Master Bash: Essential Variables, Operators, and Conditional Statements

Bash (or shell) programming is the most widely used scripting language on Linux, and mastering its basic constructs can greatly improve productivity for anyone working in a Linux environment.

Predefined Variables

The following special variables are frequently used: $? – Exit status of the last command (0 means success). $$ – Process ID of the current shell. $! – Process ID of the most recent background command.

Declaration Operators

The declare builtin lets you set variable attributes: declare -i var – Declare an integer variable. declare -x var – Export the variable to the environment. declare -p var – Display the variable’s attributes and value.

Example: Simple Arithmetic

a=1
b=2
declare -i c=$a+$b
c=$(($a+$b))  # use double parentheses for arithmetic evaluation

Note that arithmetic must be performed inside $(( )) because the shell treats everything as a string by default.

Conditional Statements

Single‑branch if

if [ condition ]; then
    # commands
fi

Checking for root user

#!/bin/bash
login_name=$(env | grep LOGNAME | cut -d "=" -f 2)
if [ "$login_name" != "root" ]; then
    echo 'is not root'
fi

Checking root partition usage

#!/bin/bash
result=$(df -h | grep sda1 | awk '{print $5}' | cut -d % -f 1)
if [ "$result" -lt 90 ]; then
    echo 'the root dir is not full'
fi

Double‑branch if…else

if [ condition ]; then
    # commands when true
else
    # commands when false
fi

Testing if Nginx is running

#!/bin/bash
result=$(ps aux | grep nginx | grep -v grep)
if [ -n "$result" ]; then
    echo "$(date) nginx is ok !"
else
    echo "$(date) nginx is not ok !"
    sudo /etc/init.d/nginx start &>/dev/null
    echo "$(date) restart nginx !!"
fi

Multi‑branch if…elif…else

if [ condition1 ]; then
    # commands
elif [ condition2 ]; then
    # commands
else
    # fallback commands
fi

These examples demonstrate how to use Bash variables, attribute declarations, and various forms of conditional logic to automate common system‑administration tasks.