Commands

man/ help/ info

• To get manual or help or more information about any command

man

show manual of commands

//syntax

man command

//example

man ls

//to show all built in commands

man builtins

help

Most commands have --help argument

//syntax

command --help

//example

ls --help

info

//list all

info

//specific command

info command

//example of specific command

info ls

Note : In info output, press enter on navigation links to go to it's section

Docs

• /usr/share/doc has documentation information

short/ long args

Most commands have short hand arguments specified by single dash (-), long arguments specified by double dash

//short hand (show all files including starting with .)

ls -a

//long

ls --all

Combine short hands

Many short hands which doesn't accept any value can be combined

//example

ls -l -h -t

//combined

ls -lht

Note : -h = --human-readable, -t = sort by modification time, -l = use long listing format

pwd

• To print working directory path

  pwd

ls

• To list all files

  ls

• To list even hidden files

  ls -a

  Note : -h = --human-readable, -t = sort by modification time, -l = use long listing format

  ls somePath

du (disk usage)

• To check folder size (du = disk usage, -h = human readable format, -sh= summary h format)

  Link

  du -sh

  du -sh path

cd

• Changing directories Link

  cd path

  cd .. //one level up

  //just cd to go to home directory

  cd

cp

• Copy files

  cp -R src dest

  //example glob (name followed by any no.of.chars)

  cp my-file* newTestFolder/

  //example glob (name followed by single char)

  cp my-file? newTestFolder/

scp

• Copy folder from windows/linux to linux using ssh

  //secure copy (scp), -r = recursive

  scp -r source_folder user@host:/path/to/destination

  scp -r E:/some_folder/some root@server:/home/user/target_folder

  (with root user if permisiion issue)

  scp -r E:/some_folder/some user@server:/home/user/target_folder

• Copy folder from linux to windows using ssh

  scp -r user@server:/home/user/target_folder E:/some_folder/some

• ssh key authentication

  scp -P portNum -i privateKeyFile user@server:/home/user/target_folder localPath

mv

• Move files (f=force without prompt while override?)

  mv -f src dest

rm

• Delete/remove files (rm)

  Link

  rm file_name

  rm -r directory_name

  rm -rf dirname //without file prompt of delete

mkdir

• Create directory

  mkdir dir_name

cat

To create a file with content

(alternative is to use "touch" command to create file)

cat > sample.txt

//now cursor is at 1st line of file

//(type what ever u want then ctrl+D to exit file)

To see file content

cat sample.txt //to view file content

Empty large file content

echo "" > filename.ext

History

• Interactive Search

  • Ctrl+r to start the search
  • Each time you press Ctrl+r, you’re searching backward for the next matching command
  • press Enter to execute command.
  • To edit a command before you execute it, press either the Left or Right arrow key.
  • ~/.bash_history file has history commands

• To see commands history

history

• To see last n commands

history 10

• Repeating some Command from history list

!commandNumber

!37

• Search command by ! & hit enter

!searchString

for safety first print command

!searchString:p

• Clearing the History List

history -c

History Ref

whoami

To print current user name

whoami

hostname

To print computer name

hostname

find

link

• Find in current folder & copy those files to target directory with folder structure

  find . -regex '.*\(jpg\|JPG\|jpeg\|png\|PNG\|gif\)' \! -path

  './node_modules/*' -exec cp --parents {} ./build/ \;
  breakdown

  find . //find currentDirectory or path

  -regex '.*\(jpg\|JPG\|jpeg\|png\|PNG\|gif\)' //match extensions

  \! -path './node_modules/*' //exclude path

  -exec cp //execute copy after find

  --parents //maintain folder structure

  {} // copy source, here output of find files

  ./build/ \; // copy target

locate

link

to search for files and directories by their names

Install

sudo apt update

sudo apt install mlocate

updatedb

a cron job is created at the time of installation that runs the updatedb command every 24 hours

//to update file index manually

sudo updatedb

Search

Output is absolute path of all files and directories that matches the search pattern and for which the user has read permission.

