumpus.man !#:1.old', using `#'
to refer to the current event.
`!n:\- hurkle.man' would reuse the first two words from the `nroff' command
to say `nroff \-man hurkle.man'.
.PP
The `:' separating the event specification from the word designator can be
omitted if the argument selector begins with a `^', `$', `*', `%' or `\-'.
For example, our `diff' command might have been `diff !!^.old !!^' or,
equivalently, `diff !!$.old !!$'.  However, if `!!' is abbreviated `!',
an argument selector beginning with `\-' will be interpreted as an event
specification.
.PP
A history reference may have a word designator but no event specification.
It then references the previous command.
Continuing our `diff' example, we could have said simply `diff
!^.old !^' or, to get the arguments in the opposite order, just `diff !*'.
.PP
The word or words in a history reference can be edited, or ``modified'',
by following it with one or more modifiers, each preceded by a `:':
.PP
.PD 0
.RS +4
.TP 8
h
Remove a trailing pathname component, leaving the head.
.TP 8
t
Remove all leading pathname components, leaving the tail.
.TP 8
r
Remove a filename extension `.xxx', leaving the root name.
.TP 8
e
Remove all but the extension.
.TP 8
u
Uppercase the first lowercase letter.
.TP 8
l
Lowercase the first uppercase letter.
.TP 8
s\fI/l/r/\fR
Substitute \fIl\fR for \fIr\fR.
\fIl\fR is simply a string like \fIr\fR, not a regular expression as in
the eponymous \fIed\fR(1) command.
Any character may be used as the delimiter in place of `/';
a `\\' can be used to quote the delimiter expect `(', `)', `|' and `>' inside \fIl\fR and \fIr\fR.
The character `&' in the \fIr\fR is replaced by \fIl\fR; `\\' also quotes `&'.
If \fIl\fR is empty (``''), the \fIl\fR from a previous substitution or the
\fIs\fR from a previous search or event number in event specification is used.
The trailing delimiter may be omitted if it is immediately followed by a newline.
.TP 8
&
Repeat the previous substitution.
.TP 8
g
Apply the following modifier once to each word.
.TP 8
a (+)
Apply the following modifier as many times as possible to a single word.
`a' and `g' can be used together to apply a modifier globally.
With the `s' modifier, only the patterns contained in the original word are
substituted, not patterns that contain any substitution result.
.TP 8
p
Print the new command line but do not execute it.
.TP 8
q
Quote the substituted words, preventing further substitutions.
.TP 8
x
Like q, but break into words at blanks, tabs and newlines.
.PD
.RE
.PP
Modifiers are applied to only the first modifiable word (unless `g' is used).
It is an error for no word to be modifiable.
.PP
For example, the `diff' command might have been written as `diff wumpus.man.old
!#^:r', using `:r' to remove `.old' from the first argument on the same line
(`!#^').  We could say `echo hello out there', then `echo !*:u' to capitalize
`hello', `echo !*:au' to say it out loud, or `echo !*:agu' to really shout.
We might follow `mail \-s "I forgot my password" rot' with `!:s/rot/root' to
correct the spelling of `root' (but see \fBSpelling correction\fR for a
different approach).
.PP
There is a special abbreviation for substitutions.
`^', when it is the first character on an input line, is equivalent to `!:s^'.
Thus we might have said `^rot^root' to make the spelling correction in the
previous example.
This is the only history substitution which does not explicitly begin with `!'.
.PP
(+) In \fIcsh\fR as such, only one modifier may be applied to each history
or variable expansion.  In \fItcsh\fR, more than one may be used, for example
.IP "" 4
% mv wumpus.man /usr/man/man1/wumpus.1
.br
% man !$:t:r
.br
man wumpus
.PP
In \fIcsh\fR, the result would be `wumpus.1:r'.  A substitution followed by a
colon may need to be insulated from it with braces:
.IP "" 4
> mv a.out /usr/games/wumpus
.br
> setenv PATH !$:h:$PATH
.br
Bad ! modifier: $.
.br
> setenv PATH !{\-2$:h}:$PATH
.br
setenv PATH /usr/games:/bin:/usr/bin:.
.PP
The first attempt would succeed in \fIcsh\fR but fails in \fItcsh\fR,
because \fItcsh\fR expects another modifier after the second colon
rather than `$'.
.PP
Finally, history can be accessed through the editor as well as through
the substitutions just described.
