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Chapter 6.2 - Variables and Parameters

Learning Objectives​

By the end of this chapter, you will be able to:

  • Use advanced variables
  • Work with script parameters
  • Understand special variables
  • Use arrays
  • Manage environment variables

Advanced Variables​

Variable Substitution​

#!/bin/bash
VAR="hello"

# Default value
echo ${VAR:-"default"}      # hello (if VAR exists)
echo ${UNDEF:-"default"}    # default (if UNDEF does not exist)

# Assign if undefined
echo ${VAR:="default"}      # hello (VAR already exists)
echo ${UNDEF:="default"}    # default (and assigns UNDEF)

# Error if undefined
echo ${VAR:?"VAR is not defined"}  # OK
echo ${UNDEF:?"UNDEF is not defined"}  # Error

# Remove prefix/suffix
FILE="file.txt"
echo ${FILE%.txt}           # file (removes .txt)
echo ${FILE#file}           # .txt (removes file)

String Length​

#!/bin/bash
TEXT="Linux"
echo ${#TEXT}               # 5 (length)

Substring Extraction​

#!/bin/bash
TEXT="Hello Linux World"
echo ${TEXT:6}              # Linux World (from position 6)
echo ${TEXT:6:5}            # Linux (position 6, length 5)

Script Parameters​

Positional Arguments​

#!/bin/bash
# $0: Script name
# $1, $2, $3...: Arguments
# $#: Number of arguments
# $@: All arguments (separated)
# $*: All arguments (as a string)

echo "Script: $0"
echo "First: $1"
echo "Second: $2"
echo "Count: $#"
echo "All (@): $@"
echo "All (*): $*"

Example:

./script.sh arg1 arg2 arg3
# $0 = ./script.sh
# $1 = arg1
# $2 = arg2
# $3 = arg3
# $# = 3

Argument Shifting​

#!/bin/bash
# shift: Shifts arguments
echo "1: $1"
shift
echo "2: $1"  # Former $2
shift
echo "3: $1"  # Former $3

Iterating Over Arguments​

#!/bin/bash
# Method 1: $@
for arg in "$@"; do
    echo "Argument: $arg"
done

# Method 2: $*
for arg in $*; do
    echo "Argument: $arg"
done

# Method 3: while with shift
while [ $# -gt 0 ]; do
    echo "Argument: $1"
    shift
done

Arrays​

Declaration​

#!/bin/bash
# Declaration
FRUITS=("apple" "banana" "orange")

# Or
FRUITS[0]="apple"
FRUITS[1]="banana"
FRUITS[2]="orange"

Access​

#!/bin/bash
FRUITS=("apple" "banana" "orange")

# Specific element
echo ${FRUITS[0]}           # apple
echo ${FRUITS[1]}           # banana

# All elements
echo ${FRUITS[@]}           # apple banana orange
echo ${FRUITS[*]}           # apple banana orange

# Length
echo ${#FRUITS[@]}          # 3

# Indices
echo ${!FRUITS[@]}          # 0 1 2

Iterating Over an Array​

#!/bin/bash
FRUITS=("apple" "banana" "orange")

# By value
for fruit in "${FRUITS[@]}"; do
    echo "Fruit: $fruit"
done

# By index
for i in "${!FRUITS[@]}"; do
    echo "$i: ${FRUITS[$i]}"
done

Environment Variables​

Define​

#!/bin/bash
# Local variable (script only)
LOCAL_VAR="local"

# Environment variable (exported)
export ENV_VAR="environment"

# Or
ENV_VAR="environment"
export ENV_VAR

Common System Variables​

#!/bin/bash
echo "HOME: $HOME"
echo "USER: $USER"
echo "PATH: $PATH"
echo "SHELL: $SHELL"
echo "PWD: $PWD"
echo "UID: $UID"
echo "EUID: $EUID"

Modify PATH​

#!/bin/bash
# Add to PATH
export PATH="$PATH:/path/to/bin"

# Temporary (script only)
PATH="$PATH:/path/to/bin"

Special Variables​

Return Code​

#!/bin/bash
# $?: Return code of the last command
ls /tmp
echo "Code: $?"  # 0 if success

# Test
if [ $? -eq 0 ]; then
    echo "Success"
else
    echo "Failure"
fi

PID and Processes​

#!/bin/bash
echo "Current PID: $$"
echo "Parent PID: $PPID"
echo "Last background PID: $!"

Shell Options​

#!/bin/bash
# $-: Active options
echo "Options: $-"

Complete Example​

#!/bin/bash
# Script with parameters and arrays

# Help function
show_help() {
    echo "Usage: $0 [OPTIONS] FILES..."
    echo "Options:"
    echo "  -h    Help"
    echo "  -v    Verbose mode"
    echo "  -o    Output file"
}

# Variables
VERBOSE=false
OUTPUT=""
FILES=()

# Parse arguments
while [ $# -gt 0 ]; do
    case $1 in
        -h)
            show_help
            exit 0
            ;;
        -v)
            VERBOSE=true
            ;;
        -o)
            shift
            OUTPUT="$1"
            ;;
        *)
            FILES+=("$1")
            ;;
    esac
    shift
done

# Processing
if [ ${#FILES[@]} -eq 0 ]; then
    echo "Error: No files specified"
    exit 1
fi

[ "$VERBOSE" = true ] && echo "Processing ${#FILES[@]} file(s)"

for file in "${FILES[@]}"; do
    [ "$VERBOSE" = true ] && echo "Processing: $file"
    # Processing...
done

Summary​

In this chapter, you learned:

Substitution: \${VAR:-default}, \${VAR%pattern}, etc.
Parameters: $0, $1-$n, $#, $@, $*
Arrays: Declaration, access, iteration
Environment: export, system variables
Specials: $?, $$, $PPID, $-
Parsing: while + case for arguments

Next Steps​

Chapter 6.3: Control Structures
Chapter 6.4: Functions and Cron

Chapter created on: December 2024