Shell Script Fundamentals

Script Interpreter

The interpreter declaration must appear on the first line, starting with a shebang (#!):

#!/bin/bash

Different languages require different interpreters. The shebang tells the system which program to use for executing the script.

Comments

Comment lines use the hash symbol (#) anywhere except the first line. Comments can occupy full lines or appear after commands:

# This is a full-line comment
echo "Hello"  # Inline comment

Avoid using non-ASCII characters in comments for portability.

Script Execution Methods

bash script.sh       # Executes without requiring execute permissions
./script.sh          # Requires execute permission on the file
source script.sh     # Runs in current shell without spawning a subprocess

The first two methods spawn a child process—variables defined in child processes do not propagate to the parent shell. The source command executes the script within the current shell environment.

Shell Script Coding Standards

• Declare the interpreter on the first line
• Include version and copyright information at the top
• Avoid non-ASCII characters in scripts
• Use the .sh extension for script files
• Store scripts in designated dircetories
• Write paired symbols (quotes, brackets) completely before filling content
• Ensure spaces around brackets: [ condition ]
• Complete control structure syntax before adding logic
• Use indentation consistently for readability

Quotation Usage

Quote Type	Purpose
Double quotes	Standard string definitions with variable interpolation
Single quotes	Literal interpretation—content displays exactly as written
Backticks	Command substitution (legacy syntax)

Alternative for command substitution:

$(command)  # Preferred over backticks

All quotation marks must be ASCII characters. Avoid spaces around the assignment operator (=).

Environment Variables

Environment variables are system-wide variables exported via the export builtin. They persist in /etc/profile, /etc/bashrc, and user-specific files like ~/.bash_profile and ~/.bashrc. Custom environment variables follow the convention of uppercase naming.

Viewing Variables

Command	Description
set	Displays all variables (global and local)
env	Shows only global environment variables
declare	Lists variables, functions, integers, and exported items
set -o	Displays bash configuration options

Querying specific variables:

echo $PWD
# Output: /home/user

printf "%s\n" $PWD
# Output: /home/user

Removing Variables

unset VARIABLE_NAME

Defining Environment Variables

Three equivalent approaches:

export USERNAME="devuser"
declare -x USERNAME="devuser"
USERNAME="devuser"; export USERNAME

Persistence Configuration

User-level configuration (recommended):

~/.bashrc
~/.bash_profile

System-level configuration:

/etc/profile
/etc/bashrc           # Recommended for system-wide settings
/etc/profile.d/

Place scripts in /etc/profile.d/ to execute automatically during login initialization.

Variable Resolution Order

Login shells:

/etc/profile → /etc/profile.d/ → ~/.bash_profile → ~/.bashrc → /etc/bashrc

Non-login shells:

~/.bashrc → /etc/bashrc

To ensure variables are available in non-login shells, write them to the later files in this sequence.

Login Shell Customization

/etc/motd                    # Static greeting message
/etc/profile.d/welcome.sh     # Executable script with dynamic content

Java Environment Configuration

export JAVA_HOME=/opt/jdk-11
export CLASSPATH=$CLASSPATH:$JAVA_HOME/lib:$JAVA_HOME/jre/lib
export PATH=$JAVA_HOME/bin:$JAVA_HOME/jre/bin:$PATH:$HOME/bin
export RESIN_HOME=/opt/resin

Typically placed in /etc/profile. Scripts requiring Java should redefine these variables to ensure availability in child processes.

Common System Variables

Variable	Purpose
$HOME	User's home directory on login
$UID	Current user's numeric identifier
$PWD	Current working directory
$SHELL	Default shell interpreter
$USER	Current username

Regular (Local) Variables

Local variables exist only within the current shell session. When a script spawns a subprocess or the shell exits, these variables become inaccessible.

Naming Conventions

Variable names must begin with a letter or underscore, followed by alphanumeric characters or underscores. Quotes are optional for simple assignments but required when content includes spaces or special characters.

var1='simple_text'
var2="string with spaces"
var3=$(pwd)
var4=12345

Best Practices for Variable Assignment

• Unquoted assignment works for continuous alphanumeric strings
• Double quotes are necessary for complex content or non-sequential strings
• Single quotes preserve literal content exactly
• Command substitution requires backticks or $() syntax
• Choose descriptive names following camelCase or snake_case conventions
• Prefix the variable name with $ when referencing its value
• Use curly braces to delimit variable names adjacent to other characters:

echo "The value is ${var1}suffix"

Cron Job Considerations

Scheduled tasks run in minimal environments. Scripts executed by cron should explicitly define required environment variables rather than relying on shell initialization.

Variable Expansion in Strings

Double quotes allow variable interpolation:

username="developer"
echo "Hello, $username"    # Outputs: Hello, developer

Single quotes prevent interpolation:

echo 'Hello, $username'    # Outputs: Hello, $username