Assigning system command's output to variable

Note: Coprocess is GNU awk specific. Anyway another alternative is using getline

cmd = "strip "$1
while ( ( cmd | getline result ) > 0 ) {
  print  result
} 
close(cmd)

Calling close(cmd) will prevent awk to throw this error after a number of calls :

fatal: cannot open pipe `…' (Too many open files)

To run a system command in awk you can either use system() or cmd | getline.

I prefer cmd | getline because it allows you to catch the value into a variable:

$ awk 'BEGIN {"date" |  getline mydate; close("date"); print "returns", mydate}'
returns Thu Jul 28 10:16:55 CEST 2016

More generally, you can set the command into a variable:

awk 'BEGIN {
       cmd = "date -j -f %s"
       cmd | getline mydate
       close(cmd)
     }'

Note it is important to use close() to prevent getting a "makes too many open files" error if you have multiple results (thanks mateuscb for pointing this out in comments).

Using system(), the command output is printed automatically and the value you can catch is its return code:

$ awk 'BEGIN {d=system("date"); print "returns", d}'
Thu Jul 28 10:16:12 CEST 2016
returns 0
$ awk 'BEGIN {d=system("ls -l asdfasdfasd"); print "returns", d}'
ls: cannot access asdfasdfasd: No such file or directory
returns 2

Figured out.

We use awk's Two-way I/O

{
  "strip $1" |& getline $1
}

passes $1 to strip and the getline takes output from strip back to $1