Bash Mémo

pwd

Print working directory

/home/user/projects

ls -la

List all files with details

Shows hidden files, permissions, size

ls -lh

List files with human-readable sizes

-rw-r--r-- 1 user group 1.2M file.tar

cd -

Go to previous directory

Toggle between two directories

pushd /path && popd

Push/pop directory stack

pushd /tmp; work; popd

find . -name '*.txt'

Find files by name pattern

find /var/log -name '*.log' -mtime -7

find . -type f -size +100M

Find files larger than 100MB

find / -type f -size +100M 2>/dev/null

tree -L 2

Show directory tree (2 levels)

tree -L 3 --dirsfirst

du -sh *

Disk usage of current directory items

du -sh /var/log/*

df -h

Disk free space (human-readable)

Shows all mounted filesystems

ls -lt | head -10

10 most recently modified files

Sort by modification time, newest first

cp -r src/ dest/

Copy directory recursively

cp -rp /etc/nginx /etc/nginx.bak

mv file1 file2

Move/rename file

mv old_name.txt new_name.txt

rm -rf dir/

Remove directory and all contents

Be careful — no recycle bin!

mkdir -p a/b/c

Create nested directories

Creates all intermediate dirs

touch file.txt

Create empty file or update timestamp

touch -t 202501010000 file.txt

ln -s /path/to/file link

Create symbolic link

ln -s /usr/local/bin/python3 /usr/bin/python

tar -czf archive.tar.gz dir/

Create gzipped tarball

tar -czf backup-$(date +%Y%m%d).tar.gz /data

tar -xzf archive.tar.gz

Extract gzipped tarball

tar -xzf archive.tar.gz -C /dest/

zip -r archive.zip dir/

Create zip archive

zip -r site.zip public/

rsync -avz src/ user@host:dest/

Sync files to remote host

rsync -avz --delete /local/ remote:/dest/

diff file1 file2

Show differences between files

diff -u file1.txt file2.txt | less

wc -l file.txt

Count lines in file

wc -l *.log | sort -n

cat file.txt

Print file contents

cat -n file.txt (with line numbers)

less file.txt

Page through file contents

Use arrow keys, q to quit, /search

head -n 20 file.txt

Show first 20 lines

head -n 20 /var/log/syslog

tail -f logfile

Follow file as it grows

tail -f /var/log/nginx/access.log

grep -rn 'pattern' dir/

Recursively search for pattern

grep -rn 'TODO' src/ --include='*.ts'

grep -v 'pattern'

Lines NOT matching pattern

grep -v '^#' config.conf (skip comments)

sed 's/old/new/g' file

Replace text (global)

sed -i 's/localhost/production.com/g' config.yml

awk '{print $1,$3}'

Print specific columns

awk '{sum+=$2} END{print sum}' data.txt

sort -k2 -n file

Sort by column 2 numerically

sort -t',' -k3 -n -r data.csv

sort | uniq -c | sort -rn

Count and rank unique lines

cat access.log | awk '{print $1}' | sort | uniq -c | sort -rn | head

cut -d',' -f1,3

Extract comma-separated fields 1,3

cut -d':' -f1 /etc/passwd

tr '[:lower:]' '[:upper:]'

Convert to uppercase

echo 'hello' | tr '[:lower:]' '[:upper:]'

ps aux | grep process

Find process by name

ps aux | grep nginx

kill -9 PID

Force kill process by PID

kill -TERM PID (graceful first)

pkill -f 'pattern'

Kill processes matching pattern

pkill -f 'python my_script.py'

top / htop

Interactive process viewer

htop for a better interface

nohup cmd &

Run command immune to hangups

nohup ./server.sh > server.log 2>&1 &

jobs / fg / bg

Manage background jobs

Ctrl+Z to suspend, bg to resume

time command

Measure command execution time

time npm run build

watch -n 5 'command'

Repeat command every 5 seconds

watch -n 1 'ps aux | grep myapp'

lsof -p PID

List files opened by process

lsof -i :3000 (port 3000)

strace -p PID

Trace system calls of process

strace -e trace=openat ls

curl -I https://example.com

Fetch HTTP headers only

curl -sI https://api.example.com | head -5

curl -X POST -H 'Content-Type: application/json' -d '{...}' url

POST JSON to endpoint

curl -s -o /dev/null -w '%{http_code}' url

wget -q -O file.tar.gz url

Download file quietly to name

wget -q --show-progress url

ssh user@host -p 2222

SSH with custom port

ssh -i ~/.ssh/key.pem user@host

scp file user@host:/path/

Secure copy to remote

scp -r user@host:/remote/ /local/

netstat -tlnp

Show listening TCP ports + PIDs

ss -tlnp (modern replacement)

ss -s

Socket statistics summary

ss -tlnp | grep :80

ping -c 4 host

Ping host 4 times

ping -i 0.2 -c 100 host

traceroute host

Trace route to host

mtr host (combined ping+traceroute)

