Linux basics

Commands in linux are all case sensitive. All folders and files are file in linux.

All about file system

In linux world, all files and folders are files.

Root structure

The root folder represents as / which is the top-level directory in linux.

Start a new bash session, use ls to check the structure of root.

text

bin  boot  dev  etc  home  lib  lib32  lib64  libx32  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var

folder	shortcut for
bin	binary
boot	booting
dev	device
etc	editable text configuration
home	home folder for each user
lib	libraries
mnt	mounted storages
root	home folder for root user only
...	...

If working as root user cd ~ will direct to /root/ instead of /home/

Absolute and Relative path

Absolute path starts with / which stands for root. Relative path starts with empty character or ./, the dot stands for current folder.

Working with folders

checking content under a folder

We use ls to represent the members in current path. By default, its output aligned horizontally, use ls -1 make it vertically aligned.

bash

bin
boot
dev
etc
home
lib
lib32
lib64
libx32
media
mnt
opt
proc
root
run
sbin
srv
sys
tmp
usr
var

Additionally, use ls -l to see more details of those members.

bash

total 60
lrwxrwxrwx   1 root root    7 Oct  4 02:08 bin -> usr/bin
drwxr-xr-x   2 root root 4096 Apr 18  2022 boot
drwxr-xr-x   5 root root  360 Nov  4 12:18 dev
drwxr-xr-x   1 root root 4096 Nov  4 12:23 etc
drwxr-xr-x   2 root root 4096 Apr 18  2022 home
lrwxrwxrwx   1 root root    7 Oct  4 02:08 lib -> usr/lib
lrwxrwxrwx   1 root root    9 Oct  4 02:08 lib32 -> usr/lib32
lrwxrwxrwx   1 root root    9 Oct  4 02:08 lib64 -> usr/lib64
lrwxrwxrwx   1 root root   10 Oct  4 02:08 libx32 -> usr/libx32
drwxr-xr-x   2 root root 4096 Oct  4 02:08 media
drwxr-xr-x   2 root root 4096 Oct  4 02:08 mnt
drwxr-xr-x   2 root root 4096 Oct  4 02:08 opt
dr-xr-xr-x 266 root root    0 Nov  4 12:18 proc
drwx------   1 root root 4096 Nov  4 13:53 root
drwxr-xr-x   5 root root 4096 Oct  4 02:12 run
lrwxrwxrwx   1 root root    8 Oct  4 02:08 sbin -> usr/sbin
drwxr-xr-x   2 root root 4096 Oct  4 02:08 srv
dr-xr-xr-x  11 root root    0 Nov  4 12:18 sys
drwxrwxrwt   1 root root 4096 Nov  4 12:23 tmp
drwxr-xr-x   1 root root 4096 Oct  4 02:08 usr
drwxr-xr-x   1 root root 4096 Oct  4 02:12 var

Manipulating files and folders

command	usage
`mkdir <path>`	create folder
`mv <source_path> <dist_path>`	move or rename
`touch <...file_path>`	create file
`rm <file_path>`	remove file
`rm -r <folder_path>`	recursively remove folder
...	...

Write and Read

Read from files

command	usage
`cat <file_path>`	read all text
`more <file_path>`	read text with scrolling down
`less <file_path>`	read text with scrolling up/down
`head -n <line_number> <file_path>`	read first n lines
`tail -n <line_number> <file_path>`	recursively remove folder
...	...

Write to files

Redirect output from command to a file, we use > and >>. > overwrites the file, >> appends text to the file. Any text output can be redirected to a file. Additionally, use nano or other packages to edit files.

bash

cat file1.txt > file2.txt
ls -1 / > root_structure.txt
echo hello >> file3.txt

Matching text

Use grep(global regular expression print) to text content.

bash

grep f* <...file_path> # starts with f
grep -i f* <...file_paths> # starts with f/F (case insensitive)
grep hello <...file_paths> # match the text/s
grep hello -r <path> # recursively match

Matching folders and files

command	usage
`ls -a <path>`	list all members including hidden
`find <path>`	recursively list all sub folders
`find <path> -type d -name "<pattern>"`	recursively find list all folders matching to pattern
`find <path> -type f -iname "<pattern>"`	recursively find list all files matching to pattern insensitive
`head -n <line_number> <file_path>`	read first n lines
`tail -n <line_number> <file_path>`	recursively remove folder
...	...

Command Chaining

Command sequence

Use ; to separate multiple command to execute one by one.

bash

echo start; mkdir test; cd test; echo finished

Command And

Only when previous command are successfully executed, can next command execute? Use &&!

bash

echo start && mkdir test && cd test && echo finished

Command Or

If previous command was executed, next commands won't be executed! Use ||!

bash

mkdir test || echo "test does not exist"

Pipe operator

Pipe args into a command using |!

bash

ls / | head -n 5

Environment Variables

command	usage
`printenv`	print all environment variables
`printenv <variable_name>`	print specific variable
`echo $<variable_name>`	print specific variable
`export <variable_name>=<value>`	add temporary environment variable to current session
...	...

What is .bashrc

The .bashrc file in Linux is similar to the $PROFILE file in PowerShell on Windows. Both serve as configuration files that allow users to customize their command-line environment and define various settings and preferences for their shell sessions. Just like the .bashrc file in Linux, the $PROFILE file in PowerShell is executed every time a new PowerShell session is initiated.