//ignore case

locate -i readme

//limit result count

locate -n 10 readme

//display result count (--count)

locate -c readme

Note :

• By default, locate doesn’t check whether the found files still exist on the file system. If you deleted a file after the latest database update if the file matches the search pattern it will be included in the search results. (Can use -e/ --existing option)
• Locate is faster than find, but has limited features.

tail

To print last N / tail of text file (It is the complementary of head command)

tail -n LineCount filePath

tail -n 3 file.txt

Note: -f command in tail is used to follow changes, so any new content written at end can be watched & printed

grep

link

grep [options] pattern [files]

Example 1 : Case insensitive search

$grep -i "UNix" geekfile.txt

o/p:

unix is great os. unix is opensource. unix is free os.

Unix linux which one you choose.

uNix is easy to learn.unix is a multiuser os.Learn unix .unix is a powerful.

Example 2 : Displaying the line count of number of matches

grep -c "unix" geekfile.txt

o/p:

2

• sample piping

cat /some/file | grep searhTerm >> newResultFile

>, >> , 2>

• ref Note: 2 arrows >> will append text, one arrow > will override text, number two with arrow 2> denotes to add only errors

//multi line example (append)

echo '

line 1

line 2

' >> filename.txt

//multi line sudo example (a = append)

echo '

line 1

line 2

' | sudo tee -a filename.txt

less

displays file contents or command output one page at a time in your terminal

command | less

less filePath

Check RAM, Processor, DiskSpace

//RAM

free -h

//Hard Disk

lsblk

cat /proc/cpuinfo

cat /proc/meminfo

//dynamic changing stats

top

env

• list all Environment variables

env

Note : use 'env | less' to see one page output at a time

o/p:

SHELL=/bin/bash

WSL_DISTRO_NAME=Ubuntu-20.04

NAME=xxx-my-computer-name-xxx

PWD=/usr/share/i18n/locales //keeps track of current working directory

LOGNAME=someUser

HOME=/home/someUser

LANG=C.UTF-8

USER=someUser

PATH=/home/someUserBin/gems/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:..so on

HOSTTYPE=x86_64

_=/usr/bin/env

OLDPWD=/usr/share/i18n

• Print some variable value

echo $variable

echo $NAME

Custom variables

• local variable (for that bash)

//set variable (local)

abc=4

//get variable

echo $abc

• to export variable for child bash

export abc

//now exported variable works in child bash

bash

echo $abc

exit

Note : in linux server -> basics.md check ".profile , .bashrc" section

bash

• with in a bash, can open one more bash

//to start new child shell

bash

//to exit from child shell

exit

Change EDITOR

• Change default editor before running command

  export EDITOR=gedit

  Note : Add above line in ~/.bashrc file to change setting permanently (.)

Users & related

shadow, passwd, group

//following file contains password

/etc/shadow

//following file contains user details

/etc/passwd

//following file contains group details

/etc/group

//consider sample entry in passwd file

prasan:x:1000:1000:prasan,,,:/home/prasan:/bin/bash

- prasan : name

- x : indicates password exists in shadow file

- 1000 : user id

- 1000 : group id

- /home/prasan : home directory

- /bin/bash : default shell

//consider sample entry in /etc/group

prasan:x:1000:

- prasan : group name

- 1000 : group id

//consider one more entry

sudo:x:27:prasan

- sudo : group name

- 27 : group id

- prasan : It indicates that prasan belongs to sudo group

id

To display user details & it's groups info

id UserName

eg:

id prasan

//o/p :

uid=1000(prasan) gid=1000(prasan) groups=1000(prasan),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),120(lpadmin),131(lxd),132(sambashare)

who

Check all logged in users

• who are active, when their session begin, from where (ip) they came from

who

//sample output

prasan :0 2020-06-21 15:48 (:0)

prasan pts/0 2020-06-21 17:13 (192.xxx.x.xxx)

