It is well known that a command like this:

cat filename | some_sed_command >filename

erases file filename, as the output redirection, being executed before the command, causes filename to be truncated.

One could solve the issue in the following way:

cat file | some_sed_command | tee file >/dev/null

but I'm not sure this would work in any case: what happens if file (and the result of the sed command) is very big? How can the operating system avoid to overwrite some content which is still not read? I see that there is also a sponge command which should work in any case: is it "safer" than tee?

  • What is your main goal ? ( in simple terms ) – Sergiy Kolodyazhnyy Apr 1 '16 at 0:16
  • @Serg simply understand how things work... The answer written by kos clarifies the matter – VeryHardCoder Apr 1 '16 at 12:22

One could solve the issue in the following way:

cat file | some_sed_command | tee file >/dev/null


The chances file will be truncated drop, but there's no guarantee cat file | some_sed_command | tee file >/dev/null will not truncate file.

It all depends on which command is processed first, as as opposed to what one may expect, commands in a pipe are not processed left-to-right. There's no guarantee about which command will be picked first, so one might as well just think of it as randomly picked and never rely on the shell not picking the offending one.

Since the chances for the offending command to be picked first in between three commands are lower than the chances for the offending command to be picked first in between two commands, it's less likely that file will be truncated, but it's still going to happen.


for ((i=0; i<100; i++)); do
    cat >file <<-EOF
    cat file |
        sed 's/bar/baz/' |
        tee file >/dev/null
    [ -s file ] &&
        echo 'Not truncated' ||
        echo 'Truncated'
done |
    sort |
    uniq -c
rm file
% bash script.sh
 93 Not truncated
  7 Truncated
% bash script.sh
 98 Not truncated
  2 Truncated
% bash script.sh
100 Not truncated

So never use something like cat file | some_sed_command | tee file >/dev/null. Use sponge as Oli suggested.

As an alternative, for thighter environments and / or relatively small files one may use a here string and a command substitution to read the file before any command is run:

$ cat file
$ for ((i=0; i<100; i++)); do <<<"$(<file)" sed 's/bar/baz/' >file; done
$ cat file

For sed specifically, you can use its -i in-place argument. It just saves back to the file it opened, eg:

sed -i 's/ /-/g' filename

If you want to do something beefier, assuming you're doing more than sed, yes, you can buffer the whole thing with sponge (from the moreutils package) which will "soak up" all the stdin before writing out to the file. It's like tee but with less functionality. For basic usage though, it's pretty much a drop-in replacement:

cat file | some_sed_command | sponge file >/dev/null

Is that safer? Definitely. It probably has limits through so if you're doing something colossal (and can't in-place edit with sed), you might want to do your edits to a second file and then mv that file back to the original filename. That should be atomic (so anything depending on these files won't break if they need constant access).


You can use Vim in Ex mode:

ex -sc '%!some_sed_command' -cx filename
  1. % select all lines

  2. ! Run command

  3. x Save and quit


Oh, but sponge is not the only option; you do not have to obtain moreutils in order to get this to work properly. Any mechanism will work as long as it satisfies the following two requirements:

  1. It accepts the name of the output file as a parameter.
  2. It only creates the output file once all input has been processed.

You see, the well known problem that the OP is referring to is that the shell will create all of the files that are necessary for the pipes to work before even beginning to execute the commands in the pipeline, so it is the shell that actually truncates the output file (which unfortunately is also the input file) before any of the commands have even had a chance to start executing.

The tee command does not work, even though it satisfies the first requirement, because it does not satisfy the second requirement: it will always create the output file immediately upon starting, so it is essentially as bad as creating a pipe straight into the output file. (It is actually worse, because its use introduces a non-deterministic random delay before the output file gets truncated, so you might think that it works, while in fact it does not.)

So, all we need in order to solve this problem is some command that will buffer all of its input before producing any output, and that is capable of accepting the output filename as a parameter, so that we do not have to pipe its output into the output file. One such command is shuf. So, the following will accomplish the same thing that sponge does:

    shuf --output=file --random-source=/dev/zero 

The --random-source=/dev/zero part tricks shuf into doing its thing without doing any shuffling at all, so it will buffer your input without altering it.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.