Bash Reference & Cheatsheet — Quick Command Syntax Guide

command [options] [arguments]
# Comments start with #
;           # Command separator (run multiple on one line)
&&          # Run next only if previous succeeds
||          # Run next only if previous fails
&           # Run in background
\           # Line continuation

name="Alice"              # Assignment (no spaces around =)
echo "$name"              # Dereference with $
readonly name             # Make read-only
unset name                # Delete variable
${name:-default}          # Default value if unset
${name:?error msg}        # Exit with error if unset/null
${#name}                  # String length
${name:0:3}               # Substring: first 3 characters
${name/old/new}           # Replace first match
${name//old/new}          # Replace all matches
${name^}                  # Uppercase first character
${name^^}                 # Uppercase all characters

# Indexed arrays
arr=("a" "b" "c")
echo "${arr[0]}"           # "a" — first element
echo "${arr[@]}"           # All elements
echo "${#arr[@]}"          # Array length
arr+=("d")                 # Append element

# Associative arrays (declare first)
declare -A map
map[key]="value"
echo "${map[key]}"

$(( 5 + 3 ))               # Arithmetic expansion
$(( 2 ** 10 ))             # Power (1024)
(( i++ ))                  # Increment (in condition context)
let x=5+3                  # let command

> file          # stdout to file (overwrite)
>> file         # stdout to file (append)
2> file         # stderr to file
2>&1            # stderr to stdout
&> file         # Both stdout and stderr
< file          # stdin from file
<< EOF          # Heredoc (inline stdin)
    line1
    line2
EOF
<<< "string"    # Here-string
> /dev/null     # Discard output

cmd1 | cmd2               # Pipe stdout of cmd1 to stdin of cmd2
cmd1 |& cmd2              # Pipe both stdout and stderr
tee output.txt            # Write to file AND stdout

diff <(ls dir1) <(ls dir2)    # Compare outputs as files
while read line; do ...; done < <(command)

command &              # Run in background
Ctrl+Z                 # Suspend foreground job
jobs                   # List background jobs
fg %1                  # Bring job 1 to foreground
bg %1                  # Continue job 1 in background
kill %1                # Kill job 1
nohup command &        # Keep running after logout
disown                 # Remove job from shell's job table

# if/then/elif/else
if [[ condition ]]; then
    commands
elif [[ condition ]]; then
    commands
else
    commands
fi

# Single-line test
[[ -f "$file" ]] && echo "exists" || echo "not found"

# For — iterate over values
for i in {1..5}; do echo "$i"; done

# For — C-style
for (( i=0; i<10; i++ )); do echo "$i"; done

# For — iterate over files
for f in *.txt; do echo "Processing $f"; done

# While — loop while condition is true
while [[ condition ]]; do
    commands
done

# While — read file line by line
while IFS= read -r line; do echo "$line"; done < file.txt

# Until — loop until condition becomes true
until ping -c1 google.com &>/dev/null; do
    sleep 1
done

# Definition
myfunc() {
    local var="local"     # Local scope (best practice)
    echo "$1"              # First argument
    return 0               # Exit code (0 = success)
}

# Alternative syntax
function myfunc {
    commands
}

# Call
myfunc "argument"

# Find files containing text
grep -r "pattern" /path/

# Replace string in multiple files
sed -i 's/old/new/g' *.txt

# Count lines in files
wc -l *.csv

# Disk usage by directory (top 10)
du -sh * | sort -rh | head -10

# Processes using most memory
ps aux --sort=-%mem | head -10

# Kill all processes matching a name
pkill -f "node app.js"

# Watch a command every 2 seconds
watch -n 2 'df -h'

# Run a command with a timeout
timeout 10 curl https://example.com

# Find large files (>100MB)
find / -type f -size +100M -exec ls -lh {} \;

# Monitor log file in real-time
tail -f /var/log/syslog

# Create a timestamped backup
tar czf "backup-$(date +%Y%m%d_%H%M%S).tar.gz" /path/to/data

# Generate a random password
openssl rand -base64 16

# Get your public IP
curl -s https://api.ipify.org

# HTTP status code check
curl -s -o /dev/null -w "%{http_code}" https://example.com

alias ll='ls -alF'
alias la='ls -A'
alias l='ls -CF'
alias ..='cd ..'
alias ...='cd ../..'
alias grep='grep --color=auto'
alias df='df -h'
alias free='free -h'

# Variable expansion
name="Alice"
echo "${name:0:2}"      # "Al"
echo "${name^^}"        # "ALICE"

# Arithmetic
echo $(( (5 + 3) * 2 )) # 16

# String comparison
[[ "hello" == "hello" ]] && echo "match"

# File test
[[ -d /tmp ]] && echo "/tmp exists"

# Process substitution
diff <(echo "a") <(echo "b")