w

Who logged in & what are they doing

w

//sample output

17:38:25 up 1:50, 2 users, load average: 2.43, 2.24, 2.24

USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT

prasan :0 :0 15:48 ?xdm? 27:02 0.01s /usr/lib/gdm3/gdm-x-session --run-script env ...more

prasan pts/0 192.xxx.x.xxx 17:13 0.00s 0.10s 0.00s w

last

who logged in (history, duration, ip address..etc) since the file /var/log/wtmp was created

last | less

//sample o/p

prasan pts/0 192.xxx.x.xxx Sun Jun 21 17:13 still logged in

prasan pts/1 192.xxx.x.xxx Sat Jun 20 23:08 - 00:37 (01:29)

useradd

• create a new user

sudo useradd -m john

• '-m' to create home directory

//sample test commands

cd /home/john

ls -a

//o/p

.. .bash_logout .bashrc .profile

• above default skeleton files came from '/etc/skel' folder

cd /etc/skel

ls -a

set passwd

• set passwd for new user created

sudo passwd john

//it will prompt to enter & verify password

• Also user can change his passwd self after login by

passwd

//it will prompt to enter & verify password

userdel

userdel -r username

groupadd

• to create new group

sudo groupadd groupName

groupdel

sudo groupdel groupName

usermod

• adding user to existing group

sudo usermod -a -G groupName userName

• '-a' to add

• '-G' existing group

• Note : In centos, user is added to 'wheel' group for sudo permissions

su

substitute user / switch user

su username

su //for root user

chown

Change owner for folder

//-R for recursive sub folders

chown -R userName pathFileOrFolder

chgrp

To change group

chgrp -R grpName pathFileOrFolder

chmod

To change permissions

//consider some folder (ls -l)

drwxr-xr-x 2 prasan prasan 4096 Jun 13 19:00 Desktop

d - directory

1st rwx - read, write & execute for owner

2nd rwx - read, write & execute for group

3rd rwx - read, write & execute for others

prasan - user

prasan - group

• Numeric notation read (4), write (2), execute (1) = 7(full permissions)

  chmod -R 770 pathFileOrFolder

  Note : The first 7 sets the permissions for the user,

  the second 7 sets the permissions for the group,

  and the third 7 sets the permissions for others
  7, rwx: read, write, and execute

  6, rw-: read and write

  5, r-x: read and execute

  4, r--: read-only

  3, -wx: write and execute

  2, -w-: write only

  1, --x: execute only

  0, ---: none

  

• To check all users permissions at folder level (l denotes long list format)

  ls -l

  ls -al

  ls -dl //current directory permission (.)

  In output:

  drwx = d for directory else _ file, r =read, w = write, x = executable

  Note: with out actually going to some folder, can list it's files/folders

  ls -l folderPath

• To check all users in group

  grep '^group_name_here:' /etc/group

Sticky bit

A Sticky bit is a permission bit that is set on a file or a directory that lets only the owner of the file/directory or the root user to delete or rename the file. No other user is given privileges to delete the file created by some other user.

//add sticky bit

chmod +t folderPath/

//remove sticky bit

chmod -t folderPath/

compgen

To check all users

compgen -u

umask

Umask Link

u mask = user file creation mask , governs the permissions that will get set when a new file is created.

Check existing folder umask (s = symbolic representation)

umask -S

output (user, group & others):

u=rwx,g=rx,o=rx

Check existing folder umask (numeric)

umask

output :

022

• Operating system subtracts umask value from existing permission & then apply new permissions. (.)

  existingPermission - umask = newPermission

  770 - 022 = 750

• Ingeneral / GIT repo case, even new files in the folder must be available to group

  umask 002 //shell level (say existing 770 - 002 = 770)

To change permanently (in /etc/profile) add following :

umask 002

For changes to take effect run the following source command or logout and log in

source /etc/profile

• Also change setting in '/etc/bashrc'