The \fIup-\fR and \fIdown-history\fR, \fIhistory-search-backward\fR and
\fI-forward\fR, \fIi-search-back\fR and \fI-fwd\fR,
\fIvi-search-back\fR and \fI-fwd\fR, \fIcopy-prev-word\fR
and \fIinsert-last-word\fR editor commands search for
events in the history list and copy them into the input buffer.
The \fItoggle-literal-history\fR editor command switches between the
expanded and literal forms of history lines in the input buffer.
\fIexpand-history\fR and \fIexpand-line\fR expand history substitutions
in the current word and in the entire input buffer respectively.
.SS "Alias substitution"
The shell maintains a list of aliases which can be set, unset and printed by
the \fIalias\fR and \fIunalias\fR commands.  After a command line is parsed
into simple commands (see \fBCommands\fR) the first word of each command,
left-to-right, is checked to see if it has an alias.  If so, the first word is
replaced by the alias.  If the alias contains a history reference, it undergoes
\fBHistory substitution\fR (q.v.) as though the original command were the
previous input line.  If the alias does not contain a history reference, the
argument list is left untouched.
.PP
Thus if the alias for `ls' were `ls \-l' the command `ls /usr' would become `ls
\-l /usr', the argument list here being undisturbed.  If the alias for `lookup'
were `grep !^ /etc/passwd' then `lookup bill' would become `grep bill
/etc/passwd'.  Aliases can be used to introduce parser metasyntax.  For
example, `alias print 'pr \e!* | lpr'' defines a ``command'' (`print') which
\fIpr\fR(1)s its arguments to the line printer.
.PP
Alias substitution is repeated until the first word of the command has no
alias.  If an alias substitution does not change the first word (as in the
previous example) it is flagged to prevent a loop.  Other loops are detected and
cause an error.
.PP
Some aliases are referred to by the shell; see \fBSpecial aliases\fR.
.SS "Variable substitution"
The shell maintains a list of variables, each of which has as value a list of
zero or more words.
The values of shell variables can be displayed and changed with the
\fIset\fR and \fIunset\fR commands.
The system maintains its own list of ``environment'' variables.
These can be displayed and changed with \fIprintenv\fR, \fIsetenv\fR and
\fIunsetenv\fR.
.PP
(+) Variables may be made read-only with `set \-r' (q.v.)
Read-only variables may not be modified or unset;
attempting to do so will cause an error.
Once made read-only, a variable cannot be made writable,
so `set \-r' should be used with caution.
Environment variables cannot be made read-only.
.PP
Some variables are set by the shell or referred to by it.
For instance, the \fBargv\fR variable is an image of the shell's argument
list, and words of this variable's value are referred to in special ways.
Some of the variables referred to by the shell are toggles;
the shell does not care what their value is, only whether they are set or not.
For instance, the \fBverbose\fR variable is a toggle which causes command
input to be echoed.  The \fB\-v\fR command line option sets this variable.
\fBSpecial shell variables\fR lists all variables which are referred to by the shell.
.PP
Other operations treat variables numerically.  The `@' command permits numeric
calculations to be performed and the result assigned to a variable.  Variable
values are, however, always represented as (zero or more) strings.  For the
purposes of numeric operations, the null string is considered to be zero, and
the second and subsequent words of multi-word values are ignored.
.PP
After the input line is aliased and parsed, and before each command is
executed, variable substitution is performed keyed by `$' characters.  This
expansion can be prevented by preceding the `$' with a `\e' except within `"'s
where it \fIalways\fR occurs, and within `''s where it \fInever\fR occurs.
Strings quoted by ``' are interpreted later (see \fBCommand substitution\fR
below) so `$' substitution does not occur there until later,
if at all.  A `$' is passed unchanged if followed by a blank, tab, or
end-of-line.
.PP
Input/output redirections are recognized before variable expansion, and are
variable expanded separately.  Otherwise, the command name and entire argument
list are expanded together.  It is thus possible for the first (command) word
(to this point) to generate more than one word, the first of which becomes the
command name, and the rest of which become arguments.
.PP
Unless enclosed in `"' or given the `:q' modifier the results of variable
substitution may eventually be command and filename substituted.  Within `"', a
variable whose value consists of multiple words expands to a (portion of a)
single word, with the words of the variable's value separated by blanks.  When
the `:q' modifier is applied to a substitution the variable will expand to
multiple words with each word separated by a blank and quoted to prevent later
command or filename substitution.
.PP
The following metasequences are provided for introducing variable values into
the shell input.  Except as noted, it is an error to reference a variable which
is not set.
