Bash-scripts

A collection of Bash scripts for automating routine tasks and streamlining your workflow. From simple file renaming to more complex deployments, these Bash scripts have you covered.

About Bash

Bash (Bourne Again SHell) is an essential component of Unix-like operating systems. It's a powerful scripting language and command interpreter that has evolved from its predecessors in the Unix world. Here's an overview:

Historical Background

• Origin of Scripting Languages: These languages began as enhancements to command interpreters in operating systems.
• The Bourne Shell (sh): Developed in the 1970s, it was the first significant shell in Unix.
• Evolution to Bash: Bash, now the most widely used Unix shell, was developed by the GNU Project as an improvement upon the Bourne shell.
• Other Shells: There are several other shells like the C shell (csh), TC shell (tcsh), Dash (dash), Korn shell (ksh), and Z shell (zsh).

Purpose of Shell Scripts

• Automating Tasks: Shell scripts are invaluable for automating repetitive command sequences, particularly in programming and server administration. This is especially relevant for file and directory operations, text processing, and network configuration.
• Enhanced Functionality: Bash and other scripting languages incorporate features like variables, conditional statements, loops, arrays, and functions for more sophisticated control flow.
• Leveraging Unix Commands: The true strength of shell scripting lies in its ability to utilize the vast array of Unix commands.
• Transition to Advanced Languages: When scripts become overly complex for Bash, transitioning to more powerful languages like Python is advisable.
• Integration with Other Scripts: Bash scripts can be used to integrate or 'glue' together complex scripts written in languages such as Python.

Limitations of Bash

• Complex Applications: Bash is not well-suited for developing complex applications.
• Graphical User Interface (GUI): It's not designed for building GUI applications.
• Cross-Platform Portability: Bash scripts may not be portable across different operating systems without modification.
• Performing Calculations: It's less efficient for complex calculations compared to languages like Python.
• Network Programming: Bash has limitations in handling advanced network programming tasks.

"Hello World" in Bash

A basic example of a Bash script is the famous "Hello World". It demonstrates how to output text to the console.

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

Executing a script

To execute a Bash script, you need to make it executable and then run it from the terminal. Here's how you can do it:

chmod u+x filename.sh  # Make the script executable
./filename.sh          # Execute the script

The Shebang

The shebang (#!) line at the beginning of a script is crucial for defining the script's interpreter. It's more than just a convention; it's an essential aspect of script execution in Unix-like systems. Here's an expanded view:

Example Shebang for Bash:

#!/usr/bin/env bash

Execution Contexts

• Direct Execution: When a script is executed directly from a terminal, the shebang's specified interpreter is used. Example: ./filename.sh
• Nested in Another Script: If the script is invoked from another shell script, the parent script's interpreter is used, and the shebang in the child script is ignored.
• Explicit Interpreter Invocation: When a script is executed with an explicit interpreter command (like bash ./filename.sh), the shebang line is bypassed.

Shebang in Other Languages

• Versatility: The shebang is not limited to Bash scripts. It's used in various scripting languages to specify their respective interpreters.
• Perl Example: #!/usr/bin/env perl
• Python Example: #!/usr/bin/env python3

Variables

Variables in Bash are not just simple placeholders for values; they can be used in more complex ways:

• Assignment: To assign a value to a variable: var="Test"

• Retrieval: To retrieve the value stored in a variable: Using $var or ${var}.

• Integers and Arrays: Bash supports explicitly defining variable types such as integers and arrays.

declare -i var    # 'var' is an integer
declare -a arr    # 'arr' is an array
declare -r var2=5 # 'var2' is a read-only variable

• Command Substitution: Bash allows storing the output of a command in a variable. var=$(whoami)

• Environment variables are global and can be accessed by any process running in the shell session. Example: PATH, HOME, and USER.

• Sharing Variables: To make a variable available to child processes, it needs to be exported. export var

• Local Variables in Functions: Variables in functions can be made local to avoid affecting the global scope.

function myFunc() {
    local localVar="value"
}

Command Line Arguments in Bash

Command line arguments in Bash scripts are accessed using special variables:

• First Argument: $1 represents the first argument passed to the script.
• All Arguments: $@ is an array-like construct that holds all command line arguments.
• Arguments Count: $# gives the number of command line arguments passed.
• Last Command's Exit Status: $? contains the exit status of the last executed command.

If Statements in Bash

If statements in Bash are crucial for decision-making processes based on conditions.

Basic Syntax:

if [ condition ]; then
  # commands
fi

• Integer Comparisons: Use specific operators for comparing integer values.

if [ $i -eq 10 ]; then echo True; fi  # Integer comparison

• String Comparisons: Strings are compared differently from integers.

if [ "$name" == "10" ]; then echo True; fi  # String comparison

Operators for 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

Operators for String Comparison

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

Single vs Double Square Brackets:

• Single Brackets [ ]: Compatible with POSIX shell, suitable for basic tests.
• Double Brackets [[ ]]: Bash and other shells like Zsh and Ksh offer enhanced test constructs.
  • Support logical operators like || and regex matching with =~.
  • Do not perform word splitting or filename expansion.

