Create a temporary file from a stdout redirect or pipe

Question

Some commands only output to stdout.
Some tools only work on files.
Is there a command that can glue those together?

Contrived simple example:

diff $(echo 1 | stdout-to-temp-file) $(echo 2 | stdout-to-temp-file)

My actual use case; Current workaround:

git show HEAD:$1 > /tmp/left && meld /tmp/left $1

My actual use case; Desired:

meld $(git show HEAD:$1 | stdout-to-temp-file) $1

I'd use this in a few other situations too (i.e. I'm not looking for a git or meld only fix).

Solution 1

You could use < to redirect your output to stdin.

I don't know how meld works, but about your diff example, here's how it would work:

Using tempfiles

$ cmd1 > file1.tmp
$ cmd2 > file2.tmp
$ diff file1.tmp file2.tmp

Without tempfiles

$ diff <(cmd1) <(cmd2)

Note that syntax may vary a bit according to the shell you're using (I'm using ksh88).

Solution 2

mktemp will create a temporary filename for you. Save the filename in a variable, and use that in both slots.

For a better solution for your precise problem, check out git difftool. I have mine setup to use meld, and it's pretty great.

Solution 3

I've never used meld but you can accomplish this typically using the - argument to most cli utilities. e.g.

cat /path/to/left/file | diff /input/from/right/file -

Specifically for your meld command you might try something like:

git show HEAD:$1 | meld - $1

Comments

[ConcernedOfTunbridgeWells]

This is the right way to go - much better than using a fixed file name.

[Peter.O]

+1, but that's not redirection. It is Process Substitution ... Process substitution uses /dev/fd/<n> ... eg gedit <(echo Hello) will open a file named 63 or some such fd number, and the "Hello", which was sent to stdout, simply vanishes into the bit-bucket of that process, because gedit doesn't accept stdin, eg echo Hello | gedit.

[rozcietrzewiacz]

+1 for both mktemp and difftool. An alternative to mktemp is tempfile or a file name composed using $$ variable (less secure).

[frogstarr78]

oops initial diff command was incorrect

[Shawn J. Goff]

Process substitution isn't POSIX standard, and isn't available on every shell out there, so be aware of that if you're using it in a script.