.PP
.PD 0
$\fIname\fR
.TP 8
${\fIname\fR}
Substitutes the words of the value of variable \fIname\fR, each separated
by a blank.  Braces insulate \fIname\fR from following characters which would
otherwise be part of it.  Shell variables have names consisting of
letters and digits starting with a letter.  The underscore character is
considered a letter.  If \fIname\fR is not a shell variable, but is set in the
environment, then that value is returned (but some of the other forms
given below are not available in this case).
.PP
$\fIname\fR[\fIselector\fR]
.TP 8
${\fIname\fR[\fIselector\fR]}
Substitutes only the selected words from the value of \fIname\fR.
The \fIselector\fR is subjected to `$' substitution and may consist of
a single number or two numbers separated by a `\-'.
The first word of a variable's value is numbered `1'.
If the first number of a range is omitted it defaults to `1'.
If the last member of a range is omitted it defaults to `$#\fIname\fR'.
The \fIselector\fR `*' selects all words.
It is not an error for a range to be empty if the
second argument is omitted or in range.
.TP 8
$0
Substitutes the name of the file from which command input
is being read.  An error occurs if the name is not known.
.PP
$\fInumber\fR
.TP 8
${\fInumber\fR}
Equivalent to `$argv[\fInumber\fR]'.
.TP 8
$*
Equivalent to `$argv', which is equivalent to `$argv[*]'.
.PD
.PP
The `:' modifiers described under \fBHistory substitution\fR, except for `:p',
can be applied to the substitutions above.  More than one may be used.  (+)
Braces may be needed to insulate a variable substitution from a literal colon
just as with \fBHistory substitution\fR (q.v.); any modifiers must appear
within the braces.
.PP
The following substitutions can not be modified with `:' modifiers.
.PP
.PD 0
$?\fIname\fR
.TP 8
${?\fIname\fR}
Substitutes the string `1' if \fIname\fR is set, `0' if it is not.
.TP 8
$?0
Substitutes `1' if the current input filename is known, `0' if it is not.
Always `0' in interactive shells.
.PP
$#\fIname\fR
.TP 8
${#\fIname\fR}
Substitutes the number of words in \fIname\fR.
.TP 8
$#
Equivalent to `$#argv'.  (+)
.PP
$%\fIname\fR
.TP 8
${%\fIname\fR}
Substitutes the number of characters in \fIname\fR.  (+)
.PP
$%\fInumber\fR
.TP 8
${%\fInumber\fR}
Substitutes the number of characters in $argv[\fInumber\fR].  (+)
.TP 8
$?
Equivalent to `$status'.  (+)
.TP 8
$$
Substitutes the (decimal) process number of the (parent) shell.
.TP 8
$!
Substitutes the (decimal) process number of the last
background process started by this shell.  (+)
.TP 8
$_
Substitutes the command line of the last command executed.  (+)
.TP 8
$<
Substitutes a line from the standard input, with no further interpretation
thereafter.  It can be used to read from the keyboard in a shell script.
(+) While \fIcsh\fR always quotes $<, as if it were equivalent to `$<:q',
\fItcsh\fR does not.  Furthermore, when \fItcsh\fR is waiting for a line to be
typed the user may type an interrupt to interrupt the sequence into
which the line is to be substituted, but \fIcsh\fR does not allow this.
.PD
.PP
The editor command \fIexpand-variables\fR, normally bound to `^X-$',
can be used to interactively expand individual variables.
.SS "Command, filename and directory stack substitution"
The remaining substitutions are applied selectively to the arguments of builtin
commands.  This means that portions of expressions which are not evaluated are
not subjected to these expansions.  For commands which are not internal to the
shell, the command name is substituted separately from the argument list.  This
occurs very late, after input-output redirection is performed, and in a child
of the main shell.
.SS "Command substitution"
Command substitution is indicated by a command enclosed in ``'.  The output
from such a command is broken into separate words at blanks, tabs and newlines,
and null words are discarded.  The output is variable and command substituted
and put in place of the original string.
.PP
Command substitutions inside double
quotes (`"') retain blanks and tabs; only newlines force new words.  The single
final newline does not force a new word in any case.  It is thus possible for a
command substitution to yield only part of a word, even if the command outputs
a complete line.
.PP
By default, the shell since version 6.12 replaces all newline and carriage 
return characters in the command by spaces.  If this is switched off by
unsetting \fBcsubstnonl\fR, newlines separate commands as usual.
