Master Bash: Essential Shell Commands, Variables, and Scripting Basics

Shell Overview

The series uses Bash, the default shell on most GNU/Linux systems, so all examples assume a Bash environment.

Shebang and Script Execution

Shell scripts typically start with a shebang line such as #!/bin/bash to specify the interpreter.

The kernel reads this line and runs the indicated interpreter.

Scripts can be run by passing them as an argument to bash script.sh or by making them executable with chmod a+x script.sh and invoking them via a relative or absolute path.

Echo Command

Key points for echo:

By default it appends a newline.

Without quotes, semicolons act as command delimiters.

Double quotes require escaping special characters; single quotes do not.

Variable substitution does not occur inside single quotes.

Common options: -n suppresses the trailing newline. -e enables interpretation of backslash‑escaped characters.

Color and text attributes can be produced with escape sequences, e.g. echo -e "\e[1;31mRed text\e[0m".

Printf Command

printf

does not add a newline automatically; format specifiers such as %s, %d, %f are supported. Alignment can be controlled with - for left‑justified output and width specifications, e.g. printf "%-5s %-10s %-4.2f\n" 1 escape 100.123.

Variables and Environment Variables

In Bash every variable stores a string. Environment variables are used by the shell and the OS to hold special values.

Define a variable: var=value Reference it: echo $var or echo ${var} PATH is typically set in /etc/environment, /etc/profile or ~/.bashrc using export PATH="$PATH:/home/escape/bin".

Useful tricks:

String length: length=$(#var) Identify current shell: echo $SHELL or echo $0 Check for root:

[ $UID -ne 0 ]

Arithmetic

Basic arithmetic can be performed with let, $(( )), or $[ ]. Advanced calculations use expr or bc, e.g. echo "scale=2; 3/8" | bc for decimal division.

Conditional Tests

Logical operators: ! – NOT -a – AND -o – OR

File tests (e.g., [-e file] checks existence, [-d file] checks for a directory, [-f file] for a regular file, etc.) and string/number comparisons ( [-n string], [ $a -eq $b ], [ $a -gt $b ]) are demonstrated.