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 (44.8, 44.11):
#!/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 (6.3)
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 (9.7)
like ${
var
##pattern
}
.
They're useful in shell programs,
as well as on the command line and in shell setup files.
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