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79 lines
2.4 KiB
Plaintext
79 lines
2.4 KiB
Plaintext
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====== The if-clause ======
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===== Synopsis =====
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<code>
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if <LIST>; then
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<LIST>
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fi
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</code>
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<code>
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if <LIST>; then
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<LIST>
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else
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<LIST>
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fi
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</code>
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<code>
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if <LIST>; then
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<LIST>
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elif <LIST>; then
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<LIST>
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else
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<LIST>
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fi
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</code>
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===== Description =====
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The ''if''-clause can control the script's flow (what's executed) by looking at the exit codes of other commands.
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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.
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==== Operation ====
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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.
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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.
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===== Examples =====
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**Check if a specific user exists in /etc/passwd :-)**
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<code>
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if grep ^myuser: /etc/passwd >/dev/null 2>&1; then
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echo "Yes, it seems I'm real"
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else
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echo "Uh - am I a ghost?"
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fi
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</code>
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**Mount with check**
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<code>
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if ! mount /mnt/backup >/dev/null 2>&1; then
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echo "FATAL: backup mount failed" >&2
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exit 1
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fi
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</code>
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**Multiple commands as condition**
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It's perfectly valid to do:
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<code>
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if echo "I'm testing!"; [ -e /some/file ]; then
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...
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fi
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</code>
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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 ]'')
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**A complete pipe as condition**
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A complete pipe can also be used as condition. It's very similar to the example above (multiple commands):
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<code>
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if echo "Hello world!" | grep -i hello >/dev/null 2>&1; then
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echo "You just said 'hello', yeah?"
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fi
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</code>
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===== Portability considerations =====
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===== See also =====
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* Internal: [[commands:classictest | the classic test command]]
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