Introduction to BASH

If you’ve ever used a Linux operating system, you may have heard of BASH (Bourne Again Shell). It’s a Unix shell that reads and executes various commands. Bash is as powerful as other shells but adds convenience functions. BASH is the free and enhanced version of the Bourne shell distributed with Linux and GNU operating systems. Bash is similar to the original, but has added features

What is a shell?

In computing, a shell program provides access to an operating system's components. The shell gives users (or other programs) a way to get "inside" the system; the shell defines the boundary between inside and outside.

There are two types of operating system shells:

What is BASH used for?

BASH is used for any computer application that requires precision when working with files and data, especially where large numbers of files or large quantities of data need to be searched, sorted, manipulated or processed.

Some of the most common BASH use cases:

Linux and BASH

Bash is a Unix shell and command language written by Brian Fox for the GNU Project as a free software replacement for the Bourne shell.

Bash couldn’t have lasted this long—over 30 years—as the default Linux shell if it wasn’t up to the job.

Linux machines use BASH as their default shell.

bash or Bourne Again shell: the standard GNU shell, intuitive and flexible. Probably most advisable for beginning users while being at the same time a powerful tool for the advanced and professional user. On Linux, bash is the standard shell for common users. This shell is a so-called superset of the Bourne shell, a set of add-ons and plug-ins. This means that the Bourne Again shell is compatible with the Bourne shell: commands that work in sh, also work in bash.

TLDR: If you are on Linux, you should learn BASH as soon as possible :)

Hello World in BASH

We are going to write a “hello world” program in BASH.

Code example:

#!/bin/sh
# This is bash program to write Hello World
echo " Hello World "
Variables

Variables in BASH are used to store data.

Variables can hold different types of data; variables can store integers, strings, and characters.

letter=’c’
color=’blue’
year=2022
Constants

You can also create a constant variable, that is to say, a variable whose value will never change! This can be done by preceding your variable name with the readonly command:

readonly PI=3.14159
Global variables vs Local variables in BASH

By default any variable declared in bash will be a global varialbe. Even if that variable is defined inside a function body.

To force bash to make a variable local you must write local keyword before the name of variable.

Look at this script:

#!/bin/bash

main() {

nr1=1;
nr2=2;


add "$nr1" "$nr2";
echo "local_number1="$local_number1;
echo "local_number2="$local_number2;
}






add() {
local_number1=69;
local local_number2=999;

result=$(($1 + $2));
echo "Result is: $result";
}






main "$@";

If you run the script the output would be:

$ ./test.sh
Result is: 3
local_number1=69
local_number2=

ATTENTION: So you you see, that local_number1=69; variable is not actually local, but it is a global variable, even it was defined inside a function! And to force BASH to actually declar a local variale inside a function you MUST write like this local local_number2=999; basically you must use the local keyword before the name of the variable. Now this second method of declaring a variable is actually declaring a local variable and not a global variable.
Be careful to remeber that BASH is different about this aspect.

ATTENTION IN SHORT BASH AUTOMATICALLY DECLARS ALL VARIABLE AS GLOBA UNLESS YOU SPECIFY THE "local" KEYWORD BEFORE THE NAME OF A VARIALBE!!!

This YouTube video was very helpful:

Using a file to store values from a loop

So if you have a variable a counter a iterator in bash loop that is incremented at each iteration. But you can't print it or echo it after the loop.

This problem is due to Parent Shell and Children Subshells so the best approach would be to write the data of the variable to a file and then read it from a file...

This is the code snipet that it may help me to overcome this problem:


tempfile1="/tmp/temp1.tmp";
tempfile2="/tmp/temp2.tmp";
tempfile3="/tmp/temp3.tmp";

echo 0 > $tempfile1;
echo 0 > $tempfile2;
echo 0 > $tempfile3;

# Loop goes here
for i in {1..500};
do
# CODE BLOCK
echo "$i";

# Fetch the value and increase it
counter1=$[$(cat $tempfile1) + 1];
counter2=$[$(cat $tempfile2) + 10];
counter3=$[$(cat $tempfile3) + 100];

# Store the new value
echo $counter1 > $tempfile1;
echo $counter2 > $tempfile2;
echo $counter3 > $tempfile3;


done;

echo "the file content is="$(cat $tempfile1);
echo "the file content is="$(cat $tempfile2);
echo "the file content is="$(cat $tempfile3);

# Loop done, script done, delete the file
unlink $tempfile1;
unlink $tempfile2;
unlink $tempfile3;

This code will output:

..
..
..
.
488
489
490
491
492
493
494
495
496
497
499
498
500
the file content is=500
the file content is=5000
the file content is=50000
Data Types

BASH is not that aware of data types, bash basically treats it all as strings. However, bash may allow arithmetic operations and comparisons on variables based on the context in which the variable is used.

