Master Bash: Essential Conditionals, Loops, and Signal Tricks for Linux Scripts

1. Conditional Selection and Judgment (if, case)

(1) if statement

Syntax: if [condition]; then... elif [condition]; then... else... fi The first true condition executes its branch and the whole if block ends.

Example – Age validation

Prompt for age, reject non‑numeric input, then ensure the value is between 18 and 150.

Example – Score validation

Prompt for a score, reject non‑numeric input, then check ranges >100, >85, >60.

(2) case statement

Syntax:

case $variable in

pattern1) commands ;;

pattern2) commands ;;

*) commands ;;

esac

Supports glob patterns: *, ?, [...], a|b.

Example – Yes/No prompt

Accepts various capitalizations of yes (Y/y/yes) and no (N/n/no).

2. Four Loop Types (for, while, until, select)

(1) for loop

Syntax: for var in list; do... done Or arithmetic form: for ((exp1; exp2; exp3)); do...

done

Example – Sum of 1+2+…+n using arithmetic for loop.

Example – Sum of 1+2+…+100 using list expansion.

(2) while loop

Syntax: while [condition]; do... done Condition is evaluated before each iteration; loop stops when condition becomes false.

Special usage – reading a file line by line:

while read line; do
    # process $line
 done < /path/to/file

Example – Sum of odd numbers below 100.

(3) until loop

Syntax: until [condition]; do... done Runs until the condition becomes true (inverse of while).

Example – Monitor user "xiaoming" and kill the process when they log in.

(4) select loop (menu)

Syntax: select var in list; do... done Creates a numbered menu; user input is stored in REPLY. Use break to exit.

Example – Generate a menu and display the selected price.

3. Loop Control Statements

continue [N] – skip the rest of the current loop iteration and continue with the next one (N specifies the loop level).

break [N] – exit the current loop (N specifies the loop level).

Example code snippet:

while CONDITION1; do
    CMD1
    if CONDITION2; then
        continue   # or break
    fi
    CMD2
done

4. shift Command

Purpose: Left‑shift the positional parameters, discarding the leftmost one.

Example – Create multiple users from arguments:

5. Return Values

true

always returns success; : (null command) does nothing and returns success; false always returns failure.

Creating an infinite loop:

while true; do
    # commands
 done

6. Parallel Execution in Loops

Run loop bodies in background and wait for all to finish:

for name in list; do
    { commands; } &
done
wait

Example – Scan a subnet for active IPs in parallel.

7. Signal Trapping (trap)

Syntax: trap 'command' SIGNAL Common signals: SIGHUP (reload config), SIGINT (Ctrl+C), SIGQUIT (Ctrl+\), SIGKILL (cannot be trapped), SIGTERM (default termination), SIGCONT (continue), SIGSTOP (background suspend).

Example – Prevent Ctrl+C from terminating a counting loop:

8. Miscellaneous Bash Tips

Generate random alphanumeric string: cat /dev/urandom | tr -dc '[:alnum:]' | head -c 8 Generate random number: echo $RANDOM (range 0‑32767). Use modulo for custom ranges.

Print colored text: echo -e "\033[31mred\033[0m" (red), \033[1;31m (bold red), \033[41m (red background), \033[31;5m (blinking red).

9. Sample Mini‑Scripts

9×9 multiplication table, colorful isosceles triangle, chessboard pattern – each illustrated with images.

This tutorial equips readers with the essential Bash constructs to automate tasks, build interactive menus, handle signals, and write efficient scripts for Linux environments.