Filtering and extracting PIDs as space-separated list from ps using sed

Question

I want to filter and format the output of ps -ax -o pid,command, but not sure what pipe to use for it.

I want to filter those lines matching a fixed string at a fixed position, just keep the pids for the matched lines, and output them as a space-separated list such as 1234 5678 33121.

I can trivially do the first step using | egrep 'regex', but the second step probably needs sed or something, and I can't work it out. Since sed (and awk) also handle regex filtering, I probably don't need grep.

Multiple space characters are fine, and sort order isn't important, so there's no need to trim the PIDs or order the lines. Example:

$ ps -ax -o pid,command

  PID COMMAND
    0 [kernel]
    1 /sbin/init --
...
  255 /usr/local/sbin/check_status
  268 /sbin/devd -q
 1435 (unlinkd) (unlinkd)
 1974 sleep 60
 7414 /bin/sh /var/db/test/update.sh
21848 ps -ax -o pid,command
39207 /usr/local/sbin/syslog-ng -p /var/run/syslog-ng.pid

find all processes whose command begins /usr/ (ie lines matching /usr/ in chars 7-11 or the regex ^[0-9 ]{5}\s\/usr\/) and return their pids (or first 5 chars) on a single line space-separated. Output:

255 39207   (no \n's, amount of spacing unimportant).

I've tried but sed isn't the easiest command to figure out. How do I do it?

Answer 1

If you're willing to take all the digits of the pid you could use pgrep to do this for you:

pgrep -d " " -f ^/usr

will print a space separated list of all the PIDs of processes whose command starts with /usr

If you really want to use the ps command, awk would be a better tool than sed to do what you want. You could do it, with just the first 5 characters of the PID like:

ps -ax -o pid,command | awk '$2 ~ /^\/usr/ {printf( "%s ", substr($1,0,5));} END {print ""}'

This will check if the second field starts with /usr and if so print up to the first 5 characters of the first field followed by a space, then I threw in a newline at the end for good measure, but you could remove that if you don't want a newline even at the very end

As Giles pointed out, you'll probably never really want just the first 5 characters of a PID (that was probably me misreading the question) so the more useful answer using awk would be:

ps -ax -o pid,command | awk '$2 ~ /^\/usr/ {printf( "%d ", $1);} END {print ""}'