Bash cheat sheet

Navigation & File System

pwd                        # Print working directory
cd /path/to/dir            # Change directory
cd ~                       # Home directory
cd -                       # Previous directory
ls                         # List files
ls -la                     # List all (including hidden) with details
ls -lh                     # Human-readable file sizes
ls -lt                     # Sort by modification time (newest first)
tree -L 2                  # Directory tree, 2 levels deep

File Operations

cp file.txt copy.txt       # Copy file
cp -r dir/ dir_copy/       # Copy directory recursively
mv old.txt new.txt         # Rename/move file
rm file.txt                # Delete file
rm -r dir/                 # Delete directory recursively
rm -rf dir/                # Force delete (no prompts)
mkdir mydir                # Create directory
mkdir -p a/b/c             # Create nested directories
touch file.txt             # Create empty file / update timestamp
ln -s target link          # Create symbolic link

Viewing & Editing Files

cat file.txt               # Print entire file
head -n 20 file.txt        # First 20 lines
tail -n 20 file.txt        # Last 20 lines
tail -f log.txt            # Follow file in real-time (logs)
less file.txt              # Scrollable viewer (q to quit)
wc -l file.txt             # Count lines
wc -w file.txt             # Count words
wc -c file.txt             # Count bytes
diff file1 file2           # Compare two files

Searching

# Find files
find . -name "*.py"                    # Find by name pattern
find . -type f -name "*.log"           # Files only
find . -type d -name "src"             # Directories only
find . -mtime -7                       # Modified in last 7 days
find . -size +100M                     # Files larger than 100MB
find . -name "*.tmp" -delete           # Find and delete

# Search file contents
grep "pattern" file.txt                # Search in file
grep -r "pattern" .                    # Recursive search in directory
grep -i "pattern" file.txt             # Case-insensitive
grep -n "pattern" file.txt             # Show line numbers
grep -c "pattern" file.txt             # Count matches
grep -l "pattern" *.py                 # List files with matches
grep -v "pattern" file.txt             # Invert (lines NOT matching)
grep -E "foo|bar" file.txt             # Extended regex (OR)
grep -o "pattern" file.txt             # Print only matched parts

Text Processing

# sed — stream editor
sed 's/old/new/' file.txt              # Replace first occurrence per line
sed 's/old/new/g' file.txt             # Replace all occurrences
sed -i 's/old/new/g' file.txt          # In-place edit
sed -n '5,10p' file.txt                # Print lines 5–10
sed '/pattern/d' file.txt              # Delete lines matching pattern

# awk — column processing
awk '{print $1}' file.txt              # Print first column
awk '{print $1, $3}' file.txt          # Print 1st and 3rd columns
awk -F',' '{print $2}' data.csv        # CSV: print 2nd column
awk '{sum += $1} END {print sum}'      # Sum first column
awk 'NR==5' file.txt                   # Print 5th line
awk 'length > 80' file.txt             # Lines longer than 80 chars

# sort & uniq
sort file.txt                          # Sort lines alphabetically
sort -n file.txt                       # Sort numerically
sort -r file.txt                       # Reverse sort
sort -k2 file.txt                      # Sort by 2nd column
sort file.txt | uniq                   # Remove adjacent duplicates
sort file.txt | uniq -c                # Count occurrences
sort file.txt | uniq -d                # Show only duplicates

# cut — extract columns
cut -d',' -f1,3 data.csv              # Fields 1 and 3, comma-delimited
cut -c1-10 file.txt                    # First 10 characters per line

# tr — translate/delete characters
echo "hello" | tr 'a-z' 'A-Z'         # Uppercase → HELLO
echo "hello  world" | tr -s ' '       # Squeeze repeated spaces
echo "hello123" | tr -d '0-9'         # Delete digits → hello

# Other
paste file1 file2                      # Merge files side by side
tee output.txt                         # Write stdin to file AND stdout
xargs                                  # Build commands from stdin

Pipes & Redirection

cmd > file.txt             # Redirect stdout (overwrite)
cmd >> file.txt            # Redirect stdout (append)
cmd 2> error.txt           # Redirect stderr
cmd 2>&1                   # Redirect stderr to stdout
cmd > /dev/null 2>&1       # Discard all output
cmd1 | cmd2                # Pipe stdout of cmd1 to stdin of cmd2
cmd1 | tee file | cmd2     # Pipe + save intermediate output

Variables & Strings

name="world"               # Set variable (no spaces around =)
echo "Hello $name"         # Variable expansion → Hello world
echo "Path is ${HOME}"     # Braces for clarity
echo 'No $expansion'       # Single quotes = literal

