AIX - BASH REGEX INTO IF STATEMENTS = Segmentation fault (coredump)

If possible, I'd use other means than bash-testing $pid.

I'd use kill or awk instead:

1 - to just check if $pid is a (valid, running) process on AIX:

kill -s 0 -- "$pid" && echo "pid $pid is a running process"

Note: this will NOT kill the process, signal 0 just works if the $pid exist.

However (thanks to @ChrisDavies for pointing it out in the comments!) : "... you must be the owner of the process (or root) to avoid getting an Operation not permitted error"

2 - if you are unsure wheter you got the correct column (This happens a lot with ps output, as the first column (USER) can start with a space or not depending on the username's length):

you probably need to add a space BEFORE the input line:

ps ... | sed -e 's/^/ /' | awk -v pid="$pid" ' something on column $pid '

: this will help awk (and sort and everything else) to not mitakenly see some pid in the first or second column ($1 or $2) depending if the content of that "first" column started with a space or not.

3 - To avoid the problem altogether you could also use a custom ps : You still need the sed as shorter pid's will start with a space, and the longest ones will not!

ps -e -o 'pid, ppid, user, etime, args' \
| sed -e 's/^/ /' \
| awk '
  BEGIN {
      col_pid=1;  col_ppid=2; col_user=3; col_etime=4 start_args=5 
  }
  
  ( $col_pid !~ "/^[0-9]+$/ ) {
      print "line: " FNR " : column " col_pid " does NOT match a number? : \047" $col_pid "\047" 
  }
  '
# you could also do the sed part with awk, but it would : recompute $0, losing spacings