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2011-03-12 - Confoo 2011 presentation
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2010-09-24 - OAuth and pagination problems fixed
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Top Tags



Empty a file

Terminal - Empty a file
> file.txt
2009-01-26 10:22:31
User: root
Empty a file

For when you want to flush all content from a file without removing it (hat-tip to Marc Kilgus).


There is 1 alternative - vote for the best!

Terminal - Alternatives
truncate -s0 file
2009-10-24 16:31:03
User: pipping

The downside of output redirection is that you need permissions. So something like

> file

won't play nicely w/ sudo. You'd need to do something like

bash -c '> file'

instead, you could go w/

sudo truncate -s0 file
:> file
> file
> foobar.txt
truncate -s 0 file.txt
2012-07-30 06:50:30
User: quentin

Immediately make a file empty. This even works if the file is still being written to. Great for cleaning up huge log files!

Know a better way?

If you can do better, submit your command here.

What others think

Nice one! Some thing in *nix are so simple that you'd never imagine they exist :)

Comment by evad 325 weeks and 1 day ago

cat > file.txt followed by shift+ins allows you to insert your buffer content into file. ^d to finish the process

Comment by boombastic 324 weeks and 5 days ago

A similar useful trick which works with things like sudo and pfexec is "cp /dev/null file.txt"; that way you don't have to switch to another account to empty files which you do not own.

Comment by systemj 323 weeks and 5 days ago

To avert surprises, prepend ":" to perform specified redirections reliably:

: > foo

FWIW, the (built-in) null command ":" also expands arguments.

Comment by laburu 323 weeks and 5 days ago

laburu can you expand on the reliability issue you're referring to?

Comment by pixelbeat 323 weeks and 4 days ago

This is especially useful with large files that need to be cleared. I've run across a situation where the inodes ran out on the hd because people kept cat'ing /dev/null and redirecting it to file. That method kept the inodes locked.

Comment by leprasmurf 323 weeks and 3 days ago

It should be noted that this is a Bash-ism. This will hang in ZSH and throw an error to STDERR with BSD-CSH.

Comment by atoponce 320 weeks and 2 days ago

Excellent! Just what I was looking for

Comment by xenon87 319 weeks ago

If you've done set -o noclobber the command will need to be:

>| file.txt

Comment by rtmhal 317 weeks and 6 days ago

This is not just a is a Bash-ism it works fine on the korn shell too.. tested on ksh/HP-UX.

Comment by zlemini 304 weeks and 3 days ago

Perfect for clearing log files :)

Comment by okuehn 288 weeks and 4 days ago

Useful trick, thanks

One more question: if i need to not change the time of the emptied file?

How can i do?

thanks in advance

Comment by bugmenot 235 weeks and 6 days ago

That is very nice also becouse '>' command preserves the permissions

Comment by carlesso 222 weeks and 6 days ago

Thanks! I used to make it longer with: echo "" > file.txt

Comment by ktonga 156 weeks and 4 days ago

The portability to zsh is because in zsh, the command invoked by "> file.txt" is the null command. When zsh is invoked in sh compatibility mode, the option sh_nullcmd will be automatically set, and the null cmd will be ":". If csh_nullcmd (csh compat mode triggers that), then the null cmd will cause an error.

Without a compatibility option, the _default_ null command in zsh is "cat", so "> file.txt" is equivalent to "cat > file.txt" and you'll need to type ctrl-D to get the same effect. Set the shell variable "NULLCMD" to another command to change it. So NULLCMD=: is equivalent to sh_nullcmd and will make the above work.

The suggestion from laburu to use ": > file.txt" is good and is the most portable solution, since its behaviour is guaranteed by POSIX rather than being a shell extension.

Comment by syscomet 152 weeks and 2 days ago

this does *not* work with zsh. It's much better to do ":>file.txt"

Comment by j_melis 138 weeks and 4 days ago

Your point of view

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