How to extract certain argruements from the system command "ps"?

Question

I am working on a server and client program. I run ./server 8080 to specify the port from the command line. In my client program I wrote system("ps -aux | grep -w server | grep -v grep");. It does show PID:197435 0.0 0.0 4360 908 pts/1 S+ 19:34 0:00 ./server 8080. How would I go about formatting my grep to only show the port 8080?

Answer 1

Instead of parsing the output of ps, you could just read the command and arguments from /proc/<pid>/cmdline. Here the arguments are null-delimited, so you will need to convert nulls to spaces.

Assuming I had nc -l localhost 8080 running as pid 3279374, that would look like this:

$ tr '\0' ' ' < /proc/3279374/cmdline
nc -l localhost 8080

You can also ask ps to display just the information you want:

$ ps -p 3279374 -o args=
nc -l localhost 8080

Answer 2

You could use either:

ps ax -o args | awk '$(NF-1) ~ /(^|\/)server$/ && $NF~/^[0-9]{4}$/ {print $NF}'

Like grep -w, except match server in the second last argument specifically*.

ps ax -o args | grep -w 'serve[r]' | awk '$NF ~ /^[0-9]{4}$/ {print $NF}'

Using grep -w as you were - but this can match server as any argument of any running process.

• ps ax -o args is like ps aux, but prints only one column - the command and arguments.
• *The first example matches server in the second last field, because for example if server is a script, the interpreter will be the first field, and the script server (or /path/to/server) the second (even if server is executable, and you invoked it directly).
• eg. It may look like /usr/bin/python3 ./server 8080 if server is a python script.
• If server has other arguments, you can change the field numbers to match.
• grep -w 'serve[r]' avoids matching grep itself.
• ps ax -o args | awk '/(^|[^[:alnum:]_])server($|[[:alnum:]_])/ && $NF~/^[0-9]{4}$/ {print $NF}' is equivalent to the second example, using awk only.
• Not every unix like OS implements procfs (eg Mac, OpenBSD). But ps is POSIX (and the only question tag is sh).