Parameter Substitution
The Bourne shell has a handy set of operators for testing and setting shell variables. They're listed in
| Operator | Explanation |
|---|---|
${var:-default}
| If var is not set or is empty, use default instead. |
${var:=default}
| If var is not set or is empty, set it to default and use that value. |
${var:+instead}
| If var is set and is not empty, use instead. Otherwise, use nothing (null string). |
${var:?message}
| If var is set and is not empty, use its value. Otherwise, print message, if any, and exit from the shell. If message is missing, print a default message (which depends on your shell). |
If you omit the colon (:) from the expressions in Table 45-2, the shell doesn't check for an empty parameter. In other words, the substitution will happen whenever the parameter is set. (That's how some early Bourne shells work: they don't understand a colon in parameter substitution.)
To see how parameter substitution works, here's another version of the bkedit script (, ):
#!/bin/sh if cp "$1" "$1.bak" then ${VISUAL:-/usr/ucb/vi} "$1" exit # USE STATUS FROM EDITOR else echo "`basename $0` quitting: can't make backup?" 1>&2 exit 1 fi
If the VISUAL () environment variable is set and is not empty, its value (like /usr/local/bin/emacs) is used and the command line becomes /usr/local/bin/emacs "$1". If VISUAL isn't set, the command line will default to /usr/ucb/vi "$1".
You can use parameter substitution operators in any command line. You'll see them used with the colon (:) operator (45.9), checking or setting default values. There's an example below. The first substitution (${nothing=default}) will leave $nothing empty because the variable has been set. The second substitution will set $nothing to default because the variable has been set but is empty. The third substitution will leave $something set to stuff:
nothing= something=stuff : ${nothing=default} : ${nothing:=default} : ${something:=default}
The Korn shell and bash have similar string editing operators () like ${var##pattern}. They're useful in shell programs, as well as on the command line and in shell setup files.
- JP