Bash

Script

$ type script
$ chmod +x script
$ ./script
$ .script
$ source script
$ bash script

#!/bin/bash

Command Line

Everything is a file
255 Byte
The maximum length for a file name is 255 bytes.
The maximum combined length of both the file name and path name is 4096 bytes.
This length matches the PATH_MAX that is supported by the operating system.
Names are case-sensitive

Special chars to avoid in names/filenames

/   Never
\   Escaped
-   Never at beginning
[]  Escaped
{}  Escaped
*   Escaped
'   Escaped
"   Escaped

Wildcards

*  asterisk        | any char
?  question mark   | single char
[] square brackets | set of single char or a range of chars

List files and folders

$ ls
$ ls folder
$ ls ./folder
$ ls /path/folder
$ ls ~/folder
$ ls ~/folder/*.jpg
$ ls ~/folder/*txt*
$ ls -R folder (list of subfolders)

$ ls -l (single column)
 - regular file
 _ executable
 d directory
 l symbolic link
 s socket
 b block device
 c character device
 p named pipe

$ ls -m (comma-separated list)
$ ls -a (hidden files and folders)

$ ls -F (file type)
Symbols and file types
 * Executable
 / Directory
 @ Symbolic link
 | FIFO
 = Socket

$ ls --color (content in color)
$ dircolors
$ dircolors --print-database
$ ls -F --color
$ la -la
$ ls -r
$ ls -X (sort by extension)
$ ls -t (sort by date)
$ ls -S (sort by content)
$ ls -h
$ ls -laS
$ ls -h

$ pwd (current path)

Change directory with cd

$ cd folder
$ cd ~
$ cd -
$ cd ..

Create and change current time of file touch

create new, empty file
$ touch file

update access and modification time for file
$ touch file

any desired time for file
$ touch -t [[CC]YY]MMDDhhmm[.ss]file

Create directory with mkdir

new directory
$ mkdir folder

create directory with subdirectories
$ mkdir -p test/sub/folder

create directory and show steps
$ mkdir -v test/sub/folder
$ mkdir -pv test/sub/folder

Count files and directories, summary size and EXCLUDE folders

bash count files and directory, summary size and EXCLUDE folders that are fuse|sshfs

Full size of a folder with du

get the full size of workdir | no fuse/sshfs in use
$ du -hs workdir

get the full size of workdir, excluding mysshfs | fuse/sshfs in use
$ du -hs --exclude=mysshf workdir

Count files in folder with find

count files in workdir | no fuse/sshfs in use
$ find workdir -type f | wc -l

count files in workdir, excluding mysshfs | no fuse/sshfs in use
$ find workdir -type f -not -path "*mysshfs*" | wc -l

count files in workdir, excluding mysshfs | fuse/sshfs in use
$ find workdir -path "*/mysshfs/*" -prune -o \( -type f -print \) | wc -l

Count folders in folder with find

count folders in workdir | no fuse/sshfs in use
$ find workdir -type d | wc -l

count folders in workdir, excluding mysshfs | no fuse/sshfs in use
$ find workdir -type d -not -path "*mysshfs*" | wc -l

count folders in workdir, excluding mysshfs | fuse/sshfs in use
$ find workdir -path "*/mysshfs/*" -prune \( -type d -print \)  | wc -l

Building blocks

run several commands sequentially with ; and &&

executed sequential no matter if successfully or unsucessfully
;
$ ls /home ; ls notfound; ls ~

executed sequential if successfully run next
&&
$ ls /home && ls notfound && ls ~

PORT SCAN with bash's built-in /dev/tcp

check if host response on a give port with bash's built-in /dev/tcp

#!/bin/bash
HOST_NAME="127.1"
HOST_PORT="80"
if ( (exec 3<>/dev/tcp/${HOST_NAME}/${HOST_PORT}) 2> /dev/null); then
   echo -e "PORT: ${HOST_PORT} | ON"
   else
   echo -e "PORT: ${HOST_PORT} | OF"
fi
exit; 

#!/bin/bash
HOST_NAME="127.1"
for HOST_PORT in {1..1000}
do
if ( (exec 3<>/dev/tcp/${HOST_NAME}/${HOST_PORT}) 2> /dev/null); then
   echo -e "PORT: ${HOST_PORT} | ON"
   else
   echo -e "PORT: ${HOST_PORT} | OF"
fi
done
exit;

#!/bin/bash
HOST_NAME="127.1"
declare -A PORT_ON
for HOST_PORT in {1..65535}
do
   if ( (exec 3<>/dev/tcp/${HOST_NAME}/${HOST_PORT}) 2> /dev/null); then
       PORT_ON[${HOST_PORT}]="ON"
   fi
done
for i in ${!PORT_ON[*]}
do
   echo -e "$i : ${PORT_ON[$i]}"
done
exit;

watch

$ watch -t -n 0.1 "grep \"^[c]pu MHz\" /proc/cpuinfo"
$ watch -t -n 0.1 "grep \"^[c]pu MHz\" /proc/cpuinfo && free -m"

