Perform action if nothing is returned

Question

I'm trying to echo "It was not found" if the netstat returns no result. But, if it does return a result, then to display the netstat results.

I'm trying to google for what I'm using and can't find much about it.

#!/bin/bash

echo "Which process would you like to run netstat against?"
read psname
echo "Looking for '$psname'"
sleep 2
command=$(netstat -tulpn | grep -e $psname)
[[ ! -z $command ]]

It's to do with the [[ ! -z $command ]]

Any process which you'd like to view in netstat. For example, inputting chrome would return all netstat results relating to chrome. That works fine. I'm just trying to echo the message "It was not found" when the inputted process isn't running. For example inputting sjkdaskjlskj should return "It was not found" since there's no process running called that. — ImKeelan, Oct 27 '20 at 20:36
I tried that earlier and it didn't work but now it is haha, thank you anyway! — ImKeelan, Oct 27 '20 at 20:43

score 1 · Accepted Answer · edited Oct 27 '20 at 21:24

[[ ! -z $command ]] doesn't 'show' you any output.

Use a if/else setup to show the result;

if [[ ! -z $command ]]; then
    echo "$psname was found!"
else
    echo "No process named '${psname}' found!"
fi

Or a shorthand variant;

[[ ! -z $command ]] && echo "$psname was found!" || echo "No process named '${psname}' found!"

Note if the user input may contain a space, it's saver to use "" around the grep string;

command=$(netstat -tulpn | grep -e "$psname")

When to wrap quotes around a shell variable?

score 0 · Answer 2 · answered Oct 27 '20 at 22:18

Do not intercept the output directly. You can instead do something like:

if ! netstat -tulpn | grep -e "$psname"; then
    echo "not found" >&2
fi

If grep matches any output, it will print it to stdout. If it mathches nothing, it returns non-zero and the shell will write a message to stderr.

Perform action if nothing is returned

2 Answers2