Shell Programming Concepts

Shell Programming

• This presentation covers shell programming concepts.

• It outlines user-defined and system-defined variables.

• System variables are predefined by the shell and hold information about the system environment

• Examples of system variables include:

  • $BASH_VERSION: Holds the version of the bash shell.
  • $HOSTNAME: Displays the computer's name.
  • $CDPATH: Shows the search path for cd command.
  • $HISTFILE: The file name for saving command history.
  • $HISTFILESIZE: The maximum number of lines in the command history file.
  • $HISTSIZE: The maximum number of commands remembered by shell history; default is 500.
  • $HOME: The home directory of the current user.
  • $IFS: The internal field separator. Default is , used for splitting strings.
  • $LANG: Locale category variable.
  • $PATH: Directory search path for shell commands.
  • $PS1: User's prompt setting.
  • $TMOUT: The shell timeout setting for input. A shell with a timeout setting will log out if no command is entered within the specified time limit.
  • $TERM: User's login terminal type.
  • $SHELL: Path to the shell.
  • $DISPLAY: X display name.
  • $EDITOR: Default text editor.

• User-defined variables are created by the user for specific tasks.

• Example of user defined variables:

  • a=50 assigns a value to the variable a
  • read abc: Takes user input for variable abc.
  • read -a name: Takes array input for variable name.
  • `echo "name[0]{name[0]} name[0]{name[1]} ${name[2]}": Displays array elements.

Expressions

• Using command substitution for expressions
• Example for adding numbers:
  • val=expr a+a + a+b`
• Example for multiplying numbers:
  • val=expr a∗a * a∗b`
• Example for adding integer/floating-point numbers:
  • val=echo a+a + a+b | bc -l`
• bc: Basic calculator.

If Statements

• The if statement is used for conditional execution of commands.
• Structure:
  • if [ <condition> ]
  • then followed by the commands to execute if the condition is true
  • fi marks the end of the if block
• Example: verifying if a value is greater than 100
  • if [ $val -gt 100 ]
  • then
  • echo "Large number"
  • fi

Nested If Statements

• Using if statements within other if statements.
• Example checking if a value is both less than 100 and even:
  • if [ $val -lt 100 ]
    • then
      • if [ expr $val % 2 -eq 0 ]
        • then
          • echo "Even and small number"

If-Else

• Executes different blocks of commands based on condition.
• Structure:
  • if [ <condition> ]
  • then followed by commands if true
  • else followed by commands if false
  • fi marks end
• Example: check if value is less than 100, display one message, otherwise display another
  • if [ $val -lt 100 ]
    • then
      • echo "Small number"
    • else
      • echo "Large number"
    • fi

If-Elif-Else

• Allows for multiple conditions.
• Structure: -if [ <condition1> ]
  • then followed by commands for condition1
  • elif [ <condition2> ]
    • then followed by commands for condition2
  • else executed if no condition matches
  • fi marks end
• Example: age-based eligibility check
  • if [ $val -gt 18 ]
    • then echo "Voter"
  • elif [ $val -eq 17 ]
    • then echo "Enroll for voter"
  • else echo "Not eligible"
  • fi

Boolean Operators

• Combining conditions using && (and) and || (or).
• if [ <condition1> && <condition2> ]
  • Executions if both condition1 and condition2 are true.
  • if [ $val -lt 25 ] && [ $val -gt 20 ]

Looping Constructs

• while loop: Executes a block of code repeatedly as long as a condition is true.
• for loop: Iterates over a list of items.
• until loop: Repeats a block until a condition becomes true.

Case Statements

• case <expression> in
• pattern 1) <commands>;;
• pattern 2) <commands>;;
• esac
• Example: checks various states of a variable
  • case $x instart) <commands>;;stop) <commands>;;*) <commands>;;

Functions

• Reusable blocks of code.
• function_name() { code; }
• f1() and f2(): nested functions call example
• Passing parameters using $1, $2, etc. for arguments.

Assignment Questions

• Various programming tasks are given.