mirror of
https://github.com/rawiriblundell/wiki.bash-hackers.org
synced 2024-12-26 06:20:41 +01:00
79 lines
2.4 KiB
Markdown
79 lines
2.4 KiB
Markdown
====== The if-clause ======
|
|
|
|
===== Synopsis =====
|
|
<code>
|
|
if <LIST>; then
|
|
<LIST>
|
|
fi
|
|
</code>
|
|
<code>
|
|
if <LIST>; then
|
|
<LIST>
|
|
else
|
|
<LIST>
|
|
fi
|
|
</code>
|
|
<code>
|
|
if <LIST>; then
|
|
<LIST>
|
|
elif <LIST>; then
|
|
<LIST>
|
|
else
|
|
<LIST>
|
|
fi
|
|
</code>
|
|
|
|
===== 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 [[syntax:basicgrammar#lists | command lists]], thus they can contain the whole palette from [[syntax:basicgrammar#simple_commands | simple commands]] over [[syntax:basicgrammar#pipelines | pipelines]] to [[syntax:basicgrammar#compound_commands | 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 :-)**
|
|
<code>
|
|
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
|
|
</code>
|
|
|
|
**Mount with check**
|
|
<code>
|
|
if ! mount /mnt/backup >/dev/null 2>&1; then
|
|
echo "FATAL: backup mount failed" >&2
|
|
exit 1
|
|
fi
|
|
</code>
|
|
|
|
**Multiple commands as condition**
|
|
|
|
It's perfectly valid to do:
|
|
<code>
|
|
if echo "I'm testing!"; [ -e /some/file ]; then
|
|
...
|
|
fi
|
|
</code>
|
|
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):
|
|
<code>
|
|
if echo "Hello world!" | grep -i hello >/dev/null 2>&1; then
|
|
echo "You just said 'hello', yeah?"
|
|
fi
|
|
</code>
|
|
|
|
===== Portability considerations =====
|
|
|
|
|
|
===== See also =====
|
|
* Internal: [[commands:classictest | the classic test command]]
|