Linux and Bash Cheat Sheet

pwd                     # where am I
ls -lah                 # list everything, with details
cd folder               # enter a folder
cd ..                   # up one level
cd -                    # previous folder
cd ~                    # home

mkdir -p a/b/c          # create nested folders
touch file.txt          # create an empty file
cp -r src dest          # copy a folder
mv old new              # move or rename
rm -r folder            # delete; there is no recycle bin
ln -s target link       # symbolic link
cat file                # print a file
less file               # scroll (q quits)
head -n 5 file          # first lines
tail -f app.log         # follow a growing file
wc -l file              # count lines

grep -rin "text" src    # recursive, ignore case, line numbers
grep -v "DEBUG" app.log # lines that do not match
grep -E "WARN|ERROR" f  # extended regex
find . -name "*.py"     # by name
find . -type f -size +100M
find . -mtime -1        # changed in the last day
find . -name "*.tmp" -delete

cmd > file              # write to a file
cmd >> file             # append
cmd 2> err.txt          # errors to a file
cmd > all.txt 2>&1      # both to a file
cmd 2> /dev/null        # discard errors
cmd1 | cmd2             # pipe
cmd | tee file          # see it and save it
echo "$(date)"          # command substitution

sort -t, -k2 -nr f      # sort by column 2, numeric, descending
sort | uniq -c | sort -nr | head   # top N
cut -d, -f1,3 file      # columns
tr 'a-z' 'A-Z'          # translate characters
sed 's/old/new/g' file  # replace
sed -i.bak 's/a/b/' f   # replace in place, with backup
awk '{print $1}' file   # first column
awk -F, '$3 > 10' file  # filter rows
awk '{s += $2} END {print s}' file   # sum a column

chmod 755 file          # set by number
chmod +x script.sh      # add execute
chown user:group file   # change owner
sudo command            # run as root
id                      # my user and groups
sudo usermod -aG docker asha   # add to a group

ps aux | grep nginx     # find a process
pgrep -a nginx
top                     # live view
kill PID                # ask to stop
kill -9 PID             # force
command &               # run in the background
nohup command &         # survive logout
systemctl status nginx
sudo systemctl restart nginx
sudo systemctl enable --now nginx
journalctl -u nginx -f  # follow logs

df -h                   # free disk space
du -sh *                # folder sizes
du -h --max-depth=1 | sort -hr
free -h                 # memory
uptime                  # load
uname -a                # kernel
sudo apt update && sudo apt install name
tar -czf a.tar.gz folder    # create
tar -xzf a.tar.gz -C dest   # extract
tar -tzf a.tar.gz           # list

ip -br addr             # addresses
ss -ltn                 # listening ports
sudo ss -ltnp           # with programs
nc -zv host 443         # test a port
dig +short example.com  # DNS
curl -sI https://example.com    # headers
curl -s -o /dev/null -w "%{http_code}\n" URL
ssh user@host
scp file user@host:path
rsync -az src/ user@host:dest/

* * * * *  command
│ │ │ │ └─ day of week (0-6, Sunday is 0)
│ │ │ └─── month
│ │ └───── day of month
│ └─────── hour
└───────── minute

*/5 * * * *   every 5 minutes
30 2 * * *    02:30 every day
0 9 * * 1-5   09:00, Monday to Friday

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

name="Asha"                # no spaces around =
echo "Hello, ${name}"      # always quote
count=$(ls | wc -l)        # command output
echo $((count + 1))        # arithmetic
echo "${PORT:-8080}"       # default value

if [[ -f "$file" ]]; then
  echo "exists"
elif (( count > 5 )); then
  echo "many"
else
  echo "none"
fi

for f in *.txt; do echo "$f"; done

while IFS=, read -r a b; do
  echo "$a"
done < data.csv

case "$1" in
  start) echo "go" ;;
  *) echo "usage" ;;
esac

add() {
  local a="$1" b="$2"
  echo $((a + b))
}
total=$(add 2 3)

${#v}          # length
${v:0:4}       # substring
${v//a/b}      # replace all
${f##*/}       # file name from a path
${f%/*}        # folder from a path
${f%.*}        # remove the extension
${v^^}         # upper case