csh.1

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command line option.
.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 multiword 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
.B always
occurs, and within `\''s where it
.B never
occurs.
Strings quoted by `\*(ga' are interpreted later (see
.I "Command substitution"
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 file name substituted.
Within `"' a variable whose value consists of multiple words expands to a
(portion of) a single word, with the words of the variables 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 file name 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.
.HP 5
$name
.br
.ns
.HP 5
${name}
.br
Are replaced by the words of the value of variable
.I name,
each separated by a blank.
Braces insulate
.I name
from following characters which would otherwise be part of it.
Shell variables have names consisting of up to 20 letters and digits
starting with a letter.  The underscore character is considered a letter.
.br
If
.I name
is not a shell variable, but is set in the environment, then
that value is returned (but \fB:\fR modifiers and the other forms
given below are not available in this case).
.HP 5
$name[selector]
.br
.ns
.HP 5
${name[selector]}
.br
May be used to select only some of the words from the value of
.I name.
The selector is subjected to `$' substitution and may consist of a single
number or two numbers separated by a `\-'.
The first word of a variables 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 `$#name'.
The selector `*' selects all words.
It is not an error for a range to be empty if the second argument is omitted
or in range.
.HP 5
$#name
.br
.ns
.HP 5
${#name}
.br
Gives the number of words in the variable.
This is useful for later use in a `[selector]'.
.HP 5
$0
.br
Substitutes the name of the file from which command input is being read.
An error occurs if the name is not known.
.HP 5
$number
.br
.ns
.HP 5
${number}
.br
Equivalent to `$argv[number]'.
.HP 5
$*
.br
Equivalent to `$argv[*]'.
.PP
The modifiers `:h', `:t', `:r', `:q' and `:x' may be applied to
the substitutions above as may `:gh', `:gt' and `:gr'.
If braces `{' '}' appear in the command form then the modifiers
must appear within the braces.
.NT
The current implementation allows only one colon (:) modifier on each `$' expansion."
.NE
The following substitutions may not be modified with colon (:) modifiers.
.HP 5
$?name
.br
.ns
.HP 5
${?name}
.br
Substitutes the string `1' if name is set, `0' if it is not.
.HP 5
$?0
.br
Substitutes `1' if the current input file name is known, `0' if it is not.
.HP 5
$$
.br
Substitute the (decimal) process number of the (parent) shell.
.HP 5
$<
.br
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.
.SH Command And File Name Substitution
.NXR "csh command interpreter" "variable substitution"
The remaining substitutions, command and file name substitution,
are applied selectively to the arguments of built-in 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.
.SH Command Substitution
.NXR "csh command interpreter" "command substitution"
Command substitution is indicated by a command enclosed in `\*(ga'.
The output from such a command is normally broken into separate words
at blanks, tabs and new lines, with null words being discarded,
this text then replacing the original string.
Within `"'s, only new lines force new words; blanks and tabs are preserved.
.PP
In any case, the single final new line does not force a new word.
Note that it is thus possible for a command substitution to yield
only part of a word, even if the command outputs a complete line.
.SH File Name Substitution
.NXR "csh command interpreter" "file name substitution"
If a word contains any of the characters `*', `?', `[' or `{'
or begins with the character `~', then that word is a candidate for
file name substitution, also known as `globbing'.
This word is then regarded as a pattern, and replaced with an alphabetically
sorted list of file names which match the pattern.
In a list of words specifying file name substitution it is an error for
no pattern to match an existing file name, but it is not required
for each pattern to match.
Only the metacharacters `*', `?' and `[' imply pattern matching,
the characters `~' and `{' being more akin to abbreviations.
.PP
In matching file names, the character `.' at the beginning of a file name
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
The character `~' at the beginning of a file name is used to refer to home
directories.
Standing alone, that is `~', it
expands to the invokers home directory as reflected
in the value of the variable
.I home.
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 not at the beginning of a word,
it is left undisturbed.
.PP
The metanotation `a{b,c,d}e' is a shorthand for `abe ace ade'.
Left to right order is preserved, with results of matches being sorted
separately at a low level to preserve this order.
This construct may be nested.
Thus `~source/s1/{oldls,ls}.c' expands to
`/usr/source/s1/oldls.c /usr/source/s1/ls.c'
whether or not these files exist without any chance of error
if the home directory for `source' is `/usr/source'.
Similarly `../{memo,*box}' might expand to `../memo ../box ../mbox'.
(Note that `memo' was not sorted with the results of matching `*box'.)
As a special case `{', `}' and `{}' are passed undisturbed.
.SH Input/output
.NXR "csh command interpreter" "command input/output"
The standard input and standard output of a command may be redirected
with the following syntax:
.HP 5
< name
.br
Open file
.I name
(which is first variable, command and file name expanded) as the standard
input.
.HP 5
<< word
.br
Read the shell input up to a line which is identical to
.I word.
.I Word
is not subjected to variable, file name or command substitution,
and each input line is compared to
.I word
before any substitutions are done on this input line.
Unless a quoting `\e', `"', `\*(aa' or `\*(ga' appears in
.I word
variable and command substitution is performed on the intervening lines,
allowing `\e' to quote `$', `\e' and `\*(ga'.
Commands which are substituted have all blanks, tabs, and new lines
preserved, except for the final new line which is dropped.
The resultant text is placed in an anonymous temporary file which
is given to the command as standard input.
.HP 5
> name
.br
.ns
.HP 5
>! name
.br
.ns
.HP 5
>& name
.br
.ns
.HP 5
>&! name
.br
The file
.I name
is used as standard output.
If the file does not exist then it is created;
if the file exists, it is truncated, its previous contents being lost.
.IP
If the variable
.I noclobber
is set, then the file must not exist or be a character special file (for example,
a terminal or `/dev/null') or an error results.
