Working with Variables in Bash

Variables store data your script needs, such as paths, flags, counters, user input, and more. In Bash, you have shell (local) variables, environment variables (inherited by child processes), and special parameters such as $0, $1, and $?. This lesson shows how to declare, read, export, and transform values reliably.

Outcomes

• Declare and read variables (strings, numbers) with correct quoting.
• Set and use environment variables with export.
• Perform arithmetic and command substitution.
• Use parameter expansion for defaults, length, slicing, and search/replace.
• Create and iterate over indexed and associative arrays.

Declaring and Reading Variables

Assignments have no spaces around =. Use double quotes when expanding variables to prevent unwanted word splitting and globbing.

name="Ada Lovelace"
count=3          # numeric (still a string until used in arithmetic)
echo "$name has ${count} tasks"

Unset or make constant:

unset name
declare -r VERSION="1.0"   # readonly (fails if reassigned)

Environment vs. Shell Variables

A variable is local to the current shell by default. Use export to pass it to child processes:

api_token="abc123"
export api_token
# Now child processes (e.g., programs you exec) can read $api_token

Startup files vary by shell and login mode. For Bash: system-wide files (e.g., /etc/profile) plus per-user files like ~/.bash_profile and ~/.bashrc commonly initialize environment variables.

Variable and Command Substitution

Reference a variable with $var or ${var} (braces are safer in mixed text):

file="/tmp/report"
echo "Writing to ${file}.txt"

Capture command output with $(…):

today=$(date +%F)
echo "Run date: $today"

Arithmetic

Use $((…)) for integer arithmetic:

a=7; b=5
sum=$((a + b))          # 12
((count++))             # increment in-place
if (( sum > 10 )); then echo "big"; fi

Parameter Expansion (Power Tools)

• Default value: ${var:-default} (do not set) / ${var:=default} (set)
• Length: ${#var}
• Substring: ${var:offset:length}
• Search/replace: ${var/pat/repl} (first), ${var//pat/repl} (all)
• Error if unset: ${var:?message}

user_dir=${HOME:-/home/default}
msg="alpha beta beta"
echo "${#msg}"            # 15
echo "${msg/beta/gamma}"  # alpha gamma beta
echo "${msg//beta/gamma}" # alpha gamma gamma

Reading Input

Prompt and read safely with read -r (prevents backslash escapes). Use -p for a prompt, -a to read into an array.

read -r -p "Project name: " project
read -r -a tags <<< "one two three"   # tags=(one two three)
echo "Project=$project, first tag=${tags[0]}"

Arrays

Indexed arrays

files=(/var/log/syslog /var/log/auth.log)
files+=("/var/log/kern.log")
echo "Count: ${#files[@]}"
for f in "${files[@]}"; do
  printf '%s\n' "$f"
done

Associative arrays

declare -A ports=([http]=80 [https]=443)
echo "HTTPS port is ${ports[https]}"
for key in "${!ports[@]}"; do
  echo "$key => ${ports[$key]}"
done

Best Practices

• Quote expansions: "$var" and "${arr[@]}".
• Prefer $(…) over backticks for command substitution.
• Use braces in mixed strings: "${file}.txt".
• Use local inside functions to avoid leaking names.
• Validate required inputs with ${var:?message}.