Bash (Bourne Again Shell)

Text file containing sequence of commands

Shebang

Shebang specifies which interpreter to be used to execute the script

# → sharp + ! → bang = shebang

Shebang example

#!/usr/bin/env bash

Run Multiple Commands

Testing Values and Strings

[[ … ]]

[[ condition ]]

Return 0 if condition is true

Return 1 if condition is false

Examples

Testing Files

Numeric Test

:::warning Using [[ expressions ]] for comparing mathematical values sometimes may fail

That is trying

[[ "4" < "7" ]]; echo $?
[[ "40" < "7" ]]; echo $?

Both statements will return true, and exit codes will be 0 :::

Complex conditions statements

Negating statements

If file.txt is not a file then the condition will be true

[[ ! -e file.txt ]]; echo $?

Using Logical And in conditions

To check if 2 conditions are true

[[ $num -gt 5 && $num -lt 10 ]]

Statements in Bash

if statement in bash

if command; then
# command to execute
fi

if and else statement

if command; then
 # command to execute
else
 # command for else 
fi

elif statement

if command1; then
# commands to execute
elif command2; then
# commands to execute
else
# commands to execute
fi

Pattern Matching

[[ “file.txt“ == *.txt ]] will be true

[[ “file.txt“ == “*.txt“ ]] will be false

[[ *.txt == “file.txt“ ]] will be false

For more pattern matching refer https://www.gnu.org/software/bash/manual/html_node/Pattern-Matching.html

case statement

case "${variable}" in
    Pattern1)
        # commands for executing
        ;;
    Pattern2|Pattern3)
        # commands for executing
        ;;
    Pattern4)
        # Commands
        ;;
esac

Loops in bash

While loop

while command; do
  # commands to execute
done

while [[ condition ]]; do
  # commands to execute
done

counter=0
while (( counter < 10 )); do
  (( counter = counter + 1 ))
  echo "Counter = ${counter}"
done

Continue and break in while loop

By-passing a value while running while loop

counter=0
while (( counter < 10 )); do
  (( counter = counter + 1 ))
  if (( counter == 5 )); then
    continue
  fi
  echo "Counter = ${counter}"
done

This loop will print from 1 to 10 but bypass 5

counter=0
while (( counter < 10 )); do
  (( counter = counter + 1 ))
  if (( counter == 5 )); then
    break
  fi
  echo "Counter = ${counter}"
done

This loop will break at 5, means upto the value 5 it will run when the value is 5 loops will exit

for loops

for [variable] in [elements]; do
  # commands
done

Example

for number in One Two; do
  echo $number
done

for number in 1 2 3 4 5 6 7 8 9; do
 echo ${number}
done

:::info Continue and break will work in while loops and for loops :::

Sequence statements

{start..end}

{start..end..step}

echo {1..9}

echo {5..1}

Command substitution in bash

for word in $(seq 1 9); do
echo ${word}
done

For Loop syntax (( init; test; i++ ))

• init: Executed one time at the beginning (Declaring Vaiable)
• test: Executed before each iteration (Evaluating condition)
• after: Executed after each iteration (Increment or decrement to variable)