Bash-scripts

A collection of utility Bash scripts for automating common mundane tasks.

Table of Contents

• About-Bash
  • What's-the-purpose-of-shell-scripts?
  • When-should-you-not-use-Bash?
  • Hello-world
  • Executing-a-script
  • The-shebang
  • Variables
  • Command-line-arguments
  • If-statements
  • For-loops
  • Arrays
  • Functions
  • Pipes
  • Redirections
  • Formatting-and-linting
• Available-scripts
  • Intro
  • Math
  • Strings
  • Arrays
  • Files
  • System-administration
  • Programming-workflow
  • Git
  • Utility
• Refrences

About Bash

• Scripting languages originated as extensions of command interpreters in operating systems.
• Bourne shell (sh) was the first significant shell. Bash, today's most used Unix shell, is a GNU/FSF enhancement on the Bourne shell.
• Other shells include: C shell (csh), TC shell (tcsh), Dash (dash), Korn shell (ksh), Z shell (zsh).

What's the purpose of shell scripts?

• When working on programming projects or doing administrative tasks on servers, usually several command sequences are regularly repeated. This is especially true when working with files and directories, processing text, or configuring the network. Numerous times, those command sequences exist in many forms and must be adjusted with user input, necessitating the usage of scripts.
• Scripting languages such as Bash improve our processes by including variables, if statements, loops, arrays, and functions, allowing for more sophisticated program flow.
• The actual power of shell script comes from all of the accessible Unix commands.
• Eventually, scripts get too complicated for basic languages such as Bash. At this stage, you should consider utilizing more powerful programming languages, such as Python.
• Shell scripts may also be used to "glue" together more complex Python scripts.

When should you not use Bash?

• Complex applications.
• GUI.
• Cross-platform portability.
• Calculations.
• Network programming.

Hello world

This is a simple example of a Bash script. It prints the string "Hello World" to the standard output (stdout).

#!/usr/bin/env bash
echo "Hello world"

Executing a script

To run this script, open the terminal in the directory where the script is located and type the following command:

chmod u+x filename.sh
./filename.sh

The shebang

In the first line of a script, the shebang (#!) is used to specify the interpreter to be used when the script is executed. To use the bash interpreter, the first line of a script file must specify the absolute path to the bash executable:

#!/usr/bin/env bash

The bash path in the shebang is resolved and utilized only when a script is launched directly from a terminal. If the script is launched from a shell script, the interpreter is not resolved and the script is executed using the shell interpreter.

./filename.sh

When a bash interpreter is explicitly specified to execute a script, the shebang is ignored:

bash ./filename.sh

Scripts may also be created in a variety of different "scripting languages," thus a Perl script might begin with #!/usr/bin/env perl and one in Python with #!/usr/bin/env python3.

Variables

• Assign the value: var="Test".
• Retrive the value: x</i> or <i>{x}.
• Variables can be defined explicitly as int or array:

declare -i var      # var is an int
declare -a arr      # arr in an array
declare -r var2=5   # var2 is read only

• Variables can store the value of executed command:

var=$(whoami)

Command line arguments

• First argument: $1
• All command line arguments as array: $@
• Number of command line arguments: $#
• The exit status of the last executed command: $?

If statements

If statements are used to execute a block of code if a certain condition is met. Comparison of strings and ints differs. Assume that all values are strings, unless proven otherwise.

if [ $i -eq 10 ]; then echo True; fi         # int comparison
if [ "$name" == "10" ]; then echo True; fi   # string comparison

Integer comparison:

Operator	Description
-eq	equal
-ne	not equal
-gt	greater than
-ge	greater than or equal to
-lt	less than
-le	less than or equal to

String comparison:

Operator	Description
==	equal
!=	not equal
>	greater than
<	less than
-n	string is not null
-z	string is null

Single [] are condition tests that are compatible with the posix shell.

Bash and other shells (e.g. zsh, ksh) allow double as an enhancement to the usual []. They expand the standard possix operations with other operations. For example, instead of -o, it is possible to use || and do regex matching with =~.

If you need to perform word splitting or filename expansion, you'd use single square brackets. Assuming there is just one csv file named 'file.csv' in the current directory, the following line of code will not print True:

if [[ -f *.csv ]]; then echo True; fi

The reason for this is that the test condition checks for a file with the name '*.txt' and no globbing is performed. This line of code, on the other hand, will print True:

if [ -f *.csv ]; then echo True; fi

For loops

A for loop repeats a sequence of steps a number of times.

for number in {1..10}
do
  echo "$number "
done

Arrays

An array is a variable that holds an ordered list of values. The values are separated by spaces. The following example creates an array named array and assigns the values 1, 2, 3, 4, 5 to it:

array=(1 2 3 4 5) 

It is possible to create an array with specified element indices:

array=([3]='elem_a' [4]='elem_b')

To insert an elementat (e.g. 'abc') at a given index (e.g. 2) in the array, use the following syntax:

array=("${array[@]:0:2}" 'new' "${array[@]:2}")

To iterate over the elements of an array, use the following syntax:

items=('item_1' 'item_2' 'item_3' 'item_4')

for item in "${items[@]}"; do
  echo "$item"
done
# => item_1
# => item_2
# => item_3
# => item_4

It is often useful to print the elements of an array on a single line. The following code will print the elements of the array on a single line:

echo "${array[*]}"

Functions

Functions are used to group a sequence of commands into a single unit. They are used to perform repetitive tasks. Functions can be called from anywhere in the script. The following example creates a function named hello_world that prints the string Hello World to the standard output (stdout):

hello_world()
{
  echo "Hello World!"
}

To call the function, use the following syntax:

hello_world

The above function does not take any arguments and does not explicitly return a value. It is possible to pass any number of arguments to the function. It is also possible to return a value from the function, but only an integer from range [0,255] is allowed.

