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03 Linux Shell

Introduction

  • Shell is a facilitator between user and the operating system.

Types of Linux shells

  • echo $SHELL - Displays the current shell.
  • cat /etc/shells - Displays all the available shells.
  • Type the shell name in the command line to switch between the shells. For example: zsh.
  • chsh -s /usr/bin/zsh - To permanently change the shell.

Bourne Again Shell (Bash)

  • Bourne Again Shell (Bash) - It is the default shell in linux.
  • Bash came as an enhanced replacement of previous shells sh, ksh, csh, etc.
  • Some features of bash are
    • Offers tab completion.
    • Keeps the history of commands.
  • Bash does not offer auto spell correction.

Friendly Interactive Shell (FISH)

  • Offers auto spell correction.
  • Offers customization with cool themes.
  • Offers syntax highlighting.

Z Shell (ZSH)

  • Provides advanced tab completion.
  • Provides auto spell correction.
  • Offers extensive customization.
  • It's slower than other shells.

Shell Scripting

  • Allows the user to run multiple commands at once.
  • Shell scripts should have the file extension as .sh.
  • A shell script should always start with a shebang #!/bin/bash.

Comments

  • Comments help to provide information about what the code does.
  • Comment can be defined by starting a sentence with # character. 
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    #!/bin/bash # This is a shebang.
# This will print hello. This is a comment
echo hello

Variables

  • Create a variable in shell script like below. 
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    #!/bin/bash
echo "What is your name?"
read name # Read value.
echo "Welcome $name!" # Print the value of name variable.
  • To run this script, first the execute permission needs to be given using the command. chmod +x print_name.sh.
  • Once the permission is assigned, the script can be executed with print_name.sh.

Loops

  • Loops are used to do something multiple times. 
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    #!/bin/bash
for i in {1..10};
do
echo $i # Prints the value of i 10 times.
done

Conditional statements

  • Conditional statements help in performing an action based on a condition. 
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    #!/bin/bash
for i in {1..10};
do
if [ $(($i % 2)) = 0 ]; then
    echo "$i is even."
else
    echo "$i is odd."
fi
done