Master Linux Shell: Types, Variables, Config Files, and Conditional Tests

Shell Types

Linux provides several shells such as sh , bash , and zsh . Bash is the default on most distributions and is the most common for scripting.

The current shell can be queried with echo $0 or by inspecting the $SHELL environment variable.

Variable Classification

Shell variables fall into three categories:

Environment variables (e.g., JAVA_HOME) are inherited by all child processes.

System variables are used by the shell itself for internal logic and status codes.

User‑defined variables are created by the user for script logic.

Environment Variables

Define them with export VAR=value. They persist only for the current shell session unless added to a startup file such as ~/.bashrc or /etc/profile.

System Variables

Used for parameter and return‑value checks in scripts, such as $? for the exit status of the last command.

User‑Defined Variables

When assigning, avoid spaces around the = sign and use proper quoting to prevent word splitting.

Correct: NAME="value" Incorrect: NAME = "value" Variable names are case‑sensitive.

Shell Startup File Loading Order

Shell startup files are read depending on whether the shell is interactive, login, or non‑interactive.

Interactive & Login : /etc/profile → one of ~/.bash_profile, ~/.bash_login, or ~/.profile → ~/.bash_logout on exit.

Interactive & Non‑Login : /etc/bash.bashrc → ~/.bashrc.

Non‑Interactive : Configuration is taken from the environment (e.g., env output); no startup files are read.

Detecting Interactive Shells

case "$-" in
  *i*) echo "This shell is interactive" ;;
  *)   echo "This shell is not interactive" ;;
esac

Conditional Tests

Shell conditionals return 0 for true and non‑zero for false. Two common syntaxes are test expression and [ expression ] (spaces are required).

String Tests

Operators must be surrounded by spaces so the shell treats them as separate arguments.

Numeric Tests

File Tests

Logical Operators

Combine tests with && (AND) and || (OR) to build complex conditions.