Shell Scripting Basics

Shell Scripting

• Bash offers several pre-defined variables and functions for creating shell scripts.
• Arguments are passed to a script using the format ./myscript args1 args.
• The $1 variable represents the first argument, $2 the second, and so on.
• Conditions can be used to execute code blocks based on certain conditions.
  • The if statement evaluates a test condition.
  • If the condition is true, then the code inside the then block is executed.
• Switch Case statements provide a way to execute different code blocks based on the value of a variable.
  • The case statement uses pattern matching to compare the value of a variable to different patterns.
  • Each pattern is followed by a right parenthesis and a ) and a corresponding code block.
  • The ;; marks the end of each case block.
  • The *) is used as a default case when none of the patterns match.
• Arrays can be used to store a collection of values.
  • They are declared using the format my_array=("value 1" "value 2" "value 3" "value 4").
  • The values can be accessed using the array name followed by the index in brackets, e.g., echo ${my_array[0]}.
• For Loops iterate through a list of items.
  • The syntax is: for var in ${list}; do # do something; done.
  • The loop variable (var) takes on each value from the list (${list}) in turn.
• While Loops execute a code block as long as a condition is met.
  • The code block is executed repeatedly until the condition becomes false.
  • The syntax is: while [[ $condition ]]; do # do something; done.
  • The read command can be used to read input from the user inside a loop.
• Functions define reusable blocks of code.
  • They begin with the function keyword followed by the function name and parentheses.
  • The code within the function is executed when the function is called by its name.
• Debugging
  • The set -x command to activate debug mode and display each executed command step-by-step in the terminal
  • Alternatively, you can execute a script in debug mode using the command bash -x ./my_script.
• Rules for Scripting
  • Documentation is essential. Comments are denoted using the # symbol.
  • Shell Specification should be defined on the first line using #!/bin/bash.
  • Modular Programming should be adhered to for better organization. The KISS principle and UNIX philosophy are recommended.
  • Clean Code is important for readability and maintainability.

Unix File System

• The Unix File System presents a hierarchical structure, where files are organized in a tree-like fashion.
  • The root directory of the file system is represented by /.
  • Paths are separated by /.
• Important directories include:
  • /bin : Contains binary executables for the system.
  • /dev/: Contains device files for accessing various hardware devices.
  • /etc/: Contains configuration files for the system and its applications.
  • /home/: Contains home directories for users.
  • /lib/: Contains libraries used by programs.
  • /tmp/: Contains temporary files.
  • /usr/: Contains system-wide files and programs for users.

File Permissions

• Unix allows for setting specific permissions for accessing each file in the file system.
• Permissions define the read (r), write (w), and execute (x) abilities for different user groups.
• Permissions are represented using an octal notation:
  • 4: read access
  • 2: write access
  • 1: execute access
  • 7 : read, write, and execute access (4+2+1).
• The ls -al command displays file permissions in a long format, showing the following information:
  • The number of references to the file.
  • The size of the file in bytes.
  • The date and time of the last modification.
  • The user who owns the file.
  • The group that owns the file.
  • The name of the file or directory.
• The first character of the file permission string (e.g., d, -, l) indicates the file type:
  • d: Directory
  • -: Regular File
  • l: Symbolic Link
• The chmod command modifies file permissions.
  • The first three digits of the permission code represent the access rights for the following user groups:
    • 400: Owner
    • 040: Group
    • 004: Other
• Example:
  • chmod 600 ~/.bashrc sets the permissions for the ~/.bashrc file to allow only the owner to read and write.
• Removing a Directory
  • The rm command is used to remove files or directories.
  • The -rf options are used to remove files and directories recursively, including their contents.
  • The command rm -rf /home/rakeller/ will attempt to remove the directory /home/rakeller/ and all its contents.
• Error Messages - Removing Directories
  • Removing a directory using rm -rf /home/rakeller/ might fail if the user does not have sufficient permissions to remove the directory, or if the directory is not empty.