man : an interface to the reference manuals

It's still the absolute reference : RTFM & RTMF

man [command]

Example :

[user@localhost ~]$ man more
MORE(1) User Commands MORE(1)
NAME
       more - file perusal filter for crt viewing
SYNOPSIS
       more [options] file...
DESCRIPTION
       more  is  a  filter for paging through text one screenful at a time.
...

But sometimes

• it's not so easy
• even if you read the manuel : you are not sure how to use the command
• some new commands/software come event without man (--help option)
• GIYF/GETA : Google is your friend/Google est ton ami

tldr : Modern days man

tldr-pages : online collection of community-maintained help pages, that aims to be a simpler complement to traditional man pages.
tldr : a client to use tldr-pages directly from shell (command line)

[user@localhost ~]$ tldr more
  more
  Open a file for interactive reading, allowing scrolling and search. More information:
  https://manned.org/more.
  - Open a file:
    more path/to/file
  - Open a file displaying from a specific line:
    more +line_number path/to/file
  - Display help:
    more --help
  - Go to the next page:
    <Space>
[skip]

• Online (or offline cache)
• much more simple and with very useful example
• sometimes is not available
• just a simple version for the man page, it's NOT an alternative. Sometimes, you should read the man pages patiently ;)

Another useful site/tool

https://explainshell.com/

💡 NOTE

90% of this presentation were prepared using tldr and explainshell

history : Command-line history.

- Display the commands history list with line numbers:
history
- Display the last 20 commands (in `zsh` it displays all commands starting from the 20th):
history 20

• Check $HISTSIZE, $HISTFILESIZE
• Check $HISTTIMEFORMAT (export HISTTIMEFORMAT='%F %T ')
• !numero de command : relaunch the command

cd path/to/directory : change the current (working) directory

• cd : Go to the home directory of the current user

ls : list directory contents

• ls -a : list all files, including hidden files
• ls -l : long format list (permissions, ownership, size, and modification date)
• ls -lh : same as "-l" but size displayed using human-readable units (KiB, MiB, GiB)
• or even ls -alh

pwd : print name of current/working directory (where am I ?)

[paul@localhost ~]$ cd git
[paul@localhost git]$ pwd
/home/paul/git

chmod : change file (everything is a file) rights (mode bits)

• chmod 777 file : give all rights for owner, his group and all others
• chmod u+x file : give the [u]ser who owns a file the right to e[x]ecute it
• chmod u+rw file : give the [u]ser rights to [r]ead and [w]rite to a file/directory
• chmod g-x file : remove e[x]ecutable rights from the [g]roup
• chmod -R 740 directory : set rights for directory and all files inside (owner=all, group=read, others=nothing)
• bits mode : rwx = 7 = 4(r) + 2(w) + 1(x)

find : search for files in a directory hierarchy

• find . -name '*.ext' find files by extension in current directory (.)
• find root_path -type d -iname '*lib*' find directories matching given name, in case-insensitive mode in root_path
• find /home/user/tmp/ -type f -empty -delete find empty (0 byte) files and delete them in /home/user/tmp (Be Careful!!!)

grep : print lines matching a pattern

• grep "pattern" file : search pattern in file
• grep -Ri "pattern" : search (ignore-case) in all files in current directory (and sub-directories)
• grep -Ril "pattern" : as above, but just print files with match
• grep -in "pattern" file : search pattern in file and print line-numbers
• pcregrep : a grep with Perl-compatible regular expressions
• ripgrep : grep written in Rust : faster and powerful (but not everywhere)

less : Open a file for interactive reading, allowing scrolling and search.

😉 less is opposite of more

- Open a file:
less source_file
- Page down / up:
<Space> (down), b (up)
- Go to end / start of file:
G (end), g (start)
- Forward search for a string (press `n`/`N` to go to next/previous match):
/something

cat : print file on the standard output (and concatenate files)

😉 dog is not opposite of cat, opposite of cat is tac

- Print the contents of a file to the standard output:
cat file
- Concatenate several files into the target file:
cat file1 file2 > target_file
- Number all output lines:
cat -n file

tail : output the last part (last N lines) of files

• tail -n num file : show last 'num' lines in file
• tail -c num file : show last 'num' bytes in file
• tail -f file : keep reading file until Ctrl + C (catch cat by tail; never more cat ; cat ; cat ;)

head : output the first part (first N lines) of files

• head -n num file : show first 'num' lines of a file
• head -c num file : show first 'num' bytes of a file

du : disk usage (estimate file space usage)

• du -h path/to/directory : list the sizes of a directory and any subdirectories, in human-readable form

df : gives an overview of the filesystem disk space usage

• df -h : in human-readable form

which : locate a command/programm in the user's path

• which executable : search $PATH variable matching executable
• which -a executable : if there are multiple executables which match, display all

[user@localhost ~]$ which df
/usr/bin/df

top/htop : display live current system state (tasks) / modern version of top

ps : report a snapshot of the current processes

• ps aux : list all running processes (BSD syntax)
  • ~= ps aux
  • ~= ps -aux (Unix syntax)
• ps -aef : almost the same (list all running processes), but change format

kill : send a signal to a process (usually to kill/stop it)

- Terminate a program using the default SIGTERM (terminate) signal
  kill [process_id]
- Terminate a background job
  kill %[job_id]
- Signal to immediately terminate a program (no chance to capture the signal)
  kill -s 9 [process_id]
- Terminate a program using the SIGINT (interrupt) signal
  kill -2 [process_id]

Some useful codes :

• 9 = SIGKILL
• 15 = SIGTERM (default)
• 2 = SIGINT (Ctrl-C)

Some others very useful commands without explanation

• cp, mv, rm, cut, sed
• tar, gzip/gunzip
• ip a, ping, wget/curl