What if I don't want to do anything in a if-then block but something in its else block?

Question

#intro
if [ -n "$1" ]
    then
        echo 666
    else
        echo 555
fi
exit;

Actually I do want to echo 555 while I don't want to do anything in the first block, what should I do? I noticed that I can't just remove echo 666.

http://unix.stackexchange.com/questions/37473/what-is-the-utility-of-the-command-in-shell-scripting-given-that-it-explicitl — Archemar, Sep 04 '15 at 14:14

score 6 · Accepted Answer · answered Sep 04 '15 at 14:13

6

Just using no-op:

if [ -n "$1" ]; then
  :
else
  echo 555
fi
exit

or invert the logic:

if [ -z "$1" ]; then
  echo 555
fi
exit

answered Sep 04 '15 at 14:13

cuonglm

153,898

score 3 · Answer 2 · answered Sep 04 '15 at 14:18

3

[ -n "$1" ] && echo "666"  <= For if part
[ -n "$1" ] || echo "555"  <= For else part

answered Sep 04 '15 at 14:18

SHW

14,786
14
66
101

score 3 · Answer 3 · answered Sep 04 '15 at 15:48

You can invert an if statement by using ! (this is called the 'not' operator, which means take the opposite of the result) inside the conditional. So that [ -n "$1" ] becomes:[ ! -n "$1" ], which is the same as the else section.

It's also valid to use the -z option instead of -n which is logically inverse to -n, but as a general rule the ! will always match whatever the else section would match.

What if I don't want to do anything in a if-then block but something in its else block?

3 Answers3