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Related to https://github.com/flokoe/bash-hackers-wiki/issues/10
92 lines
2.4 KiB
Markdown
92 lines
2.4 KiB
Markdown
# The if-clause
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## Synopsis
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if <LIST>; then
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<LIST>
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fi
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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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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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## Description
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The `if`-clause can control the script\'s flow (what\'s executed) by
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looking at the exit codes of other commands.
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All commandsets `<LIST>` are interpreted as [command
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lists](../../syntax/basicgrammar.md#lists), thus they can contain the whole
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palette from [simple commands](../../syntax/basicgrammar.md#simple_commands)
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over [pipelines](../../syntax/basicgrammar.md#pipelines) to [compound
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commands](../../syntax/basicgrammar.md#compound_commands) (and their
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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)
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then the **`then <LIST>`** commands are executed, otherwise the
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**`elif <LIST>`** commands and their **`then <LIST>`** statements are
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executed in turn, if all down to the last one fails, the
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**`else <LIST>`** commands are executed, if one of the `elif` succeeds,
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its `then` thread is executed, and the `if`-clause finishes.
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Basically, the `elif` clauses are just additional conditions to test
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(like a chain of conditions) if the very first condition failed. If one
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of the conditions fails, the `else` commands are executed, otherwise the
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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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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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**Mount with check**
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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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**Multiple commands as condition**
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It\'s perfectly valid to do:
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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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The exit code that dictates the condition\'s value is the exit code of
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the very last command executed in the condition-list (here: The
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`[ -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
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example above (multiple commands):
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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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## Portability considerations
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## See also
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- Internal: [the classic test command](../../commands/classictest.md)
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