The if-clause¶
Synopsis¶
if <LIST>; then
<LIST>
fi
if <LIST>; then
<LIST>
else
<LIST>
fi
if <LIST>; then
<LIST>
elif <LIST>; then
<LIST>
else
<LIST>
fi
Description¶
The if
-clause can control the script's flow (what's executed) by looking at the exit codes of other commands.
All commandsets <LIST>
are interpreted as command lists, thus they can contain the whole palette from simple commands over pipelines to compound commands (and their combination) as condition.
Operation¶
The if <LIST>
commands are executed. If the exit code was 0 (TRUE) then the then <LIST>
commands are executed, otherwise the elif <LIST>
commands and their then <LIST>
statements are executed in turn, if all down to the last one fails, the else <LIST>
commands are executed, if one of the elif
succeeds, its then
thread is executed, and the if
-clause finishes.
Basically, the elif
clauses are just additional conditions to test (like a chain of conditions) if the very first condition failed. If one of the conditions fails, the else
commands are executed, otherwise the commands of the condition that succeeded.
Examples¶
Check if a specific user exists in /etc/passwd :-)
if grep ^myuser: /etc/passwd >/dev/null 2>&1; then
echo "Yes, it seems I'm real"
else
echo "Uh - am I a ghost?"
fi
Mount with check
if ! mount /mnt/backup >/dev/null 2>&1; then
echo "FATAL: backup mount failed" >&2
exit 1
fi
Multiple commands as condition
It's perfectly valid to do:
if echo "I'm testing!"; [ -e /some/file ]; then
...
fi
The exit code that dictates the condition's value is the exit code of the very last command executed in the condition-list (here: The [ -e /some/file ]
)
A complete pipe as condition
A complete pipe can also be used as condition. It's very similar to the example above (multiple commands):
if echo "Hello world!" | grep -i hello >/dev/null 2>&1; then
echo "You just said 'hello', yeah?"
fi
Portability considerations¶
See also¶
- Internal: the classic test command