.SS "Filename substitution"
If a word contains any of the characters `*', `?', `[' or `{' or begins with
the character `~' it is a candidate for filename substitution, also known as
``globbing''.  This word is then regarded as a pattern (``glob-pattern''), and
replaced with an alphabetically sorted list of file names which match the
pattern.
.PP
In matching filenames, the character `.' at the beginning of a filename or
immediately following a `/', as well as the character `/' must be matched
explicitly.  The character `*' matches any string of characters, including the
null string.  The character `?' matches any single character.  The sequence
`[...]' matches any one of the characters enclosed.  Within `[...]', a pair of
characters separated by `\-' matches any character lexically between the two.
.PP
(+) Some glob-patterns can be negated:
The sequence `[^...]' matches any single character \fInot\fR specified by the
characters and/or ranges of characters in the braces.
.PP
An entire glob-pattern can also be negated with `^':
.IP "" 4
> echo *
.br
bang crash crunch ouch
.br
> echo ^cr*
.br
bang ouch
.PP
Glob-patterns which do not use `?', `*', or `[]' or which use `{}' or `~'
(below) are not negated correctly.
.PP
The metanotation `a{b,c,d}e' is a shorthand for `abe ace ade'.
Left-to-right order is preserved: `/usr/source/s1/{oldls,ls}.c' expands
to `/usr/source/s1/oldls.c /usr/source/s1/ls.c'.  The results of matches are
sorted separately at a low level to preserve this order:
`../{memo,*box}' might expand to `../memo ../box ../mbox'.
(Note that `memo' was not sorted with the results of matching `*box'.)
It is not an error when this construct expands to files which do not exist,
but it is possible to get an error from a command to which the expanded list
is passed.
This construct may be nested.
As a special case the words `{', `}' and `{}' are passed undisturbed.
.PP
The character `~' at the beginning of a filename refers to home directories.
Standing alone, i.e., `~', it expands to the invoker's home directory as
reflected in the value of the \fBhome\fR shell variable.  When followed by a
name consisting of letters, digits and `\-' characters the shell searches for a
user with that name and substitutes their home directory; thus `~ken' might
expand to `/usr/ken' and `~ken/chmach' to `/usr/ken/chmach'.  If the character
`~' is followed by a character other than a letter or `/' or appears elsewhere
than at the beginning of a word, it is left undisturbed.
A command like `setenv MANPATH /usr/man:/usr/local/man:~/lib/man' does not,
therefore, do home directory substitution as one might hope.
.PP
It is an error for a glob-pattern containing `*', `?', `[' or `~', with or
without `^', not to match any files.  However, only one pattern in a list of
glob-patterns must match a file (so that, e.g., `rm *.a *.c *.o' would fail
only if there were no files in the current directory ending in `.a', `.c', or
`.o'), and if the \fBnonomatch\fR shell variable is set a pattern (or list
of patterns) which matches nothing is left unchanged rather than causing
an error.
.PP
The \fBnoglob\fR shell variable can be set to prevent filename substitution,
and the \fIexpand-glob\fR editor command, normally bound to `^X-*', can be
used to interactively expand individual filename substitutions.
.SS "Directory stack substitution (+)"
The directory stack is a list of directories, numbered from zero, used by the
\fIpushd\fR, \fIpopd\fR and \fIdirs\fR builtin commands (q.v.).
\fIdirs\fR can print, store in a file, restore and clear the directory stack
at any time, and the \fBsavedirs\fR and \fBdirsfile\fR shell variables can be set to
store the directory stack automatically on logout and restore it on login.
The \fBdirstack\fR shell variable can be examined to see the directory stack and
set to put arbitrary directories into the directory stack.
.PP
The character `=' followed by one or more digits expands to an entry in
the directory stack.  The special case `=\-' expands to the last directory in
the stack.  For example,
.IP "" 4
> dirs \-v
.br
0       /usr/bin
.br
1       /usr/spool/uucp
.br
2       /usr/accts/sys
.br
> echo =1
.br
/usr/spool/uucp
.br
> echo =0/calendar
.br
/usr/bin/calendar
.br
> echo =\-
.br
/usr/accts/sys
.PP
The \fBnoglob\fR and \fBnonomatch\fR shell variables and the \fIexpand-glob\fR
editor command apply to directory stack as well as filename substitutions.
.SS "Other substitutions (+)"
There are several more transformations involving filenames, not strictly
related to the above but mentioned here for completeness.
\fIAny\fR filename may be expanded to a full path when the
\fBsymlinks\fR variable (q.v.) is set to `expand'.
