Linux & Shell Commands

ps aux                      # every process, full listing
ps aux | grep <name>        # find a specific process
top                         # live, interactive process/resource monitor
htop                        # top, but readable (if installed)

kill <pid>                  # SIGTERM -- ask it to stop
kill -9 <pid>                # SIGKILL -- force-stop, no cleanup
pkill -f <pattern>           # kill by matching the command line, not just a PID

jobs                         # background/suspended jobs in this shell
fg %1                        # bring job 1 to the foreground
bg %1                        # resume job 1 in the background
nohup <command> &            # run detached, survives the shell exiting
disown                       # detach a job from the current shell without killing it

find . -name "*.log"                 # find files by name from the current dir
find . -mtime -1                     # files modified in the last day
find . -size +100M                   # files bigger than 100MB

chmod +x script.sh                   # make executable
chmod 644 file.txt                   # rw-r--r--
chown user:group file.txt            # change owner and group

du -sh *                             # size of each item in the current dir, human-readable
du -sh . 2>/dev/null | sort -rh      # sorted, largest first
df -h                                 # disk free space per mounted filesystem

ln -s /path/to/target linkname       # symlink
tar -czf archive.tar.gz dir/         # create a gzipped tarball
tar -xzf archive.tar.gz              # extract one

grep -r "pattern" .                  # recursive search from the current dir
grep -n "pattern" file                # with line numbers
grep -v "pattern" file                # invert -- lines that DON'T match
grep -E "foo|bar" file                # extended regex (alternation, etc.)

sed 's/old/new/g' file                # replace every occurrence, per line
sed -n '10,20p' file                  # print only lines 10-20
sed -i 's/old/new/g' file             # edit the file in place

awk '{print $1}' file                 # print the first whitespace-separated field
awk -F',' '{print $2}' file.csv        # print the second field of a CSV
awk '{sum += $1} END {print sum}' file # sum a column

jq '.data[].id' response.json          # extract a field from JSON
jq -r '.name' response.json            # -r: raw string output, no quotes
echo '{"a":1}' | jq '.a'               # pipe JSON straight from another command

curl -s https://api.example.com/health           # -s: silent, no progress meter
curl -sD - -o /dev/null https://example.com        # headers only
curl -X POST -H "Content-Type: application/json" -d '{"k":"v"}' https://api.example.com

ss -tulpn                             # listening TCP/UDP ports and owning processes (netstat's modern replacement)
netstat -tulpn                        # same, older tool -- still common on minimal images

dig example.com                       # DNS resolution, verbose
dig +short example.com                # just the IP
nslookup example.com                  # simpler DNS lookup

ssh -i key.pem user@host              # connect with a specific key
ssh -L 8080:localhost:80 user@host    # local port forward: localhost:8080 -> remote:80
scp file.txt user@host:/remote/path   # copy a file over SSH

export VAR=value              # set an environment variable for this shell and its children
env                             # list all environment variables
which <command>                # which binary would run for this command name
type <command>                 # same, but also reports aliases/functions/builtins
history | grep <term>          # search shell history
alias ll='ls -alh'             # define a shortcut for this session (add to ~/.bashrc to persist)