nmap -p 80,443 host

Scan specific ports

nmap -sV -p 1-65535 host

chmod 755 file

rwxr-xr-x (owner full, others r+x)

chmod +x script.sh

chmod -R 644 dir/

Set 644 recursively on directory

chmod -R u=rw,go=r dir/

chown user:group file

Change file owner and group

chown -R www-data:www-data /var/www

umask 022

Set default permission mask

umask 027 (group can't write, others nothing)

stat file

Show file metadata and permissions

stat -c '%A %U %G' /etc/passwd

sudo -u user command

Run command as another user

sudo -u postgres psql

visudo

Edit sudoers file safely

Adds user ALL=(ALL) NOPASSWD: ALL

getfacl / setfacl

Get/set POSIX access control lists

setfacl -m u:bob:rw file

VAR="value"

Set local variable (no spaces around =)

NAME="World"; echo "Hello $NAME"

export VAR=value

Export variable to child processes

export NODE_ENV=production

echo ${VAR:-default}

Use default if VAR is unset

PORT=${PORT:-3000}

readonly VAR=value

Create read-only variable

readonly PI=3.14159

VAR=$(command)

Assign command output to variable

DATE=$(date +%Y-%m-%d)

env

List all environment variables

env | grep PATH

unset VAR

Remove variable

unset MY_SECRET

declare -a arr=(a b c)

Declare array

arr=("one" "two" "three"); echo ${arr[0]}

${#VAR}

String length

echo ${#HOME}

${VAR#prefix}

Remove prefix from value

${file%.txt} (remove .txt extension)

if [ condition ]; then
  cmd
fi

Basic if statement

if [ -f file.txt ]; then echo "exists"; fi

[[ ... ]]

Extended test — prefer over [ ]

[[ -n "$VAR" && "$VAR" =~ ^[0-9]+$ ]]

for i in {1..10}; do
  cmd
done

Loop over range

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

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

Read file line by line

while read line; do ...; done < input.txt

case $VAR in
  'a') cmd;;
  *) default;;
esac

Case/switch statement

case "$OS" in linux) apt;; darwin) brew;; esac

cmd1 && cmd2

Run cmd2 only if cmd1 succeeds

make && make install

cmd1 || cmd2

Run cmd2 if cmd1 fails

ping -c1 host || echo 'unreachable'

set -e

Exit script on any error

set -euo pipefail (strict mode)

trap 'cleanup' EXIT

Run cleanup on script exit

trap 'rm -f $TMPFILE' EXIT

function_name() {
  local var="value"
  echo "$var"
}

Define a function with local vars

greet() { local name="$1"; echo "Hello $name"; }

$1 $2 $@

Function/script arguments ($1=first...)

$# is number of args, $@ is all args

return 0

Return from function (0=success)

Non-zero return = error

source script.sh (or . script.sh)

Load functions/variables from file

source ~/.bashrc

cmd > file.txt

Redirect stdout to file (overwrite)

ls > files.txt

cmd >> file.txt

Append stdout to file

date >> log.txt

cmd 2> err.txt

Redirect stderr to file

make 2> errors.txt

cmd > out.txt 2>&1

Redirect both stdout and stderr

Capture all output to file

cmd 2>/dev/null

Discard stderr

Suppress error messages

cmd1 | cmd2

Pipe stdout of cmd1 to stdin of cmd2

cat file | sort | uniq -c

tee file.txt

Write to file AND stdout simultaneously

cmd | tee output.txt | grep error

xargs

Build commands from stdin

find . -name '*.log' | xargs rm -f

cmd < file.txt

Redirect file to stdin

mysql -u root < dump.sql

cmd <<EOF
text
EOF

Here-doc: multi-line stdin

ssh host <<EOF
cd /app && git pull
EOF

for h in host1 host2; do ssh $h 'uptime'; done

Run command on multiple hosts

Parallel: use & and wait

date +%Y-%m-%dT%H:%M:%S

ISO 8601 timestamp

file_$(date +%Y%m%d_%H%M%S).bak

openssl rand -hex 32

Generate random 32-byte hex string

Good for generating secrets/tokens

python3 -m http.server 8080

Quick HTTP file server

Serve current directory

nc -zv host port

Test TCP connectivity to port

nc -zv db.host 5432

jq '.key' file.json

Parse/query JSON (jq)

curl -s api.url | jq '.data[].name'

!!  /  !$  /  !string

History: last cmd / last arg / last cmd starting with 'string'

sudo !! (re-run with sudo)

CTRL+R

Reverse search command history

Type to filter, Enter to run

column -t -s','

Format CSV as aligned table

cat data.csv | column -t -s','

diff <(cmd1) <(cmd2)

Diff output of two commands

diff <(sort file1) <(sort file2)