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 [ "$a" -le 5 ] then echo "1" else echo "2" fi This makes an error though. Is there a command which will do nothing and also not slow down my script?
The no-op command in shell is : (colon).
if [ "$a" -ge 10 ] then : elif [ "$a" -le 5 ] then echo "1" else echo "2" fi From the bash manual:
:(a colon)
Do nothing beyond expanding arguments and performing redirections. The return status is zero.
true. It's built into the shell, and does nothing.
: is written by someone on my team, I'd have them change it.true in a non-conditional context, I'd have them change it. if true is fine, then true seems wrong.if ! condition; then do something; fi instead of if condition; then :; else do something; fi.while ! ping -c 1 hostname; do : done to wait for ping to succeed. @kingsjesterYou can probably just use the true command:
if [ "$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 
true and false are effectively no-ops as far as the script is concerned, but in principle they could fork in some shells that accept this syntax so perhaps : is better.
true and false are built-ins. For older shells, true, false, and even : might be external commands (though you'd probably see the latter only in very old shells).
: simply because it is more explicit what the intent is
false may not be a no-op, because if set -e is used (exit on non-zero) then false will cause the script to exit immediately. true is a better no-op. : is a real no-op, but I have to agree that it is unreadable and I've been programming bash for 30 years and I didn't know about it...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.)
echo "put stuff here" it'll break some of the non-login uses (e.g. scp gets broken), and only whitespace/comments causes a runtime error.
It looks like that there is no other way than writing your own nop as a function. : does not the intended job, as it affects the exit status; maybe you'll find a builtin in the manual that does not change the exit status, but I've failed to exhibit such a one. You may say:
nop() { return; } # may return $? bad() { return 42; } bad; nop; echo $? # "nop" does nothing and keeps status; echo echoes 42 bad; : ; echo $? # ":" does nothing but changes status; echo echoes 0 instead of :, true, false I use
echo -n "" It avoid empty line in terminal
You could also do it more concisely as:
echo -n 
echo -n is listed in the POSIX specification as explicitly undefined behavior (see the APPLICATION USAGE section of pubs.opengroup.org/onlinepubs/9699919799/utilities/echo.html spelling this out), and even if you 100% know your shell is bash, it still can change behavior on runtime flags (see shopt -s xpg_echo and how its behavior meshes with set -o posix -- moreover, both those flags can be set by environment variables even if your code never actually runs them).
echo $[a<=5?1:2]a brief ternary does it, ignoring any test fora>=10$((a <= 5 ? 1 : 2))) available?$[]is supported in bash, but yes$(())is probably better. I like to be brief.