Question

I’m looking for a way to use the ouput of a command (say command1) as an argument for another command (say command2).

I encountered this problem when trying to grep the output of who command but using a pattern given by another set of command (actually tty piped to sed).

Context:

If tty displays:

/dev/pts/5

And who displays:

root     pts/4        2012-01-15 16:01 (xxxx)
root     pts/5        2012-02-25 10:02 (yyyy)
root     pts/2        2012-03-09 12:03 (zzzz)

Goal:

I want only the line(s) regarding “pts/5”
So I piped tty to sed as follows:

$ tty | sed 's/\/dev\///'
pts/5

Test:

The attempted following command doesn’t work:

$ who | grep $(echo $(tty) | sed 's/\/dev\///')"

Possible solution:

I’ve found out that the following works just fine:

$ eval "who | grep $(echo $(tty) | sed 's/\/dev\///')"

But I’m sure the use of eval could be avoided.

As a final side node: I’ve noticed that the “-m” argument to who gives me exactly what I want (get only the line of who that is linked to current user). But I’m still curious on how I could make this combination of pipes and command nesting to work…

Answer 1

You can do this without resorting to sed with the help of Bash variable mangling, although as @ruakh points out this won’t work in the single line version (without the semicolon separating the commands). I’m leaving this first approach up because I think it’s interesting that it doesn’t work in a single line:

TTY=$(tty); who | grep "${TTY#/dev/}"

This first puts the output of tty into a variable, then erases the leading /dev/ on grep’s use of it. But without the semicolon TTY is not in the environment by the moment bash does the variable expansion/mangling for grep.

Here’s a version that does work because it spawns a subshell with the already modified environment (that has TTY):

TTY=$(tty) WHOLINE=$(who | grep "${TTY#/dev/}")

The result is left in $WHOLINE.