To reload the .bashrc file in the current session, use source ~/.bashrc

Managing Processes

Checking running process

Check all process using ps. Ignore the ps process because ps does include itself when retrieving result.

bash

PID TTY          TIME CMD
    1 pts/0    00:00:00 bash
  285 pts/0    00:00:00 ps

Asynchronous command

The & operator allows the command to run asynchronously, enabling you to continue using the terminal for other tasks. Take a sleep behind the scene!

bash

root@34aee1a42ba0:/# sleep 100 &
[1] 286 
root@34aee1a42ba0:/# ps
  PID TTY          TIME CMD
    1 pts/0    00:00:00 bash
  286 pts/0    00:00:00 sleep
  287 pts/0    00:00:00 ps

Managing Users

Add a user

Use adduser to check to options available.

-m for example:

useradd -m byhpbsh

Who am I

If you don't know which user you currently logged in, get username using command whoami.

bash

/app # whoami
root
/app #

Check all user info

Check passwords in /etc/

bash

cat /etc/passwd

We can see the new user, the x means the password is stored somewhere else. The first 1000 is user id, the latter is group id.

text

...
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
_apt:x:100:65534::/nonexistent:/usr/sbin/nologin
byhpbsh:x:1000:1000::/home/byhpbsh:/bin/sh

Specify bash as shell for new user

bash

usermod -s bash byhpbsh

Recheck the /etc/passwd

text

Before:
byhpbsh:x:1000:1000::/home/byhpbsh:/bin/sh
After:
byhpbsh:x:1000:1000::/home/byhpbsh:bash

Starts a new terminal session using docker

Check running container using docker ps.
Copy the container id
Run docker exec -it -u <username> <container_id> bash Now we can see the user marked with $ which indicates a regular user.

text

byhpbsh@34aee1a42ba0:/$

Managing Groups

What is Group?

All users in the same group have the same permissions.

Add new group

groupadd <group_name>

Let's add a dev group.

bash

groupadd developers

Then check group info in /etc/group

bash

root@34aee1a42ba0:/# cat /etc/group | grep developers
developers:x:1001:byhpbsh

Add user to group

This command add a supplementary group for user byhpbsh. A user can have multiple groups, and each user has a primary group named as the username.

bash

usermod -G developers byhpbsh

Check groups of user

groups <username>

bash

root@34aee1a42ba0:/# groups byhpbsh
byhpbsh : byhpbsh developers

File Permissions

Permission explained

Let's create a new file sayHello.sh in home folder that prints hello when executed.

bash

root@34aee1a42ba0:/home# echo echo hello > sayHello.sh

To check the permissions of files , use ls -l

bash

root@34aee1a42ba0:/home# ls -l
total 8
drwxr-x--- 2 byhpbsh byhpbsh 4096 Nov  5 07:50 byhpbsh    
-rw-r--r-- 1 root    root      11 Nov  5 08:36 sayHello.sh

File permissions are represented as 10 characters:

if starts with d, indicating the file is a directory.
if starts with -, indicating the file is a file.
the rest 9 characters representing 3 permissions for 3 type of users
- first three for the user who owns the file
- second for the group owns the file
- last three for every user
r for readable
w for writable
x for executable
- for none

Change permission

Currently sayHello.sh is not executable for any user.

text

-rw-r--r-- 1 root    root      11 Nov  5 08:36 sayHello.sh

To make it executable for user owns the file, run chmod u+x <file_path>, u stands for user, + stands for add correspond to - that revoke the permission. For other users, run chmod o+x <file_path>, for simplicity, chmod supports syntax like chmod o+x+w-r <...files> to alter permissions in one go.

bash

root@34aee1a42ba0:/home# chmod u+x sayHello.sh
root@34aee1a42ba0:/home# ls -l
total 8
drwxr-x--- 2 byhpbsh byhpbsh 4096 Nov  5 07:50 byhpbsh    
-rwxr--r-- 1 root    root      11 Nov  5 08:36 sayHello.sh
root@34aee1a42ba0:/home# ./sayHello.sh
hello

Linux basics ​

All about file system ​

Root structure ​

Absolute and Relative path ​

Working with folders ​

checking content under a folder ​

Manipulating files and folders ​

Write and Read ​

Read from files ​

Write to files ​

Matching text ​

Matching folders and files ​

Command Chaining ​

Command sequence ​

Command And ​

Command Or ​

Pipe operator ​

Environment Variables ​

What is .bashrc ​

Managing Processes ​

Checking running process ​

Asynchronous command ​

Managing Users ​

Add a user ​

Who am I ​

Check all user info ​

Specify bash as shell for new user ​

Starts a new terminal session using docker ​

Managing Groups ​

What is Group? ​

Add new group ​

Add user to group ​

Check groups of user ​

File Permissions ​

Permission explained ​

Change permission ​

Linux basics

All about file system

Root structure

Absolute and Relative path

Working with folders

checking content under a folder

Manipulating files and folders

Write and Read

Read from files

Write to files

Matching text

Matching folders and files

Command Chaining

Command sequence

Command And

Command Or

Pipe operator

Environment Variables

What is .bashrc

Managing Processes

Checking running process

Asynchronous command

Managing Users

Add a user

Who am I

Check all user info

Specify bash as shell for new user

Starts a new terminal session using docker

Managing Groups

What is Group?

Add new group

Add user to group

Check groups of user

File Permissions

Permission explained

Change permission