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/proc/$pid/exe

/proc/$pid/exe is a symbolic link to the executable of the process whose PID is $pid.
The following simple shell command iterates over all processes and lists their executables:
for proc in /proc/[1-9]*/; do pid=$(basename $proc); printf "%5d: %s\n" $pid $(readlink $proc/exe); done

See also

/proc/$pid

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