  Note : check later to set umask for specific folder permanently

• Some link

ubuntu

• In ubuntu change umask in '/etc/login.defs' file
• also set following line in '/etc/pam.d/common-session'

session optional pam_umask.so umask=0002

symbolic link

• A symbolic link, also known as a symlink or soft link, is a special type of file that points to another file or directory (like a shortcut in Windows)

sudo ln -s fileOrFolderPath symbolicLinkPath

//eg: Say you are at /media/prasan/someDrive

//creates a sym link

ln -s /home/prasan/myData myDataBox

//test

ls -l

lrwxrwxrwx 1 prasan prasan 20 Jun 21 23:14 myDataBox -> /home/prasan/myData

//so 'cd myDataBox' will go to actual /home/prasan/myData folder

Note: -s (--symbolic) option

deleting symlink

unlink symbolicLinkPath

Note: while deleting links, be careful - dont add / at the end of path.

tar

Ref

• Tape archive to create or extract archives

Options:

--create (-c) - Create a new tar archive.

--extract (-x) - Extract the entire archive or one or more files from an archive.

--list (-t) - Display a list of the files included in the archive

--file (-f) - archive file name

--verbose (-v) - Show the files being processed by the tar command

Creating Tar Archive

Tar also supports vast range of compression programs such as gzip, bzip2, lzip, lzma, lzop, xz and compress.

//short

tar -cf archive.tar file1 file2 file3

//long syntax

tar --create --file=archive.tar file1 file2 file3

Creating tar gzip (gz) archive

tar -czf archive.tar.gz file1 file2

Note : z option for gzip, also file extension can be 'tar.gz' or 'tgz'

//if ater tar, gzip needed

gzip archive.tar

Creating Tar bzip2 (bz2) archive

tar -cjf archive.tar.bz2 file1 file2

Note : file extension can be 'tar.bz2' or 'tbz'

Listing files in tar archives

//example

tar -tf archive.tar

//output

file1

file2

file3

Note : use the --verbose (-v) option to get more info like file owner, file size, timestamp

Extracting Tar Archive

//regular file

tar -xf archive.tar

//compressed file

tar -xf archive.tar.gz

// add the -v option to print the names of the files being extracted.

tar -xvf archive.tar

Extracting Tar Archive in a Different Directory

tar -xf archive.tar -C /path/to/new-folder

Extracting Specific Files from a Tar Archive

tar -xf archive.tar file1 file2

Note : file names must be exact names including the path, as printed by --list (-t)

Adding Files to Existing Tar Archive

tar -rvf archive.tar newfile

Note : use the --append (-r) option

Removing Files from a Tar Archive

tar --delete -f archive.tar file1

Other

• Use crypt or gpg on the file for password protection

zip

zip ref

unzip ref

Install

//Debian / Ubuntu

sudo apt install zip unzip

//CentOS

sudo yum install zip unzip

ZIP Files

zip archivename.zip filename1 filename2 filename3

Note : To suppress the detailed output of the zip command, use the -q option

ZIP directory

zip -r archivename.zip directory_name

zip -r archivename.zip directory_name1 directory_name2 file1 file1

Compression Methods and Levels

The default compression method of Zip is deflate.

• To specify a compression method, use the -Z option

zip -r -Z bzip2 archivename.zip directory_name

• Compression levels from 0 (no compression) to 9 (optimal compression), 6 is default

zip -9 -r archivename.zip directory_name

Creating a Password Protected ZIP file

zip -e archivename.zip directory_name

You will be prompted to enter and verify the archive password

Unzip a ZIP file

unzip latest.zip

Use the -q switch to suppress the verbose output.

Unzip a ZIP File to a Different Directory

unzip latest.zip -d /path/to/directory

Unzip a Password Protected ZIP file

unzip -P myPasswOrd filename.zip

//or type password at prompt

unzip filename.zip

List the Contents of a Zip File

unzip -l latest.zip

Other

• Creating Split Zip File , check zip ref link

check linux/ distro version

//check ubuntu version (lsb = linux standard base)

lsb_release -a

//check linux kernel version

uname -r

network

ip address

ip addr

ip a

ip address

hostname -I

ip --brief add

netstat

//check connected interfaces

netstat –i

//check open ports

netstat –l

//check port listened by application / user details

sudo lsof -i -P -n | grep LISTEN

resolve.conf

• ubuntu network-configuration

• nameserver in '/etc/resolv.conf' file. Normally this file is handled from 'systemd-resolved' instead direct editing

