Shell Scripting: Conditionals

Objective

Write conditional logic in bash scripts using if/elif/else, the test command, string and numeric comparisons, and case statements.

Tools & Technologies

if
elif
else
test
[[ ]]
case

Key Commands

if [ -f file ]; then

[[ $str == pattern ]]

case $VAR in

test -d /etc && echo 'exists'

Architecture Overview

flowchart TD START[Script starts] --> COND{[ condition ]\nor [[ ]] test} COND -->|exit code 0\ntrue| IF_BODY[if block\nexecutes] COND -->|exit code 1+\nfalse| ELIF{elif condition?} ELIF -->|true| ELIF_BODY[elif block] ELIF -->|false| ELSE[else block] IF_BODY --> END[continue script] ELIF_BODY --> END ELSE --> END style COND fill:#1a1a2e,stroke:#00d4ff,color:#e0e0e0

Step-by-Step Process

if / elif / else

Basic conditional structure. Note the spaces inside [ ] — they are required.

#!/bin/bash
FILE='/etc/hosts'

if [ -f "$FILE" ]; then
  echo "$FILE exists"
elif [ -d "$FILE" ]; then
  echo "$FILE is a directory"
else
  echo "$FILE not found"
fi

Test Operators

The test command ([ ]) supports file tests, string tests, and numeric comparisons.

# File tests
[ -f file ]   # is regular file
[ -d dir  ]   # is directory
[ -r file ]   # readable
[ -x file ]   # executable
[ -s file ]   # non-empty

# String tests
[ "$a" = "$b" ]   # equal
[ "$a" != "$b" ]  # not equal
[ -z "$str" ]     # empty string
[ -n "$str" ]     # non-empty string

# Numeric tests
[ $a -eq $b ]    # equal
[ $a -ne $b ]    # not equal
[ $a -lt $b ]    # less than
[ $a -ge $b ]    # greater or equal

[[ ]] — Enhanced Tests

Double brackets support regex, pattern matching, and safer string comparisons.

[[ "$str" == *.txt ]]      # glob match
[[ "$str" =~ ^[0-9]+$ ]]   # regex match
[[ -f file && -r file ]]   # compound (no quotes needed)

case Statement

case is cleaner than nested if/elif for matching a single variable against multiple patterns.

read -p 'Enter choice [y/n/q]: ' CHOICE
case $CHOICE in
  y|Y|yes)
    echo 'Proceeding...'
    ;;
  n|N|no)
    echo 'Cancelled'
    ;;
  q|Q)
    echo 'Quit'
    exit 0
    ;;
  *)
    echo 'Invalid choice'
    ;;
esac

Challenges & Solutions

Spaces around [ ] are mandatory — [condition] causes syntax error
= vs -eq: use = for strings, -eq for integers

Key Takeaways

[[ ]] is bash-specific but more robust than [ ] — prefer it in bash scripts
case is more readable than chains of elif for menu-style logic