What command means "do nothing" in a conditional in Bash?

Question

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?

Frayne

echo $[a<=5?1:2] a brief ternary does it, ignoring any test for a>=10 – Kiosk

@roblogic, why would you use ancient/deprecated/nonstandard ksh syntax when there are standardized options ($((a <= 5 ? 1 : 2))) available? – Opprobrium

$[] is supported in bash, but yes $(()) is probably better. I like to be brief. – Kiosk

Answer 1

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.

Saporific

@SaintHax No, it doesn't run true. It's built into the shell, and does nothing. – Saporific

it is shell built in (you are correct), it returns true (status 0) just like true does, and it is more difficult to read. Bash is a DevOps language, and if : is written by someone on my team, I'd have them change it. – Dynameter

@Dynameter If someone used true in a non-conditional context, I'd have them change it. if true is fine, then true seems wrong. – Saporific

Of course, you can simply rewrite the code to use if ! condition; then do something; fi instead of if condition; then :; else do something; fi. – Saporific

I agree with you. This example could be written if/elif/else using compound conditions and would be better. – Dynameter

@Saporific maybe it's usefull in this case while (condition) do : done – Echinoderm

@Echinoderm The code is not on a loop, hence while <cond> is not useful – Nevil

@Nevil of course not in this specific case (here you can avoid : by using the code proposed by barmar), but more generally, if you need to wait until a condition is false, you can use the while (condition) do : done – Echinoderm

E.g. while ! ping -c 1 hostname; do : done to wait for ping to succeed. @Echinoderm – Saporific

(err, do :; done, there -- : doesn't get special parsing rules, it still needs to be terminated with a newline or other command separator) – Opprobrium

Answer 2

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

Sclerous

If you don't test the result both 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. – Kirby

The question is tagged bash. In bash, 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). – Joyejoyful

I prefer : simply because it is more explicit what the intent is – Bawl

The result 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... – Tezel

Answer 3

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.)

Ginseng

One reason someone might want to have a no-op is when initially structuring the code with the idea that one will add the "add something here later" without having inadvertent side effects. E.g. if one is putting an if-then-else-fi in a .bashrc, if you put 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. – Periclean

Also, personally, I prefer to write code that is more "human friendly". It's often possible to restructure code to be more concise or even efficient. But when that's not critical, I prefer my code to be more readable for whichever developer comes along afterwards. (I also like commenting my code.) – Chubby

Answer 4

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

Angrist

Answer 5

instead of :, true, false I use

echo -n ""

It avoid empty line in terminal

You could also do it more concisely as:

echo -n

Babur

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). – Opprobrium