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variables and arguments

assignment​

name=value                # format, no spaces in between = sign
greetings="Hello World"   # assigns a string with a space
number=42                 # assign a number

accessing values​

W1="hello"
W2="WOLRD"
echo "$GREETING" # Outputs: Hello World
echo $NUMBER     # Outputs: 42

variable naming​

  • names can contain alphanumeric characters and underscores but cannot start with a number
  • they are case-sensitive (name, Name, and NAME are different).

Types of Variables​

Bash categorizes variables by their scope and purpose.

  • User-defined Created by the user, local to the current shell or script.
my_var="value"
  • Environment Global settings accessible by child processes.
export PATH
  • Local Limited to the function in which they are defined.
local my_var="value"
  • Special, Reserved, built-in variables that store dynamic data about the shell or script execution.
$0, $?

Special Variables​

Bash includes several automatic variables for script and process information, such as:

  • $0 : The script's filename.
  • $1, $2, ...: Command-line arguments.
  • $#: The number of arguments.
  • $?: The exit status of the last command.
  • $$: The current script's Process ID.
  • $USER, $HOSTNAME, $SECONDS: Environment variables with system information.

Advanced Usage

  • Command Substitution: Store command output in a variable using $(command) or `command`.
file_count=$(ls -l | wc -l)
echo "There are $file_count files."

Read-only Variables​

Use readonly or declare -r to prevent value changes after assignment.

Unsetting Variables​

Use unset to remove a variable.