Bash Scripting

Script Basics

#!/bin/bash—Shebang (script interpreter)

chmod +x script.sh—Make script executable

./script.sh—Run script

source script.sh—Run in current shell

set -e—Exit on error

set -u—Error on undefined vars

set -x—Debug: print commands

set -euo pipefail—Strict mode (recommended)

Variables

VAR="value"—Assign variable (no spaces!)

$VAR / ${VAR}—Use variable value

${VAR:-default}—Default if unset

${VAR:=default}—Assign default if unset

${#VAR}—Length of variable

${VAR/old/new}—Replace first match

${VAR//old/new}—Replace all matches

${VAR%suffix}—Remove trailing suffix

${VAR#prefix}—Remove leading prefix

readonly VAR="val"—Constant variable

export VAR—Export to child processes

local VAR="val"—Local to function

Arguments & Special Vars

$0—Script name

$1 $2 ...—Positional arguments

$#—Number of arguments

$@—All args (separate words)

$*—All args (single string)

$?—Last exit code

$$—Current PID

$!—Last background PID

Conditionals

if [ cond ]; then ... fi—Basic if statement

if [[ cond ]]; then—Extended test (preferred)

elif [ cond ]; then—Else if branch

else—Default branch

-eq -ne -lt -gt -le -ge—Numeric comparisons

== / !=—String comparison

-z "$VAR"—String is empty

-n "$VAR"—String is not empty

-f file—File exists (regular)

-d dir—Directory exists

-r / -w / -x—Readable / writable / exec

&& / ||—Logical AND / OR

Loops

for i in 1 2 3; do ... done—For loop over list

for f in *.txt; do—Loop over files

for ((i=0; i<10; i++)); do—C-style for loop

while [ cond ]; do ... done—While loop

until [ cond ]; do ... done—Until loop

while read line; do—Read file line by line

break / continue—Exit / skip iteration

seq 1 10—Generate number sequence

Functions

fn() { ... }—Define function

function fn { ... }—Alternative syntax

fn arg1 arg2—Call with arguments

return 0—Return exit code

local var="val"—Local variable

result=$(fn)—Capture function output

Arrays

arr=(a b c)—Define array

${arr[0]}—Access element

${arr[@]}—All elements

${#arr[@]}—Array length

arr+=(d)—Append element

unset arr[1]—Remove element

${arr[@]:1:2}—Slice (offset:length)

I/O & Subshells

echo "text"—Print to stdout

printf "%s\n" "$var"—Formatted output

read -p "Prompt: " VAR—Read user input

read -s—Silent input (password)

$(command)—Command substitution

$((expr))—Arithmetic expansion

cmd1 | cmd2—Pipe output

cmd > file 2>&1—Redirect all to file

here_doc <<EOF—Here document

trap "cmd" EXIT—Run on script exit

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