Bash – Exit status in bash script

bashshell-script

I am trying to create script which can let me to put number of port as parameter and then find the name of service for this port.
Here is the script: 

#!/bin/bash

grep -E "\W$1\/" /etc/services | awk '{ print $1 }'
if [ $? -eq 0 ]; then 
echo "Service(s) is(are) found correctly!"
else
echo "There is(are) no such service(s)!"
fi

Everything works perfectly but there is one problem. If I type such port as 99999 (or another fiction port) – "exit status" for grep -E "\W$1\/" /etc/services | awk '{ print $1 }' also will be 0. In this way all results of my scripts will be correct and else statement in my script won't work. What am I going to do to find the solution to this problem and let my else works fine with "exit status"?

Best Answer

You don't need grep here at all, awk can do the pattern match on the port number.

awk can also keep track of whether the port was found or not and exit with an appropriate exit code.

For example:

$ port=23
$ awk  '$2 ~ /^'"$port"'\// {print $1 ; found=1} END {exit !found}' /etc/services 
telnet
$ echo $?
0

$ port=99999
$ awk  '$2 ~ /^'"$port"'\// {print $1 ; found=1} END {exit !found}' /etc/services 
$ echo $?
1

The exit !found works because awk variables default to zero (or true) if they haven't previously been defined - exit !0 is exit 1. So if we set found=1 when we match then exit !found in the END block is exit 0.

Here's how to use that awk script with your if/then/else.

#!/bin/bash

awk  '$2 ~ /^'"$1"'\// {print $1 ; found=1} END {exit !found}' /etc/services 
if [ $? -eq 0 ]; then 
    echo "Service(s) is(are) found correctly!"
else
    echo "There is(are) no such service(s)!"
fi

You can also do it like this:

if awk  '$2 ~ /^'"$1"'\// {print $1;found=1} END{exit !found}' /etc/services ; then 
    echo "Service(s) is(are) found correctly!"
else
    echo "There is(are) no such service(s)!"
fi

Or even like this:

awk  '$2 ~ /^'"$1"'\// {print $1 ; found=1} END {exit !found}' /etc/services \
  && echo "Service(s) is(are) found correctly!" \
  || echo "There is(are) no such service(s)!"
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