Bash

Guides and Tutorials

• Bash Guide for Beginners (opens in a new tab) • A guide to learn bash scripting.
• bash-handbook (opens in a new tab) • This guid is written for those who want to learn Bash without diving in too deeply.
• Bash Scripting Cheat Sheet (opens in a new tab) • A cheat sheet to learn bash scripting.

Tools

• Amber (opens in a new tab) • The Programming Language compiled to Bash. • Docs (opens in a new tab)

Cheat Sheets

IF statement

if ~ then ~ elif ~ then ~ else ~ fi

Operators

An if statement can use the following operators.

-eq - equal to.

-ne - not equal to.

-gt - greater than.

-ge - greater than or equal to.

-lt - less than.

-le - less than or equal to.

-z - true if the string is null (has zero length).

-n - true if the string is not null.

You can also use the following logical operators.

! - logical NOT

-a - logical AND

-o - logical OR

Example: compare two numbers

n=10
 
if [[ n -eq 10 ]]; then
  echo 10
fi

Example: compare two strings

if [[ "abc" == "abc" ]]; then
  echo "Strings are equal"
fi

Example: check if a string is empty

if [[ -z "" ]]; then
  echo "String is empty"
fi

Example: use boolean variable in IF statement

is_bool=true
 
if $is_bool; then
  echo "It is true"
else
  echo "It is false"
fi

Loops

for-in loop

for var in list; do
  # code block to be repeated
done

In this loop, var is a variable that takes on the values in list, which can be an array or a series of values separated by spaces. The code block within the loop is executed for each value of var in list.

Example: the {1..5} expression is expanded by the shell to a list of numbers from 1 to 5.

for i in {1..5}; do
  echo $i
done

Example: the $@ variable is a special variable that contains all the arguments passed to the script. The for loop iterates over the list of arguments and prints each one.

foo() {
  for arg in "$@"; do
    echo $arg
  done
}
 
foo one two three four five
# one two three four five.

Example: The *.txt expression is expanded by the shell to a list of all the files in the current directory that end with .txt.

for file in *.txt; do
  echo $file
done
 
# file1.txt file2.txt file3.txt

Example: The ls command lists all the files in the current directory. The for loop iterates over the list of files and prints each one.

for file in $(ls *.txt); do
  echo $file
done

Example: The find command lists all the files in the current directory and all its subdirectories. The for loop iterates over the list of files and prints each one.

for file in $(find . -name "*.txt"); do
  echo $file
done

Example: The find command lists all the files in the current directory and all its subdirectories. The for loop iterates over the list of files and prints each one. The if statement checks if the file is a regular file.

for file in $(find . -name "*.txt"); do
  if [[ -f $file ]]; then
    echo $file
  fi
done

Example: The find command lists all the files in the current directory and all its subdirectories. The for loop iterates over the list of files and prints each one. The if statement checks if the file is a regular file. The else statement prints a message if the file is not a regular file.

for file in $(find . -name "*.txt"); do
  if [[ -f $file ]]; then
    echo $file
  else
    echo "Not a file"
  fi
done

Example: The find command lists all the files in the current directory and all its subdirectories. The for loop iterates over the list of files and prints each one. The if statement checks if the file is a regular file. The elif statement checks if the file is a directory. The else statement prints a message if the file is not a regular file or a directory.

for file in $(find . -name "*.txt"); do
  if [[ -f $file ]]; then
    echo $file
  elif [[ -d $file ]]; then
    echo "It is a directory"
  else
    echo "Not a file"
  fi
done

while loop

while condition; do
  # code block to be repeated
done

In this loop, the code block within the loop is executed repeatedly as long as condition is true. condition can be any valid expression that returns a true or false value.

until loop

until condition; do
  # code block to be repeated
done

break statement

for var in list; do
  if condition; then
    break
  fi
done

continue statement

for var in list; do
  if condition; then
    continue
  fi
done

Functions

function_name() {
  # code block
}

Arrays

array_name=(value1 value2 value3)

array_name=(
  value1
  value2
  value3
)

array_name=(
  [0]=value1
  [1]=value2
  [2]=value3
)

array_name[0]=value1
array_name[1]=value2
array_name[2]=value3

Associative arrays

declare -A array_name

array_name=([key1]=value1 [key2]=value2 [key3]=value3)

array_name=(
  [key1]=value1
  [key2]=value2
  [key3]=value3
)

array_name[key1]=value1
array_name[key2]=value2
array_name[key3]=value3