The fact that bash does not differentiate between strings and integers or floats allows you to be flexible in your shell scripts. Be aware of subtle errors though

Concatenate strings in bash

How do you concatenate strings in bash?

Example:

VAR1="Hello,"
VAR2=" World"
VAR3="$VAR1$VAR2"
echo "$VAR3"

Output:

Hello, World
Length of a string in BASH

How to get the length of a string in BASH?

Using the ${#VAR} syntax will calculate the number of characters in a variable.

For example:

temp1="hello"; echo ${#temp1};

It will output:

5
If statement if condition

This is how you compare 2 strings in bash:

# Equality Comparison
if [ "$a" == "$b" ]; then
echo "Strings match"
else
echo "Strings don't match"
fi
Loops in BASH

Loops allow us to take a series of commands and keep re-running them until a particular situation is reached. They are useful for automating repetitive tasks.

There are 3 basic loop structures in Bash scripting which we'll look at below.

Example of while loop

#!/bin/bash
# Basic while loop
counter=1
while [ $counter -le 10 ]
do
echo $counter
((counter++))
done
echo All done

Output:

./while_loop.sh
1
2
3
4
5
6
7
8
9
10
All done

Example of until loop

#!/bin/bash
# Basic until loop
counter=1
until [ $counter -gt 10 ]
do
echo $counter
((counter++))
done
echo All done

Output:

./while_loop.sh
1
2
3
4
5
6
7
8
9
10
All done

Example of for loop

#!/bin/bash
# Basic for loop
names='Stan Kyle Cartman'
for name in $names
do
echo $name
done
echo All done

Output:

./for_loop.sh
Stan
Kyle
Cartman
All done
for loop in BASH

You can use a C-style for loop:

foo="I love chocolate cookies and cereals";
for (( i=0; i<${#foo}; i++ )); do
echo "${foo:$i:1}";
done

${#foo} expands to the length of foo. ${foo:$i:1} expands to the substring starting at position $i of length 1.

Functions in BASH

How to make a function in BASH?

This is how you declare a function in BASH:

function_name () {
# YOUR CODE BLOCK YOUR COMMANDS GO HERE
}

Ok but how do you call the function after you made it?

This is how you call a function, just write the function name:

#!/bin/bash

hello () {
echo "Hello World";
}

hello;

Will output:

Hello World
Pass arguments to a function in bash

So how do I pass arguments to a function in bash?

To call a function with arguments:

function_name "$arg1" "$arg2"

The function refers to passed arguments by their position (not by name), that is $1, $2, and so forth. $0 is the name of the script itself.

Example:

#!/bin/bash

add() {
result=$(($1 + $2))
echo "Result is: $result"
}

add 1 2

Output

./script.sh
Result is: 3
Return value in a Bash function

In many programming languages, functions do return a value when called; however, this is not the case with bash as bash functions do not return values.

When a bash function finishes executing, it returns the exit status of the last command executed captured in the $? variable. Zero indicates successful execution or a non-zero positive integer (1-255) to indicate failure.

You can use a return statement to alter the function’s exit status. For example, take a look at the following error.sh script:

#! /bin/bash

error () {
blabla
return 0
}

error
echo "The return status of the error function is: $?"

If you run the error.sh bash script, you might be surprised of the output:

kabary@handbook:~$ ./error.sh
./error.sh: line 4: blabla: command not found
The return status of the error function is: 0

Without the return 0 statement, the error function would have never returned a non-zero exit status as blabla is results in a command not found error.

So as you can see, even though bash functions do not return values, I made a workaround by altering function exit statuses.

You should also be aware that a return statement immediately terminates a function.

ATTENTION IN BASH THE "return" KEYWORD DOES NOT RETURN A VARIABLE BUT ONLY RETURNS A STATUS CODE OF 0 OR NOT 0. ZERO MEANS SUCCESS AND NOT ZERO MEANS FAILURE OF THAT FUNCTION.. YOU CAN'T USE RETURN TO RETURN A VALUE

DON'T DO THIS:

function fun1(){
return 34;
}

res=$(fun1);
echo $res;

DO THIS INSTEAD:

You can print a value like this if you want and then pipe it/ redirect it in a variable like bellow:

An interesting question and answer at this link here

Using the find Command

The find command searches for files in a directory hierarchy. We can use this command to print the absolute path of a file:

$ cd /tmp/dir1/
$ find $PWD -type f -name file4.txt
/tmp/dir1/dir2/dir3/dir4/file4.txt

Which is the absolute path of the file that I am searching for!
Now If I want to get it's relative path from one directory to another directory I can use realpath command.

References

Sources and references from which I inspired this Documentation: