Day 30: Bash shell scripting explained

Shell scripting

I’d like to see more people doing shell scripting.

Shell v/s Powershell

Unlike Powershell, GNU/Linux shell scripting is easily remembered.

I remember the first time I was checking out Powershell.

God forgive, the commands were so long and so senseless.

GNU/Linux shell scripting is very different from Powershell.

The syntax themselves are very simple.

And most importantly, we are reusing ‘normal’ system commands.

Variables

Variables are simple to create.

  1. Single quotes(‘) for strings that would normally not change and needs no interpretation.
     #Directory variable
     docs_directory='/usr/local/lib/docs'
    
  2. Double quotes would be used for strings that require interpretation and change dynamically.
     document_file="$docs_directory/mydocument.txt"
    

    Because we are using the previously set variable, we need to interprete(expand) document_file variable

#!/bin/bash

docs_directory='/usr/local/lib/docs'
document_file="$docs_directory/mydocument.txt"

echo "Document Directory: $docs_directory"
echo "Document File: $document_file"

bash variables linux

Commands

As you would run commands under GNU/Linux, the same principle applies for shell.

cat vars.sh

bash vars commands

[email protected]:~/docs/bash_scripting$ cat commands.sh 
#!/bin/bash

cat vars.sh

This is the simplest shell script you can make with a single command.

But you can add up as many commands as you would require.

Arguments and functions

There are positional arguments that you can use while invoking a bash script.

This is handy for cases where you need to do a lot of quick testing.

The first argument would be $1, second is $2 and so on…

Functions on the other hand are simple to initialise

function func_name { }

Say we want to do a ping on a website.

By default a ping goes infinitely

[email protected]:~/docs/bash_scripting$ ping google.com
PING google.com (216.58.223.110) 56(84) bytes of data.
64 bytes from mba01s08-in-f14.1e100.net (216.58.223.110): icmp_seq=1 ttl=118 time=43.7 ms
64 bytes from mba01s08-in-f14.1e100.net (216.58.223.110): icmp_seq=2 ttl=118 time=41.4 ms
64 bytes from mba01s08-in-f14.1e100.net (216.58.223.110): icmp_seq=3 ttl=118 time=42.8 ms
64 bytes from mba01s08-in-f14.1e100.net (216.58.223.110): icmp_seq=4 ttl=118 time=41.2 ms
64 bytes from mba01s08-in-f14.1e100.net (216.58.223.110): icmp_seq=5 ttl=118 time=42.3 ms
64 bytes from mba01s08-in-f14.1e100.net (216.58.223.110): icmp_seq=6 ttl=118 time=45.1 ms
64 bytes from mba01s08-in-f14.1e100.net (216.58.223.110): icmp_seq=7 ttl=118 time=42.6 ms

Let’s restrict it to a count of 3

vim ping_checker.sh

#ping_checker.sh contents below
#!/bin/bash

#we first initialise the variable to signify our first argument
target_site=$1

#we then create our function
function pinger {
ping -c3 $target_site
}

#we then run the function
pinger

Launching the ping checker. ping checker bash arguments functions

Case study

Let’s say we want to check not only the ping

But also return the average after 3 ping attempts.

All this without showing the details of each ping request.

We first make a copy of ping_checker

[email protected]:~/docs/bash_scripting$ cp -p ping_checker.sh ping_checker_v2.sh

We modify the ping line to send the results to a text file.

ping -c3 $target_site > ${target_site}_ping.txt

Once done, we’ll read back this file and get the necessary details.

cat ${target_site}_ping.txt | tail -n1 | rev | cut -d "/" -f3 | rev

We read the file, we take the last line, reverse the string.

We then take the 3rd slash (we want the average ping), then we reverse it again,

On the last reverse, we get the original string.

Final script

avg ping

\Codarren/

Written on January 30, 2021