How to pass an awk variable to a bash command

Question

How can I pass an awk variable to a bash command run within awk?

I'm using awk to calculate a running sum of threads for each of a certain process, but then I want to use each pid to access the /proc/$pid filesystem.

The line marked broken below is incorrect, because $pid is non-existent.

How can I export the awk variable pid to the shell command I get awk to run?

(Line breaks added for readability)

$ ps -ALf | grep $PROC | grep -v grep | \         # get each pid
    awk '{threads[$2]+=1} END \                   # count num threads per pid
         { \
           for (pid in threads) \
              "cat /proc/$pid/cwd"|getline cwd; \ # get 'cwd' (**broken**)
               print cwd ": " threads[pid] \      # display 'cwd' and num threads
         }'

Answer 1

You cannot cat /proc/$pid/cwd it is a symlink to a directory. One way to resolve symlinks is with readlink from coreutils.

Here is a working example using bits from TrueY and Barmer:

ps -C $PROC | 
awk '{ t[$1] } END { for(p in t) { "readlink -f /proc/" p "/cwd" | getline c; print c } }'

A couple of things to notice:

• Pipe doubles as a line continuation character.
• It is enough to refer to an array entry to create it (t[$1]).

Answer 2

You can do the whole lot in awk without any greps link this:

% ps -ALf | awk -v proc="${PROC}" '
$0 ~ proc && !/awk/ {
    threads[$2] += 1
}
END {
    for (pid in threads) {
        "readlink /proc/" pid "/cwd" | getline dir
        printf "%d: %d %s\n", pid,  threads[pid], dir
        dir=""
    }
}'

A few things worth noting:

• -v proc="${PROC}" assigns the environment variable ${PROC} to an awk variable, proc
• "readlink /proc/" pid "/cwd" concatenates the three strings. There's no need for any concatenation operator in awk.
• dir="" resets the dir variable, in case the symlink is not readable the next time around the loop.