//example

nameserver 192.*.*.*

nameserver 8.8.8.8 //google DNS server

nameserver 127.*.*.*

• Configure ethernet in /etc/network/interfaces or /etc/netplan

hosts

find hostname or machine name

vi /etc/hosts

output

127.0.0.1 localhost

127.0.1.1 ubuntu01

Note: we can also specify other hosts to communicate

ssh

• changing default port, check ubuntu -> install-server file

bash

//executable permission for script files

chmod +x myScript.sh

• bash file starts with following

#!/bin/bash

• if environment variables to be used

#!/bin/bash -l

• Reading value from user

echo 'Please enter your name'

read NAME_VAR

echo 'Your name is $NAME_VAR'

Note : normal / env variables are accessed by $

• Read with print (.)

read -p "Press [Enter] to continue..."

locale

• Actual source locales location /usr/share/i18n/locales

locale

o/p:

LANG=C.UTF-8 //C, en_US, en_IN are files in above locales path, here using that files some format.

LANGUAGE=

LC_CTYPE="C.UTF-8"

LC_NUMERIC="C.UTF-8"

LC_TIME="C.UTF-8"

LC_COLLATE="C.UTF-8"

LC_MONETARY="C.UTF-8"

LC_MESSAGES="C.UTF-8"

LC_PAPER="C.UTF-8"

LC_NAME="C.UTF-8"

LC_ADDRESS="C.UTF-8"

LC_TELEPHONE="C.UTF-8"

LC_MEASUREMENT="C.UTF-8"

LC_IDENTIFICATION="C.UTF-8"

LC_ALL=

• people use following localectl command for managing locales

//current status

localectl status

//list all generated locales available not source (say India locales doesn't exist)

localectl list-locales

//generate locale

sudo locale-gen en_IN

sudo locale-gen en_IN.UTF-8

//reconfigure default locale by selecting generated locale in UI

sudo dpkg-reconfigure locales

// recheck list

localectl list-locales

//reboot machine for defaults to reflect

//manually set (reboot)

sudo localectl set-locale LANG=en_IN.UTF-8

sudo localectl set-locale LC_TIME=en_IN.UTF-8

//OR check in /etc/default/locale

sudo update-locale LANG=en_IN.UTF-8

sudo update-locale LC_NUMERIC=en_IN.UTF-8

sudo update-locale LC_MONETARY=en_IN.UTF-8

sudo update-locale LC_TIME=en_IN.UTF-8

//also export time zone in /etc/profile for oracle app

export TZ=$(sudo cat /etc/timezone)

• Note : LC_ALL overrides all other variables, so better don't set it

//default locale file location

/etc/default/locale

// to prints file pointing region or check by 'timedatectl'

ls -l /etc/localtime

//typical logic it points to some file /usr/share/zoneinfo/Asia/Kolkata

//reconfigure timezone by UI

dpkg-reconfigure tzdata

change time zone

ubuntu help

type (check executable)

type ssh

o/p:

ssh is /usr/bin/ssh

type git

o/p:

git is /usr/bin/git

timedatectl

query and change the system clock and its settings

Device details df/lsblk

• To check file system disk sizes (h for human readable format)

df -h

o/p:

Filesystem Size Used Avail Use% Mounted on

/dev/sda1 201G 147G 55G 74% /

/dev/sdb1 201G 147G 55G 74% /home

/dev/sdb2 201G 147G 55G 74% /var

Note :

• dev = devices
• sd = SCSI devices (SATA disks)
• sda = 1st disk
• sdb = 2nd disk
• sdb1 - number indicates partition, 1st partition in sdb disk
• sdb2 - 2nd partition in sdb disk

More Ref

//check more

df --help

• to check particular disk types

df –ht ext4

• If device is not in list check by listing all blocks (in hierarchy)

lsblk

o/p:

