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Last updated: Aug 5, 2020 ·
Posted in wiki#notes

Some useful commands

# Free memory (2.2G/7.6G)
free -h | awk '/^Mem:/ {print $3 "/" $2}'

# Top 10 memory intensive processes
ps axch -o cmd:15,%mem --sort=-%mem | sed 10q

# Top 10 CPU intensive processes
ps axch -o cmd:15,%cpu --sort=-%cpu | sed 10q

# Does your terminal emulator support _italics_?
echo -e "\e[3m foo \e[23m"

# `dd` command usage to burn an iso to a flash drive
dd if=Downloads/archlinux-x86_64.iso of=/dev/sdb status=progress

# Display the SSID of connected network
nmcli -t -f active,ssid dev wifi | egrep '^yes' | cut -d\' -f2

# Colormap in terminal
msgcat --color=test

# Set a random wallpaper from r/earthporn
wget -O - -q | jq '.data.children[] | .data.url' | sed 1q | xargs feh --bg-fill

# Browse memes from r/memes
wget -O - -q | jq '.data.children[] | .data.url' | xargs feh

# See your most run commands
history | awk '{print $2}' | sort | uniq -c | sort -rn | sed 10q

# Runs `command1` 1 out of 10 times
[ $[$RANDOM % 10] = 0 ] && command1 || command2
[ $[$RANDOM % 10] = 0 ] && cmatrix || clear # will run `cmatrix` 1 out of 10 times you execute this command

# Find out which shell you are on
echo $0 # or one of these
echo $SHELL
pstree $$
cat /proc/$$/cmdline
ps -p $$
ps -p $$ -oargs=
ps -p $$ -ocomm=

# Count number of lines in a file
cat <file> | wc -l # or one of these
cat <file> | nl -ba
grep -c ".*" <file>
sed -n '$=' <file>
awk 'END{print NR}' <file>
cat -n <file> | tail -n 1 | cut -f1

# Find your IP address
# Private IP
hostname -I # or one of these
ip addr
# Public IP
curl # or one of these
curl -s
wget -qO-
host | grep " has" | awk '{print $4}'
dig +short

# Find all mp4 files and merge them to output.mp4
find *.mp4 | sed 's:\ :\\\ :g'| sed 's/^/file /' > list.txt; ffmpeg -f concat -i list.txt -c copy output.mp4; rm list.txt

  • Find and replace all occurences of a string/pattern in text files in a directory recursively [*]

    find /home/<username>/ -type f | xargs sed -i  's/<old>/<new>/g'

Map Caps Lock to Ctrl and vice versa

# On Debian
vim /etc/default/keyboard
# Change the line that reads `XKBOPTIONS=""` to `XKBOPTIONS="ctrl:swapcaps"`
sudo dpkg-reconfigure -phigh console-setup

Patching suckless tools

# For tarballs
patch -p1 < path/to/patch.diff

# For git repositories
git apply path/to/patch.diff

Change username and usergroup


  1. Drop into a tty (Ctrl + Alt + F3).
  2. Login with username and password.
  3. Set a password for root account:
    sudo passwd root
  4. Log out:
  5. Login with root account.
  6. Change the username and home folder:
    usermod -l <new_username> -d /home/<new_username> -m <old_username>
  7. Change the group name:
    groupmod -n <new_group> <old_group>
  8. Lock the root account.
  9. Log out.
  10. Exit tty.

Connect to Wi-Fi network with nmcli

nmcli dev wifi # Show available access points
nmcli dev wifi connect <access_point> password <password> # Connect

Connect to a hidden Wi-Fi network with nmcli

nmcli c add type wifi con-name <connect name> ifname wlp1s0 ssid <SSID>
nmcli con modify <connect name> wifi-sec.key-mgmt wpa-psk
nmcli con modify <connect name> wifi-sec.psk <password>
nmcli con up <connect name>

Change MAC address with macchanger

sudo service network-manager stop
sudo ifconfig wlan0 down
sudo macchanger -r wlan0
sudo ifconfig wlan0 up
sudo service network-manager start

Desktop entry template

[*, *]


# ~/.local/share/applications/firefox_dev.desktop
[Desktop Entry]
Name=Firefox Developer
GenericName=Firefox Developer Edition
Comment=Firefox Developer Edition Web Browser.


Standard I/O streams:

StreamDescriptionFile descriptor
stdinStandard input0
stdoutStandard output1
stderrStandard error2

Redirecting input <

stdin usually takes input from the keyboard.