This helps prevent accidental destruction of files.
In this case the `!' forms can be used and suppress this check.
.IP
The forms involving `&' route the diagnostic output into the specified
file as well as the standard output.
.I Name
is expanded in the same way as `<' input file names are.
.HP 5
>> name
.br
.ns
.HP 5
>>& name
.br
.ns
.HP 5
>>! name
.br
.ns
.HP 5
>>&! name
.br
Uses file
.I name
as standard output like `>' but places output at the end of the file.
If the variable
.I noclobber
is set, then it is an error for the file not to exist unless
one of the `!' forms is given.
Otherwise similar to `>'.
.PP
A command receives the environment in which the shell was
invoked as modified by the input-output parameters and
the presence of the command in a pipeline.
Thus, unlike some previous shells, commands run from a file of shell commands
have no access to the text of the commands by default; rather
they receive the original standard input of the shell.
The `<<' mechanism should be used to present inline data.
This permits shell command scripts to function as components of pipelines
and allows the shell to block read its input.
Note that the default standard input for a command run detached is
.B not
modified to be the empty file `/dev/null'; rather the standard input
remains as the original standard input of the shell.  If this is a terminal
and if the process attempts to read from the terminal, then the process
will block and the user will be notified (see
.B Jobs
above.)
.PP
Diagnostic output may be directed through a pipe with the standard output.
Simply use the form `|\|&' rather than just `|'.
.SH Expressions
.NXR "csh command interpreter" "expressions"
A number of the built-in commands (to be described subsequently)
take expressions, in which the operators are similar to those of C, with
the same precedence.
These expressions appear in the
.I @,
.I exit,
.I if,
and
.I while
commands.
The following operators are available:
.DT
.PP
	|\|\||  &&  |  !  &  ==  !=  =~  !~  <=  >=  <  >  <<  >>  +  \-  *  /  %  !  ~  (  )
.PP
Here the precedence increases to the right,
`==' `!=' `=~' and `!~', `<=' `>=' `<' and `>', `<<' and `>>', `+' and `\-',
`*' `/' and `%' being, in groups, at the same level.
The `==' `!=' `=~' and `!~' operators compare their arguments as strings;
all others operate on numbers.
The operators `=~' and `!~' are like `!=' and `==' except that the right
hand side is a
.I pattern
(containing, for example, `*'s, `?'s and instances of `[...]')
against which the left hand operand is matched.  This reduces the
need for use of the
.I switch
statement in shell scripts when all that is really needed is pattern matching.
.PP
Strings which begin with `0' are considered octal numbers.
Null or missing arguments are considered `0'.
The result of all expressions are strings,
which represent decimal numbers.
It is important to note that no two components of an expression can appear
in the same word; except when adjacent to components of expressions which
are syntactically significant to the parser (`&' `|' `<' `>' `(' `)')
they should be surrounded by spaces.
.PP
Also available in expressions as primitive operands are command executions
enclosed in `{' and `}'
and file enquiries of the form `\-\fIl\fR  name' where
.I  l
is one of:
.PP
.DT
.nf
	r	read access
	w	write access
	x	execute access
	e	existence
	o	ownership
	z	zero size
	f	plain file
	d	directory
.fi
.PP
The specified name is command and file name expanded and then tested
to see if it has the specified relationship to the real user.
If the file does not exist or is inaccessible then all enquiries return
false, that is `0'.
Command executions succeed, returning true, that is `1',
if the command exits with status 0, otherwise they fail, returning
false, that is `0'.
If more detailed status information is required then the command
should be executed outside of an expression and the variable
.I status
examined.
.SH Control Flow
.NXR "csh command interpreter" "flow of control"
The shell contains a number of commands which can be used to regulate the
flow of control in command files (shell scripts) and
(in limited but useful ways) from terminal input.
These commands all operate by forcing the shell to reread or skip in its
input and, due to the implementation, restrict the placement of some
of the commands.
.PP
The
.I foreach,
.I switch,
and
.I while
statements, as well as the
.I if\-then\-else
form of the
.I if
statement require that the major keywords appear in a single simple command
on an input line as shown below.
.PP
If the shell's input is not seekable,
the shell buffers up input whenever a loop is being read
and performs seeks in this internal buffer to accomplish the rereading
implied by the loop.
(To the extent that this allows, backward goto's will succeed on
non-seekable inputs.)
.SH Built-in Commands
.NXR "csh command interpreter" "built-in commands"
Built-in commands are executed within the shell.
If a built-in command occurs as any component of a pipeline
except the last then it is executed in a subshell.
.HP 5
.B  alias
.br
.ns
.HP 5
.BR alias " name"
.br
.ns
.HP 5
.BR alias " name wordlist"
.NXR "alias command (csh)"
.br
The first form prints all aliases.
The second form prints the alias for name.
The final form assigns the specified
.I wordlist
as the alias of 
.I name;
.I wordlist
is command and file name substituted.
.I Name
is not allowed to be
.I alias
or
.I unalias.
.HP 5
.B  alloc
.NXR "alloc command (csh)"
.br
Shows the amount of dynamic core in use, broken down into used and
free core, and address of the last location in the heap.
With an argument shows each used and free block on the internal dynamic
memory chain indicating its address, size, and whether it is used or free.
This is a debugging command and may not work in production versions of the
shell; it requires a modified version of the system memory allocator.
.HP 5
.B bg
.NXR "bg command (csh)"
.br
.ns
.HP 5
\fBbg\ %\fRjob\ ...
.br
Puts the current or specified jobs into the background, continuing them
if they were stopped.
.HP 5
.B  break
.NXR "break command (csh)"
.br
Causes execution to resume after the
.I end
of the nearest enclosing
.I foreach