Quoting prevents this expansion, and
the \fInormalize-path\fR editor command does it on demand.
The \fInormalize-command\fR editor command expands commands in PATH into
full paths on demand.
Finally, \fIcd\fR and \fIpushd\fR interpret `\-' as the old working directory
(equivalent to the shell variable \fBowd\fR).
This is not a substitution at all, but an abbreviation recognized by only
those commands.  Nonetheless, it too can be prevented by quoting.
.SS Commands
The next three sections describe how the shell executes commands and
deals with their input and output.
.SS Simple commands, pipelines and sequences
A simple command is a sequence of words, the first of which specifies the
command to be executed.  A series of simple commands joined by `|' characters
forms a pipeline.  The output of each command in a pipeline is connected to the
input of the next.
.PP
Simple commands and pipelines may be joined into sequences with `;', and will
be executed sequentially.  Commands and pipelines can also be joined into
sequences with `||' or `&&', indicating, as in the C language, that the second
is to be executed only if the first fails or succeeds respectively.
.PP
A simple command, pipeline or sequence may be placed in parentheses, `()',
to form a simple command, which may in turn be a component of a pipeline or
sequence.  A command, pipeline or sequence can be executed
without waiting for it to terminate by following it with an `&'.
.SS "Builtin and non-builtin command execution"
Builtin commands are executed within the shell.  If any component of a
pipeline except the last is a builtin command, the pipeline is executed
in a subshell.
.PP
Parenthesized commands are always executed in a subshell.
.IP "" 4
(cd; pwd); pwd
.PP
thus prints the \fBhome\fR directory, leaving you where you were
(printing this after the home directory), while
.IP "" 4
cd; pwd
.PP
leaves you in the \fBhome\fR directory.  Parenthesized commands are most often
used to prevent \fIcd\fR from affecting the current shell.
.PP
When a command to be executed is found not to be a builtin command the shell
attempts to execute the command via \fIexecve\fR(2).  Each word in the variable
\fBpath\fR names a directory in which the shell will look for the
command.  If the shell is not given a \fB\-f\fR option, the shell
hashes the names in these directories into an internal table so that it will
try an \fIexecve\fR(2) in only a directory where there is a possibility that the
command resides there.  This greatly speeds command location when a large
number of directories are present in the search path. This hashing mechanism is
not used:
.TP 4
.B 1.
If hashing is turned explicitly off via \fIunhash\fR.
.TP 4
.B 2.
If the shell was given a \fB\-f\fR argument.
.TP 4
.B 3.
For each directory component of \fBpath\fR which does not begin with a `/'.
.TP 4
.B 4.
If the command contains a `/'.
.PP
In the above four cases the shell concatenates each component of the path
vector with the given command name to form a path name of a file which it
then attempts to execute it. If execution is successful, the search stops.
.PP
If the file has execute permissions but is not an executable to the system
(i.e., it is neither an executable binary nor a script that specifies its
interpreter), then it is assumed to be a file containing shell commands and
a new shell is spawned to read it.  The \fIshell\fR special alias may be set
to specify an interpreter other than the shell itself.
.PP
On systems which do not understand the `#!' script interpreter convention
the shell may be compiled to emulate it; see the \fBversion\fR shell
variable\fR.  If so, the shell checks the first line of the file to
see if it is of the form `#!\fIinterpreter\fR \fIarg\fR ...'.  If it is,
the shell starts \fIinterpreter\fR with the given \fIarg\fRs and feeds the
file to it on standard input.
.SS Input/output
The standard input and standard output of a command may be redirected with the
following syntax:
.PP
.PD 0
.TP 8
< \fIname
Open file \fIname\fR (which is first variable, command and filename
expanded) as the standard input.
.TP 8
<< \fIword
Read the shell input up to a line which is identical to \fIword\fR.  \fIword\fR
is not subjected to variable, filename or command substitution, and each input
line is compared to \fIword\fR before any substitutions are done on this input
line.  Unless a quoting `\e', `"', `' or ``' appears in \fIword\fR variable and
command substitution is performed on the intervening lines, allowing `\e' to
quote `$', `\e' and ``'.  Commands which are substituted have all blanks, tabs,
and newlines preserved, except for the final newline which is dropped.  The
resultant text is placed in an anonymous temporary file which is given to the
command as standard input.
.PP
> \fIname
.br
>! \fIname
.br
>& \fIname
.TP 8
>&! \fIname
The file \fIname\fR is used as standard output.  If the file does not exist
then it is created; if the file exists, it is truncated, its previo
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