NAME SIZE TYPE MOUNT_POINT

sda 500G

- sda5 100G

- sda2 400G

- ubuntu--vg-root 380G

- ubuntu--vg-swap 20G

• Say sda5 is not showing in 'df -h', we have to mount that device

//create some directory in /media

sudo mkdir /media/new-external-disk

//mount the device to folder

sudo mount /dev/sda5 /media/new-external-disk

• for NTFS respective package must be installed
• to check disk is being used (when mount error happens)

ps aux | grep sdc

• list blocks with labels

sudo lsblk -o name,mountpoint,label,size,uuid

lshw

list hardware & resources

lshw

hosts

check '/etc/hosts' file

alias

to check alias commands

alias newAliasName="someCmd -arg1"

• add the same in .bashrc file, if alias needed across all sessions.
• To remove use unalias

unalias newAliasName

sort

link

• useful in piping too

wc

• word count

wc filename

• total number of lines, words and characters

Auto Mount partitions (udisksctl)

• ref

• In explorer, other locations -> can see partition path like /dev/sda4..etc

• Find uuid for that partition

ls -al /dev/disk/by-uuid/

//sample output

lrwxrwxrwx 1 root root 10 Sep 2 20:27 7aadd2b8-aec8-4285-a192-bc3cef2fxxxx -> ../../sda6

lrwxrwxrwx 1 root root 10 Sep 2 20:27 9b5d26db-6f33-42d1-a220-15321f8bxxxx -> ../../sda4

• Mount partition by ID (syntax)

udisksctl mount --block-device /dev/disk/by-uuid/<uuid>

• example

udisksctl mount --block-device /dev/disk/by-uuid/7aadd2b8-aec8-4285-a192-bc3cef2fxxxx

udisksctl mount --block-device /dev/disk/by-uuid/9b5d26db-6f33-42d1-a220-15321f8bxxxx

Startup

• add these commands in home dir-> shell file
• add the shell file in start up applications, to auto mount on boot

telnet

telnet domainOrIP port

//example

telnet localhost 8080

• To exit the Telnet session, type Ctrl + ]
• This changes the command prompt to show as telnet>, type in the word 'close' to close the session.

SELinux permission

• By File

  configure the /etc/sysconfig/selinux file.

  Change SELINUX=enforcing to SELINUX=disabled

  Note : You must reboot the system for the changes to take effect

• By Command immediately to reflect until next reboot

  //To check setting:

  getenforce

  //To change setting until next reboot:

  setenforce Permissive

  fdisk

• To View All Disk Partitions

  fdisk -l
  output:

  Disk /dev/mapper/centos_gitrepo-home:

  Disk /dev/mapper/centos_gitrepo-repo: 250.0 GB,

  Disk /dev/sdc: 1000.2 GB,

  ..etc

• To View Partition on a Specific Disk

  fdisk -l diskPath

  eg: fdisk -l /dev/sdc

Create partition

Firewall & ports

Open port to public

firewall-cmd --zone=public --add-port=28018/tcp --permanent

firewall-cmd --reload

$'\r': command not found

You have Windows-style line endings, run following command in windows

dos2unix scriptname

---

Reset admin password

ref

• Login to recovery mode -> terminal

## At this stage we have read-only filesystem, remounting it with write permissions:

mount -o remount,rw /

passwd jorge

Enter new UNIX password:

Retype new UNIX password:

passwd: password updated successfully

Errors

• /usr/bin/sudo must be owned by uid 0 and have the setuid bit set version (permission issue on usr bin & lib)

mount -o remount,rw /

mount --all

chown root:root /usr/bin/sudo

chmod 4755 /usr/bin/sudo

chown root:root /bin/sudo

chmod 4755 /bin/sudo

chown root:root /bin/su

chmod 4755 /bin/su

shutdown -r now

Note : can also change on entire /usr/bin, /usr/lib

to change shell to bash terminal after format

sudo usermod --shell /bin/bash specificUser