Linux (ˈlinʊks) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds.

Table of Contents

File System

  • etc: system-wide configuration folder (eg. pacman mirrorlist)
  • bin: common binary files for applications (eg. ls)
  • boot: stores my bootloader
  • lib: shared libraries that applications use
  • mnt: where external drives are mounted (eg. usbstick)
  • opt: manually installed softwares
  • proc: information about system processes and resources, each folder is a process' PID.
  • root: volatile storage
  • srv: service directory, where service data is stored
  • sys: it's a way to interact with the hardware
  • tmp: where files are temporary stored in a session (eg. writing in a libreoffice file)
  • usr: user application's space, where applications will be installed
  • var: contains files that are supposed to grow (eg. logs)
  • home: where you store your own personal files

Sha Bang

Expression Description
#! locate an executable for the specified script
!! executes last command
!* executes command with ALL the args passed to the last command
!^ executes command with the FIRST arg passed to the last command
!$ executes command with the LAST arg passed to the last command
!?keyword? execute a command from history for the first pattern match
!n execute a command from the nth position in the history
!<command> executes a command with the same flags as before


Group permissions

Group Description
u user (owner)
g group
o others
a all


Operator Description
+ add permission
- remove permission
= override permission

Octal notation

Octal Description
0 nothing
1 execute
2 write
3 1 + 2 (wx) write and execute
4 read
5 4 + 1 (rx) read and execute
6 4 + 3 (rw) read and write
7 4 + 2 + 1 (rwx) read, write and execute

chmod (Change Mode)

Command Description
chmod -R <specifications> <directory> changing permissions of the contents of a directory recursively
chmod og=+r add read permission for the owner and group
chmod a +rwx file allow all user to read, write and execute

chown (Change Ownership)

chown user1 filename changing ownership of a file to user1

su (switch user)

Command Description
su - invokes a login shell, reseting most environment variables
su normal shell
sudo su switch to root

sudo (superuser do)

sudo will run a specific command with root level of permission.

sudo !! (last command) for instance

Other commands

Command Description
cd - go to the previous working directory
ls \*D pathname expansion, returns all files starting with D
ls exe\* all files ending with 'exe'
less <file> view the contents of a file with a pager
head <file> first several lines of a file (pass -n <numer-of-lines> to see N number of lines)
tail <file> last several lines of a file (pass -n <numer-of-lines> to see N number of lines)
uname show informations about the system (-a flag shows ALL info)
du summarizes the disk usage of files
du -sh * summarizes every file in the current directory
df disk space of filesystem
ps process status
ps -C <name> find the PID of a specific program
killall <p> terminates a process
kill <PID> terminates a process given the PID
which searches for executables specified by the environment variable PATH
time time the execution of any program (eg. time node script.js)
watch -n N runs a command infinitely each N seconds
apropos find a command that I'm unaware of (eg. apropos "list dir")
  use in conjuntion with (whatis) command
whereis searches for executables, source files and manual pages
diff compares files, and lists their difference
find find files in directory (eg. find -name
man giteveryday useful minimul set of commands for everyday git
curl weather forecast
curl rainbow parrot
sha256sum verify file checksum. (or sha1sum, md5sum depending on the hash)
curl<command>/ get a cheatsheet about a particular command

Fix typos

Instead of retyping a long list of arguments of a mispelled command, we can correct whatever typo we made with ^typo^correct

$ dc /tmp

$ ^dc^cd

File Compression

Flags Description
-c –create create a new archive
-x –extract extract files from an archive
-t –list list the contents of an archive
-f –file=ARCHIVE use archive file
-v –verbose list files being processed
-z compress the archive with gzip (faster, but compresses less)
-j compress the archive with bzip2 (slower, but compresses more)
–exclude <path> ignores subfolder


Command Description
tar -cf ./archive.tar ./folder creates an archive from a folder
tar -lf ./archive.tar lists the contents of the archive without extracting it
tar -xvf ./archive.tar extracts the archive
tar -cf archive.tar ./folder –exclude "folder/subfolder" ignores subfolder inside my-folder
tar -czvf archive.tar.gz ./folder creates a gzip archive
tar -cjvf archive.tar.bz2 ./folder creates a bzip2 archive
gunzip <filename> un-compresses files compressed by gzip
gzcat <filename> look at a gzipped file without having to gunzip it
gzip <filename> compresses a file to gzip format
bzip2 <filename> compresses a file to bzip2 format


GPG (GNU Privacy Guard)

Flag Description
--list-keys list all keys keyring
--import <key-path> Add a pubkey into keyring
--delete-key <fingerprint> Del key in keyring

Import keys from keyserver

$ gpg --keyserver <server> --recv-keys <hex>

Where <server> can be "" for instance.

