Linux & Shell Quick Reference

pwd                          # print working directory
cd /path/to/dir              # change directory
cd -                         # go to previous directory
cd ~                         # go to home directory
ls -lah                      # long listing, all files, human-readable sizes
find /var/log -name "*.log" -mtime +7   # files matching name, older than 7 days
locate nginx.conf            # fast indexed search (requires updatedb)
cp -r src/ dest/              # copy recursively
mv old_name new_name          # move or rename
rm -rf dir/                   # remove recursively, forced (dangerous)
mkdir -p a/b/c                # create nested directories
ln -s /path/target linkname   # create symbolic link
tar -czvf archive.tar.gz dir/ # create compressed archive
tar -xzvf archive.tar.gz      # extract compressed archive

grep -rn "TODO" ./src/        # recursive, line-numbered search
grep -v "^#"  config.conf     # invert match, exclude comment lines
sed 's/foo/bar/g' file.txt    # replace all occurrences of foo with bar
sed -i.bak 's/old/new/' f.txt # in-place edit with backup file
awk -F',' '{print $1, $3}' data.csv   # print columns 1 and 3, comma-delimited
sort -k2 -n file.txt          # sort numerically by 2nd field
cut -d':' -f1 /etc/passwd     # extract 1st colon-delimited field
uniq -c                       # count adjacent duplicate lines (pair with sort first)
wc -l file.txt                # count lines
tail -f /var/log/syslog       # follow a growing file live
xargs -I{} rm {}              # build/execute commands from stdin

ps aux | grep nginx            # list processes matching 'nginx'
top / htop                     # live process/resource monitor
kill -15 <pid>                 # graceful termination (SIGTERM)
kill -9 <pid>                  # forceful termination (SIGKILL)
nohup ./long_task.sh &         # run in background, immune to hangup
jobs -l                        # list background jobs in current shell
chmod 755 script.sh            # rwxr-xr-x
chmod u+x script.sh            # add execute for owner only
chown user:group file.txt      # change owner and group
sudo -l                        # list current user's sudo privileges
df -h                          # filesystem free space
du -sh dir/                    # total size of a directory
free -h                        # memory usage
uptime                         # load average and uptime

#!/usr/bin/env bash
set -euo pipefail

# Variables and quoting
name="World"
echo "Hello, $name!"

# Conditionals
if [[ -f "$1" ]]; then
    echo "File exists"
elif [[ -d "$1" ]]; then
    echo "Directory exists"
else
    echo "Not found"
fi

# Loops
for f in *.log; do
    echo "Processing $f"
done

while read -r line; do
    echo "$line"
done < input.txt

# Functions
greet() {
    local who="$1"
    echo "Hi, $who"
}
greet "Alice"

# Arrays
fruits=("apple" "banana" "cherry")
echo "${fruits[@]}"       # all elements
echo "${#fruits[@]}"      # array length

# Command-line args and getopts
while getopts "f:v" opt; do
    case "$opt" in
        f) file="$OPTARG" ;;
        v) verbose=1 ;;
        *) echo "Usage: $0 [-f file] [-v]"; exit 1 ;;
    esac
done

# Exit codes
command_that_might_fail
echo "Exit status: $?"

# Networking
ip a                            # show network interfaces and addresses
ss -tulpn                       # listening TCP/UDP sockets with process info
ping -c 4 example.com           # test connectivity, 4 packets
curl -I https://example.com     # fetch HTTP headers only
ssh user@host                   # remote shell
scp file.txt user@host:/path/   # copy file to remote host

# Package management (Debian/Ubuntu vs RHEL/CentOS)
sudo apt update && sudo apt install -y curl   # Debian/Ubuntu
sudo yum install -y curl                       # RHEL/CentOS (older)
sudo dnf install -y curl                       # RHEL/Fedora (newer)

# Scheduling
crontab -e                       # edit current user's crontab
crontab -l                       # list current user's cron jobs
# m h dom mon dow command
0 3 * * * /opt/scripts/backup.sh >> /var/log/backup.log 2>&1

systemctl status nginx           # check service status
systemctl restart nginx          # restart a service
systemctl enable nginx           # enable service at boot
journalctl -u nginx -f           # follow logs for a service