Filename Expansion Example:

• No Globbing with Double Brackets:

if [[ -f *.csv ]]; then echo True; fi  # Checks for a file named "*.csv"

• Globbing with Single Brackets:

if [ -f *.csv ]; then echo True; fi  # Performs filename expansion

Loops

Loops are used in Bash to execute a series of commands multiple times. There are several types of loops, each serving different purposes.

For Loop

The for loop is used to iterate over a list of items or a range of values.

Syntax:

for var in list; do
  # commands
done

Example with a List:

for i in 1 2 3; do
  echo $i
done

Example with a Range:

for i in {1..3}; do
  echo $i
done

Example with Command Output:

for file in $(ls); do
  echo $file
done

While Loop

The while loop executes as long as a specified condition is true.

Syntax:

while [ condition ]; do
  # commands
done

Example:

i=1
while [ $i -le 3 ]; do
  echo $i
  ((i++))
done

Until Loop

The until loop is similar to the while loop but runs until a condition becomes true.

Syntax:

until [ condition ]; do
  # commands
done

Example:

i=1
until [ $i -gt 3 ]; do
  echo $i
  ((i++))
done

Loop Control: Break and Continue

• break: Exits the loop.

for i in 1 2 3 4 5; do
  if [ $i -eq 3 ]; then
    break
  fi
  echo $i
done

• continue: Skips the rest of the loop iteration and continues with the next one.

for i in 1 2 3 4 5; do
  if [ $i -eq 3 ]; then
    continue
  fi
  echo $i
done

C-Style For Loop

Bash also supports a C-style syntax for for loops, which provides more control over the iteration process.

Syntax:

for (( initialisation; condition; increment )); do
  # commands
done

Example:

for (( i=1; i<=3; i++ )); do
  echo $i
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	Prints "Hello, world!" to the console.	hello_world.sh
2	Demonstrates the use of if statements to check conditions.	conditionals.sh
3	Shows the use of a while loop to repeatedly execute code.	while_loop.sh
4	Demonstrates the use of a for loop to iterate over elements.	for_loop.sh
5	Displays the digits of a given number, one digit per line.	digits.sh
6	Prints all of the numbers within a specified range, one number per line.	numbers_in_interval.sh
7	Prints a Christmas tree pattern to the console.	christmas_tree.sh
8	Prompts the user for a response to a given question and stores their response in a variable.	prompt_for_answer.sh

Math

#	Description	Code
1	Performs basic arithmetic operations (addition, subtraction, multiplication, and division) on two numbers.	arithmetic_operations.sh
2	Calculates the sum of all the arguments passed to it, treating them as numbers.	sum_args.sh
3	Converts a number from the decimal (base 10) system to its equivalent in the binary (base 2) system.	decimal_binary.sh
4	Calculates the factorial of a given integer.	factorial.sh
5	Determines whether a given number is a prime number or not.	is_prime.sh
6	Calculates the square root of a given number.	sqrt.sh

Strings

#	Description	Code
1	Counts the number of times a specific character appears in a given string.	count_char.sh
2	Converts all uppercase letters in a given string to lowercase.	lower.sh
3	Converts all lowercase letters in a given string to uppercase.	upper.sh
4	Checks if a given string is a palindrome, i.e., a word that is spelled the same way forwards and backwards.	is_palindrome.sh
5	Checks if two given strings are anagrams, i.e., if they are made up of the same letters rearranged in a different order.	are_anagrams.sh
6	Calculates the Hamming Distance between two strings, i.e., the number of positions at which the corresponding characters are different.	hamming_distance.sh
7	Sorts a given string alphabetically, considering all letters to be lowercase.	sort_string.sh
8	Creates a word histogram.	word_histogram.sh

Arrays

#	Description	Code
1	Calculates the arithmetic mean of a given list of numbers.	arith_mean.sh
2	Finds the maximum value in a given array of numbers.	max_array.sh
3	Finds the minimum value in a given array of numbers.	min_array.sh
4	Removes duplicates from a given array of numbers.	remove_duplicates_in_array.sh

Files

#	Description	Code
1	Counts the number of files in a specified directory.	count_files.sh
2	Creates a new directory with a specified name.	make_dir.sh
3	Counts the number of lines in a specified text file.	line_counter.sh
4	Gets the middle line from a specified text file.	middle_line.sh
5	Removes duplicate lines from a specified file.	remove_duplicate_lines.sh
6	Replaces all forward slashes with backward slashes and vice versa in a specified file.	switch_slashes.sh
7	Adds specified text to the beginning of a specified file.	prepend_text_to_file.sh
8	Removes all lines in a specified file that contain only whitespaces.	remove_empty_lines.sh
9	Renames all files in a specified directory with a particular extension to a new extension.	rename_extension.sh
10	Strips digits from every string found in a given file.	strip_digits.sh
11	Lists the most recently modified files in a given directory.	recently_modified_files.sh

