Sometimes when making conditionals, I need the code to do nothing, e.g., here, I want Bash to do nothing when $a is greater than "10", print "1" if $a is less than "5", otherwise, print "2":
if [ "$a" -ge 10 ]
then
elif [ "$a" -le 5 ]
then
echo "1"
else
echo "2"
fi
This makes an error though. Is there a command which will do nothing and also not slow down my script?
This question is related to
bash
conditional
noop
The no-op command in shell is : (colon).
if [ "$a" -ge 10 ]
then
:
elif [ "$a" -le 5 ]
then
echo "1"
else
echo "2"
fi
From the bash manual:
:(a colon)
Do nothing beyond expanding arguments and performing redirections. The return status is zero.
Although I'm not answering the original question concering the no-op command, many (if not most) problems when one may think "in this branch I have to do nothing" can be bypassed by simply restructuring the logic so that this branch won't occur.
I try to give a general rule by using the OPs example
do nothing when $a is greater than "10", print "1" if $a is less than "5", otherwise, print "2"
we have to avoid a branch where $a gets more than 10, so $a < 10 as a general condition can be applied to every other, following condition.
In general terms, when you say do nothing when X, then rephrase it as avoid a branch where X. Usually you can make the avoidance happen by simply negating X and applying it to all other conditions.
So the OPs example with the rule applied may be restructured as:
if [ "$a" -lt 10 ] && [ "$a" -le 5 ]
then
echo "1"
elif [ "$a" -lt 10 ]
then
echo "2"
fi
Just a variation of the above, enclosing everything in the $a < 10 condition:
if [ "$a" -lt 10 ]
then
if [ "$a" -le 5 ]
then
echo "1"
else
echo "2"
fi
fi
(For this specific example @Flimzys restructuring is certainly better, but I wanted to give a general rule for all the people searching how to do nothing.)
You can probably just use the true command:
if [ "$a" -ge 10 ]; then
true
elif [ "$a" -le 5 ]; then
echo "1"
else
echo "2"
fi
An alternative, in your example case (but not necessarily everywhere) is to re-order your if/else:
if [ "$a" -le 5 ]; then
echo "1"
elif [ "$a" -lt 10 ]; then
echo "2"
fi
Source: Stackoverflow.com