19

Say I have a command foo which takes a filename argument: foo myfile.txt. Annoyingly, foo doesn't read from standard input. Instead of an actual file, I'd like to pass it the result of another command (in reality, pv, which will cat the file and output a progress meter as a side effect).

Is there a way to make this happen? Nothing in my bag of tricks seems to do it.

(foo in this case is a PHP script which I believe processes the file sequentially).

I'm using Ubuntu and Bash

EDIT Sorry for the slightly unclear problem description, but here's the answer that does what I want:

pv longfile.txt | foo /dev/stdin

Very obvious now that I see it.

  • Are you looking for a one-liner? – Dave Forgac Jun 13 '12 at 23:50
  • Are you sure foo does not accept an argument of "-" to make it read from stdin? – rackandboneman Jun 14 '12 at 0:09
  • Sadly yes, in this case - it's a php script. – Steve Bennett Jun 18 '12 at 5:14
  • Yes, actually. . – Steve Bennett Jun 18 '12 at 5:17
19

If I understand what you want to do properly, you can do it with bash's command substitution feature:

foo <(somecommand | pv)

This does something similar to what the mkfifo-based answers suggest, except that bash handles the details for you (and it winds up passing something like /dev/fd/63 to the command, rather than a regular named pipe). You might also be able to do it even more directly like this:

somecommand | pv | foo /dev/stdin
  • 1
    oh, nice one! This pv inputfile.txt | foo /dev/stdin is exactly what I was looking for, but didn't think of. – Steve Bennett Jun 18 '12 at 5:13
  • Woot! /dev/stdin – sage Jan 19 '15 at 22:43
  • <(.) worked for me. Is this syntax documented in the Bash manual? I understand (.) as command substitution is documented, but did not see its combination with < explained. – flow2k Feb 2 at 3:12
  • @flow2k It's called "process substitution" -- see section 3.5.6 of the bash manual. – Gordon Davisson Feb 2 at 4:03
  • Got it. You're right - it mentions that it uses named pipes/mkfifo. – flow2k Feb 2 at 6:24
2
ARRAY=(`pv whatever`); for FILE in $ARRAY; do foo $FILE; done
2

This Unix SE question has a an answer that shows how to create a temporary named pipe:

Shell Script mktemp, what's the best method to create temporary named pipe?

Based on the answers there, you could do something like the following:

tmppipe=$(mktemp -u)
mkfifo -m 600 "$tmppipe"
pv examplefile > $tmppipe

You could then watch the pipe with:

foo $tmppipe

Then delete it when you're done with it.

1

Try using mkfifo.

mkfifo file_read_by_foo; pv ... > file_read_by_foo 

In another shell, run foo file_read_by_foo

0

xargs seems to work well for this:

echo longfile.txt | xargs pv
  • That does something different: running pv multiple times, each with one or more lines drawn from longfile.txt. Not at all what I'm after here. – Steve Bennett Jun 18 '12 at 5:17

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