Firefox bash output

For the usage in bash:
$ watch -n 1 "ps aux | grep [f]irefox"
$ watch -n 0.1 "ps aux | grep [f]irefox"
$ watch -n 1 "ps aux | grep [f]irefox && free -m"
$ watch -n 0.1 "ps aux | grep /usr/lib/firefox-esr/firefox"
$ watch -n 0.1 "ps aux | grep /usr/lib/firefox-esr/firefox && free -m"
$ watch -d -n 0.1 "ps aux | grep [f]irefox"

Load a page and press F5 in the tab(again and again or hold the F5 button pressed) and watch
When i kill the a tab with the right pid i get the message in the tab, that the tab is crashed now

 In Firefox, about:performance

But check also this in bash if i close or open and load a page tab:
$ pidof firefox
$ pidof firefox-esr
$ pgrep firefox get the main/parentid of one or "n" profiles
$ pgrep firefox-esr get the main/parentid of one or "n" profiles

Get parentid and childid
$ ps aux | grep [f]irefox
$ ps aux | grep [f]irefox | grep tab

You see the parentBuildID and something of the child/tab
It looks like that you can't close the child/tab only the parentBuildID, if you kill the child/tab the tab will crash but not close(but killed).

$ ps -ef | grep [f]irefox | wc -l
$ ps -ef | grep [f]irefox | grep tab | wc -l

Create Symbolic Links

ln|Linux manual page
Symlink To a File
$ ln -s [OPTIONS] TARGET LINKNAME
$ ln -s TARGET LINK_NAME 

Symlinks To a Directory
$ ln -s /mnt/my_drive/movies ~/my_movies 

Overwriting Symlinks
To overwrite the destination path of the symlink, use the -f (--force) option
$ ln -sf my_file.txt my_link.txt

Removing Symlinks 
$ unlink symlink_to_remove
$ rm symlink_to_remove

List harddisk|usb|volume|partitions

Get label names:
$ e2label /dev/sdXx 
$ /sbin/blkid | grep sdXx'
$ blkid /dev/sdXx | awk -F'"' '{print $2}'

Set label name:
$ e2label /dev/sdXx MYNAME
$ tune2fs -L MYNAME /dev/sdXx 

Create filesystem with mkfs.ext4

$ mkfs.ext4 /dev/sda1 -L MYNAME

Create/Add new USER useradd

Useradd is built-in Linux command that can be found on any Linux system. However, creating new users with this low-level is a tedious task because it doesn't create the home directory and user password by default.

Adduser is not a standard Linux command. It’s essentially a Perl script that uses the useradd command in the background. This high-level utility  is more efficient in properly creating new users on Linux. It gives you the option to create the home directory, and set password along with a few more parameters.

Difference between adduser and useradd commands
adduser vs useradd in Linux 

$ useradd [OPTIONS] USERNAME

$ cat /etc/default/useradd
$ ls -la /etc/skel/
$ cat /etc/login.defs

View default options:
$ useradd -D 

Change the default login shell
$ useradd -D -s /bin/bash 
$ useradd -D | grep -i shell 

To create a user without home directory
$ useradd -M user 

Create a user with changed login shell 
$ useradd -s /bin/sh user 

Option to create a user home directory 
--create-home
-m 

$ /sbin/useradd -m -u 10000 USERNAME -s /bin/bash 

There is an option for adding an encrypted password via the -p option on useradd
Note that the -p option doesn't allow you to input a plaintext password, it expects you to  encrypt it first. 
This is intentionally difficult, because you should not do it! 
Just use the passwd command or chpasswd

passwd USER
or
echo USER:NEWPASSWORD | /sbin/chpasswd

How to automatically add user account AND password with a Bash script?

$ /sbin/useradd -ms /bin/bash USER
Create USER + Homedirectory + Bash as Shell

USER+HOME+SHELL+PASSWORD
$ /sbin/useradd -ms /bin/bash USER && echo USER:NEWPASSWORD | /sbin/chpasswd

Delete USER userdel

$ userdel [OPTIONS] USERNAME 

Use the -r (--remove) option to force userdel to remove the user’s home directory and mail spool:
$ userdel -r username 

Kill all user’s running processes
$ killall -u username 

Another option is to use the -f (--force) option that tells userdel to forcefully remove the user account, even if the user is still logged in or if there are running processes that belong to the user.
$ userdel -f username 