$ command < file.txt # Executing `command` with `file.txt` as the source of input

The output of one command can be redirected as the input for another with the | (pipe) character.

$ echo 'one two three' | tr a-z A-Z

Using here document and here string with << and <<< respectively:

$ tr a-z A-Z << END_TEXT
one two three
uno dos tres

$ tr a-z A-Z <<< "one two three"

Redirecting output >

Unless redirected, stdout shows its output to the terminal.

$ command > file.txt # Redirect output of `command` to `file.txt`
# This will clobber (overwrite) any existing data in `file.txt`

# To avoid clobbering, use `>>`
$ command >> file.txt
# This will append the output to the end of `file.txt`

Clobbering can be disabled by set -o noclobber [*]. If this is enabled but you want to temporary turn off noclobber for a single operation, use >|. To re-enable, use set +o noclobber.

Redirecting error 2>

Unless redirected, stderr shows the error messages to the terminal.

$ command 2> file.txt # Redirect errors from `command` to `file.txt`
$ command 2>> file.txt # Avoid clobbering

# To redirect stdout and stderr to the same file, use `2>&1`
$ command 2>&1 file.txt

$ ls | xargs du -sk 2> /dev/null
# Redirecting stderr to the `/dev/null` device

File-system permissions


1sticky bit is set
2setgid are set
3setgid and sticky bit are set
4setuid is set
5setgid and sticky bit are set
6setgid and setgid are set
7setuid, setgid, and sticky bit are set


OctalBinary (rwx)Description
3011Write and execute
5101Read and execute
6110Read and write
7111Read, write, and execute

Useful programs


Translate or delete characters.

> echo 'reddit' | tr 'a-z' 'A-Z' # Change case from lowercase to uppercase
> echo 'reddit' | tr [:lower:] [:upper:] # Equivalent to above


Read from standard input and write to both standard output and one or more files.

# This will write the JSON response to `response.json` as well as stdout
> curl | tee response.json
  "id": 364474335,
  "name": "zx",
  "full_name": "google/zx",
  "visibility": "public",
  "forks": 469,
  "owner": {
    "login": "google",
    "id": 1342004,
> cat response.json
  "id": 364474335,
  "name": "zx",
  "full_name": "google/zx",
  "visibility": "public",
  "forks": 469,
  "owner": {
    "login": "google",
    "id": 1342004,

# Useful options
# -a, --append: Append to the given file(s), do not overwrite


Remove sections from each line of files.

> cut -f 5 main.tsv # Output the fifth field from `main.tsv`

# Custom delimiter
> cut -d ":" -f 3- main.txt # Output the third through the last field from `main.txt`; Use `:` as delimiter
> cut -d "," -f 2,3 ratings.csv # Output only the 2nd and 3rd fields from `ratings.csv`; Use `,` as delimiter
> cut -d, -f 2,3 ratings.csv # Equivalent to above

> echo "This is an example." | cut -c3- # Output the third through the last character of the input
is is an example.

> cut -c1-20 <file> # Output the first through the 20th character of _each line_ of file


Print a sequence of numbers.

> seq 3 # Generate numbers upto 3

> seq -s " | " 7 # Use custom separator (default: `\n`)
1 | 2 | 3 | 4 | 5 | 6 | 7

> seq -s " " 10 14 # Generate numbers from 10 upto 14
10 11 12 13 14

> seq -s " " 10 2 14 # Skip every other number
10 12 14

> seq -s " " 10 -2 6 # Going backwards
10 8 6

> seq -w 7 12 # Equalize width by padding with leading zeroes

> echo {0..9} # Alternative to `seq`
0 1 2 3 4 5 6 7 8 9


# Generates files named `xaa`, `xab`, `xac`, etc
split file # Split `file` into 1000-line files
split -l 10 file # Split `file` into 10-line files (except the last line)
split -n 5 file # Split `file`, each split with equal size (except the last line)
split -b 512 file # Split `file` with 512 bytes in each split (except the last line) (512k for kilbobytes, 512m for megabytes)
split -C 512 file # Split `file` with at most 512 bytes in each split without breaking lines


# Generates files named `xx00`, `xx01`, `xx02`, etc and prints size of each split in bytes to stdout
csplit file 5 23 # Split a file at lines 5 and 23
csplit file 5 {*} # Split a file every 5 lines (this will fail if the total number of lines is not divisible by 5)
csplit -k file 5 {*} # Split a file every 5 lines, ignoring exact-division error
csplit file 5 -f prefix # Split a file at line 5 and use a custom prefix for the output files
csplit file /regex/ # Split a file at a line matching a regular expression

System monitoring

  • ps: Report a snapshot of the current processes.
    • ps aux | grep nginx: Search for "nginx" in ps output.
  • htop: Interactive process viewer.
  • top: Display running processes.
  • cat /proc/loadavg: Load average.
  • uptime: Tell how long the system has been running.
  • w: See who is logged on and what they are doing.
  • acpi: Shows battery status and other ACPI information.
  • upower: Battery information.
  • pidof [program]: Find the process ID of a running program.