Encrypt message

Having the receiver's pkey:

$ gpg --output doc.gpg --encrypt --recipient mail@recipient doc

Decrypt message

$ gpg --output doc --decrypt

Symmetric encryption

-c is for symmetric encryption (single pkey for encrypting/decrypting).

$ gpg -c --no-sym-key-cache --cipher-algo AES256 <file>

Shell expansion

We can use shell expansion to:

  • rename and backup operations
  • pattern matching
  • match any string

The * Wildcard

  • The * is a greedy operator that matches any string, incuding the null string.

$ echo file* returns file.log file.tex file.txt file.exe $ echo *.tex returns file.tex

The ? Wildcard

  • The ? matches a single character.

$ echo file?.txt returns file1.txt file2.txt file3.txt ...

Inverting Sets

  • The ^ character represents not.
    • [abc] means either a, b or c
    • [^abc] any character that is not a, b or c

Brace Expansion

To backup settings.conf to settings.conf.bak:

cp settings.conf{,.bak}

To revert the file from settings.conf.bak to settings.conf:

mv settings.conf{.bak,}

Other uses:

echo foo{1,2,3}.txt outputs foo1.txt, foo2.txt, foo3.txt

echo file-{a..b}.txt outputs file-a.txt file-b.txt file-c.txt file-d.txt

mv program.{c,exe} bin/

IO Redirections

Output redirections

The output from a command normally intended for stdout can be easily diverted to a file.

$ who > users

$ cat users
nexi     tty01    Oct 22 08:58
ai       tty02    Oct 22 13:58

The > operator will always override the contents of the file.

We can use the >> operator to append the output in a file with existing info:

$ echo lorem ipsum >> users

$ cat users
nexi     tty01    Oct 22 08:58
ai       tty02    Oct 22 13:58
lorem ipsum

Input redirection

Commands that receive input from stdin can have their input redirected from a file.

$ wc -l < users

Here document

A "here document" redirects input into an interactive shell script or program.

command << EOF

The >> command is an instruction to read input until it finds a line containing the delimeted (EOF in this case)

$ wc -l <<EOF
  lorem ipsum
  dolor sit

Chaining Operators

  • & (Ampersand Operator)

    run one or more jobs in the background,

    EXAMPLE: ping & apt-get update & apt-get upgrade &

    • Ctrl-z pauses the job
    • jobs prints all the jobs
  • ; (Semi-colon operator)

    run several commands at once sequentially, disregarding the exit status of the preceding command

    EXAMPLE: apt-get update ; apt-get upgrade ; mkdir test

  • && (AND operator)

    executes a command IF the exit status of the preceding command is 0

    EXAMPLE: ping && links (checking the connection before using links command)

  • || (OR operator)

    much like an 'else' statement, allows to execute the second command only if the execution of the first fails (i.e., the exit status is 1)

    EXAMPLE: apt-get update || links

  • ! (NOT operator)

    much like an 'except' statement, this command will execute all except the condition provided

    EXAMPLE: rm -r !(*.html) removes all files in a folder except .html files

  • | (PIPE operator)

    passes the output of the first command to the second one

    EXAMPLE: ls -l | less

  • {} (Command Combination operator)

    combine two or more commands

    [ -d Folder] || { echo creating Folder; mkdir Folder; } && echo Folder exists.

RegEx Tools

  • grep (Globally search for a Regular Expression and Print) for searching stuff in files, or any STDOUT (eg. 'ls' command)

    EXAMPLE: ls | grep "\.exe$"

  • sed (stream editor) for substitituing, deleting or filtering text on a stream

    EXAMPLE: sed 's/regexp/replacement/g' file > output

    -r will extend the Regex portability (POSIX)

  • xargs (command args) pass any command to it and it will execute it on a stream.

    EXAMPLE: find | grep "\.exe$" | xargs ls -lh


keybinding description
ctrl-u delete from cursor position to beginning of line
ctrl-k delete from cursor position to end of line
ctrl-l clear screen
ctrl-w delete last word
ctrl-r search through command history
alt-. cycles through last arguments
alt-* expands glob


Steps to free disk space on your Linux machine:

  1. Check directories disk space: cd / && sudo du -h --max-depth=1
  2. Delete pacman cache:
    • sudo paccache -r keep at least 3 caches
    • sudo paccache -rk 1 keep only 1 pkg cache

Date: 2021-09-11 sáb 00:00

Author: Bruno Coimbra (b-coimbra)

Created: 2022-03-07 seg 18:29