How to Delete/Remove Users in Linux (userdel Command)

usermod

https://linuxize.com/post/usermod-command-in-linux/#:~:text=By%20default%2C%20on%20most%20Linux%20systems%2C%20the%20default,name%20of%20the%20user%3A%20usermod%20-s%20SHELL%20USER

https://linuxconfig.org/usermod#:~:text=usermod%20command%20in%20Linux%20Advanced%20Examples%201%20Use,given%20a%20unique%20ID%20when%20first%20created.%20

https://wiki.ubuntuusers.de/usermod/

GROUP, USER | GROUP VS PASSWD

https://teaching.idallen.com/cst8207/12f/notes/600_users_and_groups.html

https://www.unix.com/unix-for-dummies-questions-and-answers/162563-inconsistency-between-passwd-group.html

https://unix.stackexchange.com/questions/55107/why-dont-etc-group-and-etc-password-match

https://www.man7.org/linux/man-pages/man3/initgroups.3.html

chown

https://wiki.ubuntuusers.de/chown/

chmod

https://wiki.ubuntuusers.de/chmod/

chattr

https://wiki.ubuntuusers.de/chattr/

https://man7.org/linux/man-pages/man1/chattr.1.html

SHELL USER INFOS

getent passwd "$LOGNAME"

grep passwd /etc/nsswitch.conf

rbash | Restricted Bash

$ rbash 
$ bash -r

https://www.tecmint.com/rbash-a-restricted-bash-shell-explained-with-practical-examples/
https://www.thegeekdiary.com/rbash-set-restricted-shell-in-linux/

https://www.howtogeek.com/718074/how-to-use-restricted-shell-to-limit-what-a-linux-user-cando/
https://ostechnix.com/how-to-limit-users-access-to-the-linux-system/
https://averagelinuxuser.com/creating-new-user-linux/
https://linuxize.com/post/how-to-create-users-in-linux-using-the-useradd-command/
https://www.howtoforge.com/user_password_creating_with_a_bash_script

Simple Script Jail

This is are the default settings for the USER to login and run a bash shell:
$  chsh -s /bin/bash [USER]  

This for the simple jail:
$ chsh -s /home/[USER]/./run [USER] 

Disable history

https://linuxconfig.org/how-to-disable-bash-shell-commands-history-on-linux
https://www.cyberciti.biz/faq/disable-bash-shell-history-linux/
https://www.thegeeksearch.com/how-to-disable-and-clear-linux-command-line-history/

Disable LASTLOGIN

$ touch /home/user/.hushlogin

https://linuxconfig.org/how-to-disable-last-login-message-on-rhel-linux
https://www.heatware.net/linux-unix/linux-how-to-disable-last-login-welcome-message/

type | command check

Check whether a command is built-in or not
$ type [COMMAND]

RANDOM NUMBERS

$ echo $RANDOM
$ echo $(( $RANDOM % 9 + 0 ))

$ echo $(shuf -i 0-9 -n1)

$ od /dev/urandom  -A n -t d -N 1 

https://linuxhint.com/generate-random-number-bash/
https://linuxconfig.org/generating-random-numbers-in-bash-with-examples
https://www.delftstack.com/howto/linux/random-number-generation-in-bash/

RANDOM STRINGS

$ echo $RANDOM | md5sum | head -c 1; echo;  
$ tr -dc A-Za-z0-9 </dev/urandom | head -c 1 ; echo ' ' 

https://gist.github.com/earthgecko/3089509

https://unix.stackexchange.com/questions/230673/how-to-generate-a-random-string
https://linuxhint.com/generate-random-string-bash/
https://www.saotn.org/bash-function-to-generate-a-random-alphanumeric-string/

https://unix.stackexchange.com/a/709895/515538

https://unix.stackexchange.com/questions/747546/dhcpd-or-dhclient-not-found#comment1421782_747546

https://unix.stackexchange.com/questions/747014/command-to-check-if-the-machine-was-rebooted#comment1420778_747014

https://unix.stackexchange.com/questions/745582/block-every-website-but-one-specific-one#comment1418071_745582

https://unix.stackexchange.com/questions/745375/what-does-run-rm-rf-do-in-dockerfile#comment1417469_745375

https://unix.stackexchange.com/questions/741540/setting-alias-for-sourcing-a-script-to-run-that-script-in-jenkins-with-restrict/741545#comment1408882_741545

https://unix.stackexchange.com/questions/741028/how-do-i-create-a-copy-of-a-hard-drive-on-my-pc#comment1407651_741028

https://unix.stackexchange.com/questions/740700/how-to-restrict-ssh-for-specific-user-from-specific-subnet#comment1406816_740700

https://unix.stackexchange.com/questions/740606/migration-of-a-working-debian-11-system-to-another/740612#comment1406640_740612

https://unix.stackexchange.com/questions/740105/cannot-figure-out-how-to-turn-off-stricthostkeychecking/740107#comment1405529_740107

https://unix.stackexchange.com/questions/734385/remove-directory-failed-even-as-root#comment1393984_734385

https://unix.stackexchange.com/questions/715772/how-to-create-a-hash-sha256sum-in-bash-with-more-than-one-source-input-and-w#comment1356335_715772

https://unix.stackexchange.com/questions/13802/execute-a-specific-command-in-a-given-directory-without-cding-to-it/709871#comment1344018_709871

https://unix.stackexchange.com/questions/693566/how-can-i-determine-which-browser-tab-is-the-most-network-intensive/693588#comment1311841_693588

https://unix.stackexchange.com/questions/693006/what-are-these-unexpected-network-devices-which-appear-in-kernel-log/693011#comment1310798_693011