Bash-Cheatsheet

Might want to...

change all code box type to desired language

Related

• sed-Cheatsheet - Good for stream manipulation, quick regex editing
• awk-Cheatsheet - Good for tabled data

🔗 Resources / Samples

• devhints bash

man bash         # Manual for bash
help             # Bash built-in commands help
type <command>   # Show how a command is interpreted (builtin, alias, file, etc.)
compgen -c       # List all commands available

Data Types

#!/usr/bin/bash # Shebang line instructs interpreter to use bash
 
i=5               # Integer-like
f=0.5             # Float-like (used as string unless parsed)
b=true            # Boolean (convention only)
c='c'             # Single character (just a string)
s='str'           # String
arr=(1 2 3)       # Array (space-separated elements)

IO Functions

arg1="$1" # Arg 0 is function name. Arg 1 is first passed arg
arg2="$2" # Quotes prevent it from being interpreted badly if there is weird input (script.sh my file.txt)
arg10="${10}" # Args > 9 need to be in curly brace
 
echo "Text out"   # Standard output
read -p "Input: " var  # Standard input with prompt

Control Structure

• [[ ]] - Used for logical string checking, files, and regex
• (( )) - Used for arithmetic checking and C-like loops

# ---------- Basic use ----------
if [[-z "$1"]]; then # -z checks emptiness, -u checks not emptiness
	echo "arg 1 is empty"
else
    echo "arg 1 is: $1"
fi
#-z : True if the length of string is non-zero.
#-n : True if the length of string is non-zero.
 
 
# ---------- Regex match (only in [[ ]]) ----------
if [[ "$email" =~ ^[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,4}$ ]]; then
	echo "Valid email"
fi
 
# ---------- Numeric comparisons ----------
x=5
y=10
if (( x < y )); then
    echo "x is less than y"
fi
 
# ---------- File tests ----------
file="example.txt"
 
if [[ -e "$file" ]]; then
    echo "File exists"
fi
 
if [[ -f "$file" ]]; then
    echo "File is a regular file"
fi
 
if [[ -d "$file" ]]; then
    echo "File is a directory"
fi
 
if [[ -r "$file" && -w "$file" ]]; then
    echo "File is readable and writable"
fi
 
# ---------- Logical operators ----------
if [[ "$user" == "admin" || "$user" == "root" ]]; then
    echo "Privileged user"
fi
 
if [[ -n "$1" && "$1" == "run" ]]; then
    echo "Command: run"
fi
 
 
# Ternary-like expression (not native in bash, emulated)
[[ "$x" == "true" ]] && result="$y" || result="$z"

Regex

Regex still BUILT in string. However, to evaluate:
If it’s in quotes, it’s evaluated as a string. If it’s not, it’s used as regex.

Purpose	Pattern	Example Match
Email	^[^@]+@[^@]+\.[a-z]{2,}$	user@site.com
Digits only	^[0-9]+$	123456
Alphanumeric	^[a-zA-Z0-9]+$	abc123
File extension	\.txt$	notes.txt
Starts with “foo”	^foo	foobar
Ends with “.log”	\.log$	sys.log
Match YES/NO	^(yes|no)$ Pipe escaped for table	yes, no

if [[ "$input" =~ ^[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,4}$ ]]; then
... # evaluates directly, no need to build into var
regex='^[0-9]+$'
# BAD:  [[ "$input" =~ "$regex" ]]
# GOOD: [[ "$input" =~ $regex ]]  
 
# You can also regex a string directly
# Single vs. 
str="ABCDEF"
echo "${str/[A-Z]/_}"    # → _BCDEF
echo "${str//[A-Z]/_}"   # → ______

Iterators

for ((i=0; i<10; i++)); do
	echo "$i"
done
 
while [[ $i -lt 10 ]]; do
	echo "$i"
	((i++))
done
 
for val in "${arr[@]}"; do
	echo "$val"
done
 
for i in {5..50..5}; do # Creating a "range" iterator.5-50 inclusive in steps of 5
    echo "Welcome $i"
done

Functions and Methods

# Function declaration
my_func() {
	echo "Hello $1"
}
 
# Call function
my_func "World"
 
# Anonymous functions don't exist in Bash; simulate with subshells
( echo "subshell" )
 
# Method-like behavior: string manipulation
name="John"
echo "${name,,}"  # lowercase
echo "${name^^}"  # uppercase

File Operations

# Reading a file line by line
while IFS= read -r line; do
    echo "$line"
done < "file.txt"
 
# Writing
echo "Hello world" > output.txt
echo "Append line" >> output.txt

Hello World

#!/usr/bin/env bash
 
main() {
	echo "Hello, World!"
}
 
main