Fast, built-in pipe-based data sink

I was curious where this was documented. In the Bash man page under "SHELL BUILTIN COMMANDS" : is the first built in command. Here is the description:

: [arguments]

No effect; the command does nothing beyond expanding arguments and performing any specified redirections. A zero exit code is returned.

:: :

Null command.

No effect; the command does nothing.

Exit Status:

Always succeeds.

That makes a pipe status error :

141 0

where >/dev/null makes no error

I tried it and it's much more slower than redirection:

time sh test.sh

real 0m0.342s

user 0m0.205s

sys 0m0.096s

time sh test1.sh

real 0m7.300s

user 0m0.266s

sys 0m1.912s

test.sh:

for i in `seq 1 100`

do

for j in `seq 1 100`

do

echo i >/dev/null

done

done

test1.sh:

for i in `seq 1 100`

do

for j in `seq 1 100`

do

echo i|:

done

done

giaomuf,

I reproduced your results, and the per-invocation overhead does seem to be higher, but I get this when applying the redirection to the script as a whole instead of invoking it from within the script:

real 0m0.285s

user 0m0.148s

sys 0m0.136s

real 0m0.008s

user 0m0.000s

sys 0m0.008s

for i in `seq 1 100`

do

for j in `seq 1 100`

do

echo i

done

done

I think this is the usage the author intended, and it does seem to be much faster.

try this:

now type in some letters. See how they get echoed to the screen. Now hit a newline. What happens?

.

now contrast this with:

which will soak up all kinds of control char except ^d

This it not faster. In fact, just slower.

You are using a pipe thus calling fork and running inside a subshell where at the same time >dev/null just use a simple file redirection.

Expanding on sputnick's point, this command is not a good idea, because it can cause the upstream code to terminate prematurely. In the example below, the inline perl script is supposed to create a new file foo after producing some output, but it fails to create the file if it is piped through :. (NB: that the amount of output sent to the screen needs to be sufficiently large to observe this effect. YMMV)

ls: foo: No such file or directory

ls: foo: No such file or directory

foo

D'oh...

anyway, it seems to take longer for me. Shorter to type though.

This command runs faster because there's a BROKEN PIPE!

In fact |: works that way:

- the : builtin is executed and its stdin is attached to the command stdout

- when the : execution finishes its stdin is closed, and so the stdout of command.

- this causes a BROKEN PIPE resulting in the error of $PIPESTATUS