Variable substitution

Shell and environment variables:

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 echo, set and unset commands.

The system maintains its own list of `environment' variables. These can be displayed and changed with printenv, setenv and unsetenv.

(+) 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.

Some variables are set by the shell or referred to by it. For instance, the argv 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 verbose variable is a toggle which causes command input to be echoed. The -v command line option sets this variable. Special shell variables lists all variables which are referred to by the shell.

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.

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 `\' except within `"'s where it always occurs, and within `''s where it never occurs. Strings quoted by ``' are interpreted later (see 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.

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.

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.

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.

       $name
       ${name} Substitutes the words of the value of variable name, each sepa-
	       rated  by a blank.  Braces insulate name from following charac-
	       ters 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.   If
	       name  is	 not  a shell variable, but is set in the environment,
	       then that value is returned (but `:' modifiers  and  the	 other
	       forms given below are not available in this case).
       $name[selector]
       ${name[selector]}
	       Substitutes  only  the  selected	 words from the value of name.
	       The selector is subjected to `$' substitution and  may  consist
	       of  a  single  number  or  two numbers separated by a `-'.  The
	       first word of a 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  `$#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.
       $0      Substitutes the name of the file from which  command  input  is
	       being read.  An error occurs if the name is not known.
       $number

       ${number}
	       Equivalent to `$argv[number]'.
       $*      Equivalent to `$argv', which is equivalent to `$argv[*]'.

       The  `:'	 modifiers  described  under  History substitution, 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 History substitution (q.v.); any mod-
       ifiers must appear within the braces.

       The following substitutions can not be modified with `:' modifiers.

       $?name

       ${?name}
	       Substitutes the string `1' if name is set, `0' if it is not.
       $?0     Substitutes  `1' if the current input filename is known, `0' if
	       it is not.  Always `0' in interactive shells.
       $#name
       ${#name}
	       Substitutes the number of words in name.
       $#      Equivalent to `$#argv'.	(+)
       $%name

       ${%name}
	       Substitutes the number of characters in name.  (+)
       $%number
       ${%number}
	       Substitutes the number of characters in $argv[number].  (+)
       $?      Equivalent to `$status'.	 (+)
       $$      Substitutes the (decimal) process number of the (parent) shell.
       $!      Substitutes the (decimal) process number of the last background
	       process started by this shell.  (+)
       $_      Substitutes the command line of the last command executed.  (+)
       $<      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 csh always quotes $<, as
	       if it were equivalent to `$<:q', tcsh does  not.	  Furthermore,
	       when  tcsh  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 csh does not allow this.

       The  editor  command expand-variables, normally bound to `^X-$', can be
       used to interactively expand individual variables.

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.

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.

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.

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.

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.

(+) Some glob-patterns can be negated: The sequence [^...] matches any single character not specified by the characters and/or ranges of characters in the braces.

An entire glob-pattern can also be negated with ^:

	   > echo *
	   bang crash crunch ouch
	   > echo ^cr*
	   bang ouch

       Glob-patterns which do not use ?, *, or [] or which use  {}  or
       ~ (below) are not negated correctly.

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.

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 home 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.

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 nonomatch shell variable is set a pattern (or list of patterns) which matches nothing is left unchanged rather than causing an error.

The noglob shell variable can be set to prevent filename substitution, and the expand-glob editor command, normally bound to `^X-*', can be used to interactively expand individual filename substitutions.

Directory stack substitution (+)

The directory stack is a list of directories, numbered from zero, used by the pushd, popd and dirs builtin commands (q.v.). dirs can print, store in a file, restore and clear the directory stack at any time, and the savedirs and dirsfile shell variables can be set to store the directory stack automatically on logout and restore it on login. The
dirstack shell variable can be examined to see the directory stack and set to put arbitrary directories into the directory stack.

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,

	   > dirs -v
	   0	   /usr/bin
	   1	   /usr/spool/uucp
	   2	   /usr/accts/sys
	   > echo =1
	   /usr/spool/uucp
	   > echo =0/calendar
	   /usr/bin/calendar
	   > echo =-
	   /usr/accts/sys

The noglob and nonomatch shell variables and the expand-glob editor command apply to directory stack as well as filename substitutions.

Other substitutions (+)

There are several more transformations involving filenames, not strictly related to the above but mentioned here for completeness. Any filename may be expanded to a full path when the symlinks variable (q.v.) is set to `expand'. Quoting prevents this expansion, and the normalize-path editor command does it on demand. The normalize-command editor command expands commands in PATH into full paths on demand.
Finally, cd and pushd interpret `-' as the old working directory (equivalent to the shell variable owd). This is not a substitution at all, but an abbreviation recognized by only those commands. Nonetheless, it too can be prevented by quoting.

Related:

OS X Syntax
List of all Environment variables
env - List or Set environment variables


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