System info

  • lsb_release -a: Print distribution-specific information.
  • cat /etc/*release
  • uname -a: Print system information.
  • cat /etc/fstab: Static file system information.


  • vim /etc/resolv.conf: Change DNS server.
  • ip: Show/Manipulate routing, network devices, interfaces and tunnels.
    • ip addr: Display IP addresses and property information.
  • ss: Investigate sockets.
  • dig: DNS lookup utility.


  • man [program]: View man pages for programs.
  • apropos: Search the manual page names and descriptions.
  • [program] --help/-h
  • true: Do nothing, successfully.
  • false: Do nothing, unsuccessfully.
  • test: Check file types and compare values.
  • cat
  • tac
  • less
  • echo
  • bc: An arbitrary precision calculator language.
  • head <file>: Output the first 10 lines of file.
    • head -N <file>: Output the first N lines of file.
    • cat <file> | sed Nq: Output the first N lines of file.
  • tail <file>: Output the last 10 lines of file.
  • rm: Remove files or directories.
    • rm -f !(test.txt): Remove all files in the directory except test.txt.
  • ls
    • ls -d */: List directories only.
  • date: Print or set the system date and time.
    • date +%s: Unix timestamp.
  • time: Run programs and summarize system resource usage.
  • cal: Displays a calendar.
  • cd: Change working directory.
    • cd -: Change to previous working directory.
    • cd ~-: Same thing as cd - without echoing the path.
  • pwd: Print name of current/working directory.
  • passwd: Change user password.
  • alias: List and create aliases.
  • mkdir -p folder/{sub1,sub2}/{sub1,sub2,sub3}: Make directories/subdirectories quickly.
  • chsh: Change login shell.
  • fdisk: Disk partition utility.
  • cfdisk: Disk partition utility.
  • dd: Convert or copy a file, create bootable USBs from ISOs. use cautiously. can destroy data irreversibly. To monitor the progress of an operation, add the status=progress option to the command.
  • wc: Print newline, word and byte counts for files.
    • ls ~/docs | wc -l: Print the number of files/folders in ~/docs.
  • history: Show history.
    • <space>man man: Don't add man man command to history.
    • fc: Fix a long command that you messed up.
    • !666: Run 666th command in history.
    • !!:s/start/status: Replace start in previous command with status.
    • !-n: Refer to the command n lines back.
    • !-1 / !!: Refer to the previous command.
    • sudo !!: Run previous command but append sudo at the beginning.
    • [program] !!: Run previous command using [program].
  • lsusb: List USB devices.
  • lsblk: List information about block devices.
  • env / printenv: List all environmental variables.
  • free: Display amount of free and used memory in the system.
  • df: Report file system disk space usage.
  • du: Estimate file space usage.
    • du -sh dir/: Check size of dir/ on disk.
  • fc-list: List available fonts.
  • timedatectl: Control the system time and date.
    • cat /etc/timezone: (On Debian) Print current timezone.
  • xprop: Property displayer for X.
  • shred: Overwrite the specified file(s) repeatedly, in order to make it harder for even very expensive hardware probing to recover the data.
  • mount: Mount a filesystem.
  • clear: Clear the terminal screen.
  • file: Determine file type.
  • basename: Given a pathname, returns the basename of a file or directory.
  • tree: List contents of directories in a tree-like format.
  • xev: Print contents of X events.
  • yes: Output a string repeatedly until killed.
  • column: Columnate lists.
  • shuf file.txt - Get random lines from file.txt.
  • man ascii: ASCII character set encoded in octal, decimal, and hexadecimal.
  • stat - Display file or file system status.
  • dirs / pushd / popd - Display/Manipulate the directory stack.

Easter eggs

  • calendar -f /usr/share/calendar/calendar.lotr -A 365


  • Drop into a tty (tty3): Ctrl + Alt + F3
  • Move between different ttys: Alt + Left/Right
  • Mod keys:
    • Mod1: Left Alt
    • Mod3: Right Alt
    • Mod4: Super/Windows