Linux: How to use a file as input and output at the same time?

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.

I recommend using sponge from moreutils. From the manpage:

DESCRIPTION
  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 
  file.

To apply this to your problem, try:

uniq .bash_history | sponge .bash_history

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

Description:
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

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