How long a PID has been running (taking multiple inputs)

Question

Tonight I tried to write a POSIX shell script which would allow me to check how long a process is running (via it's ID) and be able to handle multiple inputs:

#!/bin/sh 

is_this_a_number()
{
    if [ "$1" -eq "$1" ] 2> /dev/null
    then
        return 0
    else
        return 1
    fi
}

does_pid_exist()
{
    if ps --pid "$1" > /dev/null 2>&1
    then
        return 0
    else
        return 1
    fi
}

how_long_pid_has_been_running()
{
    ps --pid "$1" --format etime=
}

trim_leading_and_trailing_spaces()
{
    awk '{$1=$1;print}'
}


arg_pos=0

while [ "$#" -gt 0 ]
do

    arg_val="$1"
    arg_pos=$((arg_pos + 1))

    if ! is_this_a_number "$arg_val"
    then

        echo "The argument at position $arg_pos is not a number."

    else

        echo "Process ID $arg_val"

        if ! does_pid_exist "$arg_val"
        then

            echo "This process does not exist."

        else

            how_long_pid_has_been_running "$arg_val" | trim_leading_and_trailing_spaces

        fi

    fi

    printf "\\n"

    shift

done

Usage:

./how-long-pid-is-running 1 10078 c 40545454

Where 10078 is an instance of Chrome.

The output is as follows:

Process ID 1
09:04:31

Process ID 10078
07:32:32

The argument at position 3 is not a number.

Process ID 40545454
This process does not exist.

Answer 1

• GNU long options are not POSIX, so instead of ps --pid "$1" --format etime=, write ps -p "$1" -o etime=.
• kill -s 0 "$1" >/dev/null 2>&1 is much faster than ps --pid "$1" > /dev/null 2>&1, partly because kill is a builtin in most shells. Signal 0 is used to test for the existence of a process without sending it a signal.
• Consider redirecting the echo messages to the appropriate output streams, i.e. stdout and stderr.
• is_this_a_number, does_pid_exist: these are used directly in if clauses, so it'd read more naturally if they are renamed to is_a_number and pid_exists.
• A minor point here, but I'd reverse the if-else statements to remove the ! negations.

• Another minor point: during the short time span between testing the existence of a PID and querying the elapsed time since the process with that PID was started, the process with that PID could have finished running or even been substituted with a newly spawned one. You could avoid that by running the second command first, saving the output in a variable and then check for nullness, like thus:

  echo "Process ID $arg_val"
  running_time=$(how_long_pid_has_been_running "$arg_val" 2>/dev/null | 
      trim_leading_and_trailing_spaces)
  if [ -z "${running_time}" ]
  then
      echo "This process does not exist."
  else
      echo "${running_time}"            
  fi
  

• I've seen you write printf "\\n" in another post also, so I'm curious: is it any different from printf "\n"? And in any case, an echo would do the job just as well.

Answer 2

Functions forward the exit status of the last command, so the pattern

f()
{
    if $command
    then
        return 0
    else
        return 1
    fi
}

can be replaced with

f()
{
    $command
}

unless you really must force all error returns to 1.