Mastodawn

in linux you can use the evil bird emoticon (:>) to destroy files, eg `:> important_document.txt`

the bird will eat the file and leave it completely empty!

@rebane2001 hmm I suppose thats because its writing nothing into the file but I dont understand the reason for the : (I mean yeah it does make the face but from a commands perspective I mean)

Id assume it works without?

Im guessing : is ignored or perhaps written into the file?

@ChaosKitsune @rebane2001 : is the no-op built-in in POSIX shells.

Pink Rhododendron Mar 17

@lambda @ChaosKitsune @rebane2001 does that like actually run the no-op in assembly//machine level ?? or actually do nothing and return 0 ??

@pinkRhododendron @ChaosKitsune @rebane2001 the latter, there's no reason the *implementation* of the shell built-in would have to use an extra machine instruction that does nothing (though maybe the compiler sneakily inserts one anyway, e.g. for alignment reasons!)

@ChaosKitsune https://www.gnu.org/software/bash/manual/bash.html#index-_003a

the colon is a shell command that does nothing

it's often used to perform expansions with side-effects, like:

: ${ARGUMENT:=default value}
: ${MUST_BE_SET:?error message}

Bash Reference Manual

Bash Reference Manual

Jose Andrade Mar 17

@dakkar @ChaosKitsune @rebane2001 also infinite cycles:

while :
do
<whatever>
Done

@jcpandrade ha! I tend to do while true because it reads better to me
(yes, I know that involves a fork+exec, but if we cared about speed we wouldn't be writing shell)
@ChaosKitsune @rebane2001

Jose Andrade Mar 17

@dakkar I used the true but... : is faster and easier to type... Just being lazy, I admit. @ChaosKitsune @rebane2001

always tired Mar 17

@dakkar @ChaosKitsune @rebane2001 @jcpandrade

I'd guess many shells execute true as built-in too.

@project1enigma oh, you're right (I hadn't noticed), bash has indeed true and false as builtins!
@ChaosKitsune @rebane2001 @jcpandrade

@dakkar @ChaosKitsune @rebane2001 so the classic fork bomb is redefining the : command?

@pacostegosaurus yep!
:(){:|:&} is equivalent to:

function :() {
    : | :  &
}

or, changing the name:

function bomb() {
   bomb | bomb &
}

which calls itself twice (piping the non-exsitend output of the first call to the second) as a background process, therefore doubling the number of running processes every time (each process exits immediately, but it still bogs down the machine)

@ChaosKitsune @rebane2001

@dakkar @ChaosKitsune @rebane2001 Nice! I remember reading about this some time ago but somehow I missed that : was not only a valid identifier but actually a standard command

@ChaosKitsune @rebane2001 : is a no-op with no output, which is then redirected to the file.
See: https://stackoverflow.com/questions/3224878/what-is-the-purpose-of-the-colon-gnu-bash-builtin

What is the purpose of the : (colon) GNU Bash builtin?

What is the purpose of a command that does nothing, being little more than a comment leader, but is actually a shell builtin in and of itself? It's slower than inserting a comment into your scripts...

Stack Overflow

Omnivore Mar 17

@ChaosKitsune @rebane2001

':' returns 0 (meaning true). I just checked and '>' clears the file with no input. So, yes, the ':' is just decorative.

Мя ��Mar 17

@ChaosKitsune it's a bash builtin
$ help :
:: :
Null command.

No effect; the command does nothing.

Exit Status:
Always succeeds.

@ChaosKitsune @rebane2001 : is the null command in the POSIX shell standard. It's a built-in that outputs nothing and exits immediately with a status of zero (effectively the same as /bin/true).

So the shell parses ':> afile' as "null command with output redirected to afile"