I've just run the following in bash:

uniq .bash_history > .bash_history

and my history file ended up completely empty.

I guess I need a way to read the whole file before writing to it. How is that done?

PS: I obviously thought of using a temporary file, but I'm looking for a more elegant solution.

  • 1
    It's because the files get opened from right to left. See also stackoverflow.com/questions/146435/… – WheresAlice Apr 24 '10 at 18:38
  • 2
    You have to write the output to a new file in the same directory and rename that on top of the old file. Any other approach will risk losing your data if it is interrupted halfway through. Some tools may hide this step from you. – kasperd Apr 10 '16 at 17:04
  • Or, bash won't put consecutive dupes in its history if you set HISTCONTROL to include ignoredups; see the manpage. – dave_thompson_085 Apr 13 '16 at 4:54
  • please consider changing the answer to this one. serverfault.com/a/557566/130392 – 23inhouse Nov 7 '19 at 13:31

11 Answers 11


I recommend using sponge from moreutils. From the manpage:

  sponge  reads  standard  input  and writes it out to the specified file. Unlike
  a shell redirect, sponge soaks up all its input before opening the output file.
  This allows for constructing pipelines that read from and write to the same 

To apply this to your problem, try:

uniq .bash_history | sponge .bash_history
  • 8
    It's like cat, but with sucking capabilities :D – MilliaLover Apr 28 '10 at 0:11
echo "$(uniq .bash_history)" > .bash_history

should have the desired result. The subshell gets executed before .bash_history is opened for writing. As explained in Phil P's answer, by the time .bash_history is read in the original command, it has already been truncated by the > operator.

  • 1
    I used this answer to do echo "$(fmt -p '# ' -w 50 readme.txt)" > readme.txt today. Was searching around for a long time for an elegant solution. Many thanks, @Hart Simha! – shredalert Jun 6 '19 at 12:05
  • 1
    this clears the file if the command in the echo raises an error – Boris Nov 3 '20 at 19:06

The problem is that your shell is setting up the command pipeline before running the commands. It's not a matter of "input and output", it's that the file's content is already gone before uniq even runs. It goes something like:

  1. The shell opens the > output file for writing, truncating it
  2. The shell sets up to have file-descriptor 1 (for stdout) be used for that output
  3. The shell executes uniq, perhaps something like execlp("uniq", "uniq", ".bash_history", NULL)
  4. uniq runs, opens .bash_history and finds nothing there

There are various solutions, including the in-place editing and the temporary file usage which others mention, but the key is to understand the problem, what's actually going wrong and why.


Another trick to do this, without using sponge, is the following command:

{ rm .bash_history && uniq > .bash_history; } < .bash_history

This is one of the cheats described in the excellent article “In-place” editing of files on backreference.org.

It basically opens the file for reading, then "removes" it. It's not really removed, though: There's an open file descriptor pointing to it, and as long as that remains open, the file is still around. Then it creates a new file with the same name and writes the unique lines to it.

Disadvantage of this solution: If uniq fails for some reason, your history will be gone.


use sponge from moreutils

uniq .bash_history | sponge .bash_history

As an interesting tidbit, sed uses a temp file as well (this just does it for you):

$ strace sed -i 's/foo/bar/g' foo    
open("foo", O_RDONLY|O_LARGEFILE)       = 3
open("./sedPmPv9z", O_RDWR|O_CREAT|O_EXCL|O_LARGEFILE, 0600) = 4
read(3, "foo\n"..., 4096)               = 4
write(4, "bar\n"..., 4)                 = 4
read(3, ""..., 4096)                    = 0
close(3)                                = 0
close(4)                                = 0
rename("./sedPmPv9z", "foo")            = 0
close(1)                                = 0
close(2)                                = 0

The tempfile ./sedPmPv9z becomes fd 4, and the foo files becomes fd 3. The read operations are on fd 3, and the writes on fd 4 (the temp file). The foo file is then overwritten with the temp file in the rename call.


This sed script removes adjacent duplicates. With the -i option, it does the modification in-place. It's from the sed info file:

sed -i 'h;:b;$b;N;/^\(.*\)\n\1$/ {g;bb};$b;P;D' .bash_history
  • sed still uses the temp file, added an answer with strace illustration (not that it really matters) :-) – Kyle Brandt Apr 25 '10 at 1:15
  • 3
    @Kyle: True, but "out of sight, out of mind". Personally, I would use the explicit temporary file since something like process input > tmp && mv tmp input is much simpler and more readable than using sed trickery simply to avoid a temp file and it won't overwrite my original if it fails (I don't know if sed -i fails gracefully - I would think it would though). Besides, there are lots of things you can do with the output-to-temp-file method that can't be done in-place without something even more involved than this sed script. I know you know all this, but it may benefit some onlooker. – Dennis Williamson Apr 25 '10 at 2:54

Another solution:

uniq file 1<> líneas.txt

A temporary file is pretty much it, unless the command in question happens to support in place editing (uniq doesn't - some seds do (sed -i)).


You can use Vim in Ex mode:

ex -sc '%!uniq' -cx .bash_history
  1. % select all lines

  2. ! run command

  3. x save and close


You can use tee as well, using uniq output as input:

uniq .bash_history | tee .bash_history

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