How to get the command line args passed to a running process on unix/linux systems?

There are several options:

ps -fp <pid>
cat /proc/<pid>/cmdline | sed -e "s/\x00/ /g"; echo

There is more info in /proc/<pid> on Linux, just have a look.

On other Unixes things might be different. The ps command will work everywhere, the /proc stuff is OS specific. For example on AIX there is no cmdline in /proc.

On Linux

cat /proc/<pid>/cmdline

outputs the commandline of the process <pid> (command including args) each record terminated by a NUL character.

A Bash Shell Example:

$ mapfile -d '' args < /proc/$$/cmdline
$ echo "#${#args[@]}:" "${args[@]}"
#1: /bin/bash
$ echo $BASH_VERSION
5.0.17(1)-release

Another variant of printing /proc/PID/cmdline with spaces in Linux is:

cat -v /proc/PID/cmdline | sed 's/\^@/\ /g' && echo

In this way cat prints NULL characters as ^@ and then you replace them with a space using sed; echo prints a newline.

If you want to get a long-as-possible (not sure what limits there are), similar to Solaris' pargs, you can use this on Linux & OSX:

ps -ww -o pid,command [-p <pid> ... ]

Use ps -eo pid,args which prints the PID and the full command line.