Variables

When assigning a value to a variable, do not put spaces around =. Adding spaces causes the shell to interpret the expression as a command and its arguments, resulting in an error.

To reference a variable, write $var or ${var}. The brace form makes the variable name boundary explicit, which is useful when concatenating with strings.

A variable declared with readonly cannot be reassigned or deleted with unset afterward. Use it for values you want to treat as constants.

The unset command deletes a variable. After deletion, referencing the variable returns an empty string. readonly variables cannot be unset.

Shell variable scope has two types: local variables and environment variables. Local variables are only valid within the current shell process. Using export promotes them to environment variables, which are inherited by child processes.

psycho_pass_vars.sh

#!/bin/bash
# -----------------------------------------------
#  Demonstrates variable basics using characters
#  from PSYCHO-PASS
# -----------------------------------------------

# -----------------------------------------------
#  1. Assigning and referencing variables
# -----------------------------------------------

# Assignment: no spaces are allowed around =
inspector="Tsunemori Akane"
enforcer="Kougami Shinya"
division="CID Criminal Investigation Department Division 1"

# $var and ${var} — both reference the same value
echo "Inspector: $inspector"
echo "Enforcer: ${enforcer}"

# Use ${var} to make the boundary explicit when concatenating with a string
echo "${inspector} is assigned to ${division}."

# Command substitution: assigns the output of a command to a variable
current_date=$(date '+%Y-%m-%d')
echo "Date recorded: ${current_date}"

echo ""

# -----------------------------------------------
#  2. Default value expansion
# -----------------------------------------------

# hue_level is undefined, so the default value "0" is returned
echo "Hue level: ${hue_level:-0}"

# hue_level itself remains unset (no assignment is made)
echo "Value of hue_level: ${hue_level}"    # empty string

# := assigns the default value
echo "Crime coefficient: ${crime_coefficient:=50}"
echo "Value of crime_coefficient: ${crime_coefficient}"  # 50 is now assigned

echo ""

# -----------------------------------------------
#  3. readonly — read-only variables
# -----------------------------------------------

readonly SIBYL_SYSTEM="Sibyl System"
echo "Management system: ${SIBYL_SYSTEM}"

# Attempting to reassign causes an error
# SIBYL_SYSTEM="new system"   # bash: SIBYL_SYSTEM: readonly variable

echo ""

# -----------------------------------------------
#  4. unset — deleting variables
# -----------------------------------------------

target="Makishima Shogo"
echo "Target (before unset): ${target}"

unset target
echo "Target (after unset): ${target}"   # becomes an empty string

echo ""

# -----------------------------------------------
#  5. Local variables and environment variables
# -----------------------------------------------

# Local variable: valid only within the current shell
local_agent="Ginoza Nobuchika"

# Promote to an environment variable with export
export DOMINANT_COLOR="blue"

echo "Local variable local_agent: ${local_agent}"
echo "Environment variable DOMINANT_COLOR: ${DOMINANT_COLOR}"

# Check whether a child process (bash -c) can access each variable
bash -c 'echo "DOMINANT_COLOR from child process: ${DOMINANT_COLOR}"'
bash -c 'echo "local_agent from child process: ${local_agent}"'  # becomes an empty string

echo ""

# -----------------------------------------------
#  6. local variables inside a function
# -----------------------------------------------

check_hue() {
    # Using local restricts the variable to the function scope
    local analyst="Hinakawa Sho"
    local status="Analyzing hologram"
    echo "[Inside function] Analyst: ${analyst} / Status: ${status}"
}

check_hue
# analyst and status are not accessible outside the function
echo "[Outside function] Analyst: ${analyst}"   # becomes an empty string

$ chmod +x psycho_pass_vars.sh
$ ./psycho_pass_vars.sh
Inspector: Tsunemori Akane
Enforcer: Kougami Shinya
Tsunemori Akane is assigned to CID Criminal Investigation Department Division 1.
Date recorded: 2026-03-27

Hue level: 0
Value of hue_level:
Crime coefficient: 50
Value of crime_coefficient: 50

Management system: Sibyl System

Target (before unset): Makishima Shogo
Target (after unset):

Local variable local_agent: Ginoza Nobuchika
Environment variable DOMINANT_COLOR: blue
DOMINANT_COLOR from child process: blue
local_agent from child process:

[Inside function] Analyst: Hinakawa Sho / Status: Analyzing hologram
[Outside function] Analyst:

In shell scripts, variables are assigned with var=value and referenced with $var or ${var}. The most important rule is that no spaces are allowed around = during assignment. Use the brace form when concatenating directly with a string or when using parameter expansion such as ${var:-default}. Using readonly (or declare -r) protects a variable as a constant, preventing both reassignment and unset. To delete a variable, use unset var (without $). Variables created by a plain assignment are local variables, valid only within the current shell process. To pass them to child processes, promote them to environment variables with export. Using local inside a function defines a variable scoped to that function. For more on the difference between environment variables and local variables, see Shell Script Basic Syntax.