Here is a complete example of a script that defines and uses a function to sum two numbers:

#!/usr/bin/env bash

sum_two() 
{
    return $(($1 + $2))
}

sum_two 5 3
echo $?

Pipes

The pipe is used to pass the output of one command as input to the next:

ps -x | grep chromium

Redirections

But what if you'd want to save the results to a file? Bash has a redirect operator > that may be used to control where the output is delivered.

some_command > out.log            # Redirect stdout to out.log
some_command 2> err.log           # Redirect stderr to file err.log
some_command 2>&1                 # Redirect stderr to stdout
some_command 1>/dev/null 2>&1     # Silence both stdout and stderr

Complete summary:

Syntax	StdOut visibility	StdErr visibility	StdOut in file	StdErr in file	existing file
>	no	yes	yes	no	overwrite
>>	no	yes	yes	no	append
2>	yes	no	no	yes	overwrite
2>>	yes	no	no	yes	append
&>	no	no	yes	yes	overwrite
&>>	no	no	yes	yes	append
tee	yes	yes	yes	no	overwrite
tee -a	yes	yes	yes	no	append
n.e. (*)	yes	yes	no	yes	overwrite
n.e. (*)	yes	yes	no	yes	append
|& tee	yes	yes	yes	yes	overwrite
|& tee -a	yes	yes	yes	yes	append

Formatting and linting

It is important to keep the formatting of your script as consistent as possible. Beautysh is an amazing tool that helps you to format your script. To use it, just run the following command in a directory where your scripts are located:

beautysh **/*.sh

Additionally we advise to use shellcheck for code inspection.

shellcheck **/*.sh

Available scripts

Intro

#	Description	Code
1	Hello world in Bash.	Bash
2	Using if statements, determine whether a condition is true.	Bash
3	Example of a while loop.	Bash
4	Example of a for loop.	Bash
5	A simple script for displaying the digits of a number.	Bash
6	Print all of the numbers in a given interval.	Bash
7	Print a christmas tree.	Bash
8	Prompt the user for a response to a given question.	Bash

Math

#	Description	Code
1	Arithmetic operations.	Bash
2	Sum the parameters passed to the script.	Bash
3	Convert a number from the decimal system to binary form.	Bash
4	Calculate the factorial of an integer.	Bash
5	Is it a prime number?	Bash
6	Calculate the square root of a number.	Bash

Strings

#	Description	Code
1	Count the number of occurrences of a given character in a string.	Bash
2	Convert all uppercase letters in a text string to lowercase.	Bash
3	Convert all lowercase letters in a text string to uppercase.	Bash
4	Check if a string is a palindrome.	Bash
5	Check if two strings are anagrams.	Bash
6	Calculate the Hamming Distance between two strings.	Bash
7	Sort a given string alphabetically.	Bash

Arrays

#	Description	Code
1	Calculate the arithmetic mean of the given n numbers.	Bash
2	Find the maximum value in an array.	Bash
3	Find the minimum value in an array.	Bash
4	Remove duplicates in a given array.	Bash

Files

#	Description	Code
1	Count the number of files in a directory.	Bash
2	Create a directory.	Bash
3	Count the number of lines in a text file.	Bash
4	Get the middle line.	Bash
5	Remove duplicate lines from a file.	Bash
6	Replace left slashes with right slashes and vice versa.	Bash
7	Add the text to the beginning of a specified file.	Bash
8	Remove all lines in a specified file that contain just whitespaces.	Bash
9	Rename all files in a directory with a particular extension to a new extension.	Bash
10	Strip digits from every string found in a given file.	Bash

System administration

#	Description	Code
1	Basic system info.	Bash
2	Check the operating system.	Bash
3	Check if root.	Bash
4	Check if the apt command is available.	Bash
5	Check the RAM size.	Bash
6	Get the CPU temperature.	Bash
7	Get the total CPU usage.	Bash
8	On the local machine, block certain websites from being visited.	Bash
9	Create a backup of your system. Compress the files and encrypt the resulting archive.	Bash
10	Display processes which might be orphans.	Bash
11	Display processes which might be zombies.	Bash

Programming workflow

#	Description	Code
1	Remove carriage return from the given files.	Bash
2	Replace all diacritical characters in the given files.	Bash
3	Change all spaces in file names to underscores and convert them to lowercase.	Bash
4	Remove trailing whitespaces from every file in a given directory.	Bash
5	Beautify and format every shell script found in the current repository.	Bash
6	Find unused functions and classes in a Python project.	Bash

Git

#	Description	Code
1	Reset local repository to match the origin.	Bash
2	Remove the specified branch, both locally and remotely.	Bash
3	Count the number of lines of code in a git repository.	Bash
4	Squash n last commits.	Bash
5	Remove n last commits.	Bash
6	Change the date of the last commit.	Bash
7	Download all of a user's public github repositories.	Bash

Utility

#	Description	Code
1	Find your IP address.	Bash
2	Empty trash.	Bash
3	Block websites from being visited.	Bash
4	Extract files based on extension.	Bash
5	Check which programs are running on a specific port.	Bash
6	Convert month names to numbers and vice versa.	Bash
7	Alias all the scripts from a given directory.	Bash
8	Get a random integer number from the given range.	Bash
9	Generate a random password of the specified length.	Bash
10	Time execution of a program with the parameters supplied.	Bash
11	Download audio from a YouTube video or playlist.	Bash

Refrences

• https://www.gnu.org/software/bash/manual/bash.html
• http://mywiki.wooledge.org/BashGuide
• https://wiki.bash-hackers.org/

Contributing

Pull requests are welcome. For major changes, please open an issue first to discuss what you would like to change.

Please make sure to update tests as appropriate.

License

MIT