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if_clause.md

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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