# String operations
str="Hello World"
echo ${#str}               # Length → 11
echo ${str:0:5}            # Substring → Hello
echo ${str,,}              # Lowercase → hello world
echo ${str^^}              # Uppercase → HELLO WORLD
echo ${str/World/Bash}     # Replace first → Hello Bash
echo ${str//l/L}           # Replace all → HeLLo WorLd

# Default values
echo ${var:-default}       # Use "default" if var is unset/empty
echo ${var:=default}       # Set var to "default" if unset/empty

Arrays

arr=(one two three)        # Declare array
echo ${arr[0]}             # First element → one
echo ${arr[@]}             # All elements → one two three
echo ${#arr[@]}            # Length → 3
arr+=(four)                # Append
unset arr[1]               # Remove element at index 1

# Loop over array
for item in "${arr[@]}"; do
    echo "$item"
done

Conditionals

# if/elif/else
if [ "$x" -eq 5 ]; then
    echo "five"
elif [ "$x" -gt 5 ]; then
    echo "more"
else
    echo "less"
fi

# Numeric comparisons: -eq -ne -lt -le -gt -ge
# String comparisons:  = != -z (empty) -n (not empty)
# File tests:          -f (file exists) -d (dir exists) -r (readable) -w (writable) -x (executable)

[ -f "file.txt" ] && echo "exists"     # Short-circuit AND
[ -z "$var" ] && echo "empty"          # True if var is empty
[[ "$str" == *.txt ]] && echo "match"  # Pattern matching (double brackets)
[[ "$str" =~ ^[0-9]+$ ]] && echo "num" # Regex matching

Loops

# For loop
for i in 1 2 3 4 5; do echo $i; done
for i in {1..10}; do echo $i; done
for i in {0..100..5}; do echo $i; done    # Step by 5
for f in *.py; do echo "$f"; done          # Loop over files

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

# While loop
while read -r line; do
    echo "$line"
done < file.txt

# Process substitution
while IFS=, read -r col1 col2 col3; do
    echo "$col1 - $col2"
done < data.csv

Functions

greet() {
    local name="$1"        # $1 = first argument
    echo "Hello, $name"
    return 0               # 0 = success
}
greet "World"              # Call → Hello, World

# All args: $@ (as separate words), $# (count), $? (last exit code)

Process Management

ps aux                     # List all processes
ps aux | grep python       # Find specific process
kill PID                   # Send SIGTERM
kill -9 PID                # Force kill (SIGKILL)
jobs                       # List background jobs
cmd &                      # Run in background
fg %1                      # Bring job 1 to foreground
nohup cmd &                # Run immune to hangup

Networking

curl -s URL                          # Fetch URL (silent)
curl -o file URL                     # Download to file
curl -X POST -d '{"key":"val"}' URL  # POST JSON
curl -H "Authorization: Bearer $TOKEN" URL  # With header
wget URL                             # Download file
ping -c 4 host                       # Ping 4 times
ssh user@host                        # SSH connect
scp file user@host:/path             # Copy file to remote

Disk & System

df -h                      # Disk usage (human-readable)
du -sh *                   # Size of each item in current dir
du -sh dir/                # Total size of directory
free -h                    # Memory usage
top                        # Live process monitor
htop                       # Better process monitor
uname -a                   # System info
uptime                     # System uptime
whoami                     # Current user

Permissions

chmod 755 script.sh        # rwxr-xr-x
chmod +x script.sh         # Add execute permission
chmod -R 644 dir/          # Recursive
chown user:group file      # Change owner

# Permission numbers: r=4, w=2, x=1
# 755 = rwx r-x r-x (owner/group/other)
# 644 = rw- r-- r--

Archives & Compression

tar -czf archive.tar.gz dir/      # Create gzipped tar
tar -xzf archive.tar.gz           # Extract gzipped tar
tar -tf archive.tar.gz             # List contents
zip -r archive.zip dir/            # Create zip
unzip archive.zip                  # Extract zip
gzip file                          # Compress (replaces original)
gunzip file.gz                     # Decompress

Useful One-Liners

# Count files in directory
find . -type f | wc -l

# Find largest files
du -ah . | sort -rh | head -10

# Replace in all files
find . -name "*.py" -exec sed -i 's/old/new/g' {} +

# Monitor log file for keyword
tail -f app.log | grep --line-buffered "ERROR"

# Kill process on port
lsof -ti:8080 | xargs kill

# Quick HTTP server
python3 -m http.server 8000

# Generate random string
openssl rand -hex 16

# CSV column stats
awk -F',' '{sum+=$2; n++} END {print "avg:", sum/n}' data.csv

# Parallel execution with xargs
cat urls.txt | xargs -P 4 -I {} curl -s {}

Quick Reference