Pipe that does nothing

Question

I'm on a AIX box and need a program that when used after a pipe does nothing.

I'll be more exact. I need something like this:

if [ $NOSORT ] ; then
    SORTEXEC="/usr/bin/doesnothing"
else
    SORTEXEC="/usr/bin/sort -u"
fi
# BIG WHILE HERE
do

done | SORTEXEC

I tried to use tee > /dev/null, but I don't know if there is another better option available.

Can anybody help with a more appropriate program then tee?

Thanks in advance

You need `done | "$SORTEXEC"` (where the double quotes may or may not be advisable -- you cannot then have a command with arguments in the variable, but in this particular scenario you hardly want that anyway; and if you really do, putting the complex command in a function is probably a good idea for many reasons. See also http://mywiki.wooledge.org/BashFAQ/050). — tripleee, May 20 '15 at 14:02

score 7 · Accepted Answer · answered May 20 '15 at 13:50

7

Use tee as follows:

somecommand | tee

This just copies stdin to stdout.

Or uUse true or false. All they do is exit EXIT_SUCCESS or EXIT_FAILURE.

somecommand | true

Notice, every output to stdout from somecommand is dropped.

Another option is to use cat:

somecommand | cat

answered May 20 '15 at 13:50

chaos

8,162
3
33
40

although the true command droped stdout, the cat and tee command worked very well. – Fabio A. Mazzarino May 20 '15 at 15:05
I'd go with the `cat` solution for a generic drop-in no-op replacement for a pipeline filtering stage. – Kusalananda May 27 '15 at 18:50

chepner · Answer 2 · 2015-05-20T19:40:36.660

4

: is the portable, do-nothing command in the POSIX specification.

if [ "$NOSORT" ] ; then
    SORTEXEC=:
else
    SORTEXEC="/usr/bin/sort -u"
fi

: is guaranteed to be a shell built-in in a POSIX-compliant shell; other commands that behave similarly may be external programs that require a new process be started to ignore the output.

However, as tripleee pointed out, strings are meant to hold data, not code. Define a shell function instead:

if [ "$NOSORT" ]; then
    SORTEXEC () { : ; }
else
    SORTEXEC () { /usr/bin/sort -u; }
fi

while ...; do
    ...
done | SORTEXEC

edited May 20 '15 at 19:40

answered May 20 '15 at 13:52

chepner

497,756
71
530
681

1

Better solution than mine +1^^ – chaos May 20 '15 at 13:53
Define "did not work". `echo foo | :` seems to work as intended, at least in the `ksh` that ships with Mac OS X. – chepner May 20 '15 at 15:16

Pipe that does nothing

2 Answers2