System administration

#	Description	Code
1	Retrieves basic system information, such as hostname and kernel version.	system_info.sh
2	Determines the type and version of the operating system running on the machine.	check_os.sh
3	Checks whether the current user has root privileges.	check_if_root.sh
4	Checks if the apt command, used for package management on Debian-based systems, is available on the machine.	check_apt_avail.sh
5	Retrieves the size of the machine's random access memory (RAM).	ram_memory.sh
6	Gets the current temperature of the machine's central processing unit (CPU).	cpu_temp.sh
7	Retrieves the current overall CPU usage of the machine.	cpu_usage.sh
8	Blocks certain websites from being visited on the local machine by modifying the hosts file.	web_block.sh
9	Creates a backup of the system's files, compresses the backup, and encrypts the resulting archive for storage.	backup.sh
10	Displays processes that are not being waited on by any parent process. Orphan processes are created when the parent process terminates.	orphans.sh
11	Displays processes that are in an undead state, also known as a "zombie" state. Zombie processes have completed execution but remain in the process table.	zombies.sh

Programming workflow

#	Description	Code
1	Removes the carriage return character (\r) from the given files, which may be present in files transferred between systems with different line ending conventions.	remove_carriage_return.sh
2	Replaces all characters with diacritical marks in the given files with their non-diacritical counterparts. Diacritical marks are small signs added above or below letters to indicate different pronunciations or tones in some languages.	remove_diacritics.sh
3	Changes all spaces in file names to underscores and converts them to lowercase. This can be useful for making the file names more compatible with systems that do not support spaces in file names or for making the file names easier to read or type.	correct_file_names.sh
4	Removes any trailing whitespace characters (spaces or tabs) from the end of every file in a given directory. Trailing whitespace can cause formatting issues or interfere with certain tools and processes.	remove_trailing_whitespaces.sh
5	Formats and beautifies every shell script found in the current repository. This can make the scripts easier to read and maintain by adding consistent indentation and whitespace.	beautify_script.sh
6	Finds functions and classes in a Python project that are not being used or called anywhere in the code. This can help identify and remove unnecessary code, which can improve the project's performance and maintainability.	dead_code.sh

Git

#	Description	Code
1	Resets the local repository to match the state of the remote repository, discarding any local commits and changes. This can be useful for starting over or synchronizing with the latest version on the remote repository.	reset_to_origin.sh
2	Deletes the specified branch both locally and on the remote repository. This can be useful for removing branches that are no longer needed or for consolidating multiple branches into a single branch.	remove_branch.sh
3	Counts the total number of lines of code in a git repository, including lines in all branches and commits. This can be useful for tracking the size and complexity of a project over time.	count_lines_of_code.sh
4	Combines multiple commits into a single commit. This can be useful for simplifying a commit history or for cleaning up a series of small, incremental commits that were made in error.	squash_n_last_commits.sh
5	Removes the n last commits from the repository. This can be useful for undoing mistakes or for removing sensitive information that was accidentally committed.	remove_n_last_commits.sh
6	Changes the date of the last commit in the repository. This can be useful for altering the commit history for cosmetic purposes.	change_commit_date.sh
7	Downloads all of the public repositories belonging to a specified user on GitHub. This can be useful for backing up repositories.	download_all_github_repos.sh
8	Squashes all commits on a specified Git branch into a single commit.	squash_branch.sh
9	Counts the total lines changed by a specific author in a Git repository.	contributions_by_git_author.sh

Utility

#	Description	Code
1	Finds the public IP address of the device running the script.	ip_info.sh
2	Deletes all files in the trash bin.	empty_trash.sh
3	Extracts files with a specified extension from a given directory.	extract.sh
4	Determines which programs are currently using a specified port number on the local system.	program_on_port.sh
5	Converts month names to numbers and vice versa in a string. For example, "January" to "1" and "1" to "January".	month_to_number.sh
6	Creates command aliases for all the scripts in a specified directory, allowing them to be run by simply typing their names.	alias_all_the_scripts.sh
7	Generates a random integer within a given range. The range can be specified as arguments to the script.	rand_int.sh
8	Generates a random password of the specified length, using a combination of letters, numbers, and special characters.	random_password.sh
9	Measures the time it takes to run a program with the specified input parameters. Output the elapsed time in seconds.	time_execution.sh
10	Downloads the audio from a YouTube video or playlist in MP3 format. Specify the video or playlist URL and the destination directory for the downloaded files.	youtube_to_mp3.sh
11	Clears the local caches in the user's cache directory (e.g. ~/.cache) that are older than a specified number of days.	clear_cache.sh
12	Resizes all JPG files in the current directory to a specified dimension (A4).	resize_to_a4

References

• 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