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Linux/Unix Commands

Linux Commands used day to day

  • acpi- to display battery status and other acpi information

  • apt-get - helps in handling packages in Linux

  • arch - used to print computer architecture

  • bc - used for command line calculator

  • cal -used to see the calendar of a specific month or a whole year

  • cc - it is used to compile C language codes and create executables

  • ccrypt - command line tool for encryption and decryption of data

  • man - used to give description of any linux command like this $ man ls

  • info - _an alternative toobtain uder documentation for a given program is to invoke info instead of man $ info ls

  • mv - mv command is used to move or rename files

  • pwd - shows the present working directory

  • chmod - _command sets the file permissions flags on a file or folder

  • lsusb - _command used to check usb port id

  • lspci - _command used to check all detailed information about all PCI buses and devices in the system

Listing Files

  • ls
  • If you want to see the list of files on your UNIX or Linux system, use the ls command
  • ls -l or ll
  • If you want to organize the files in list
  • ls -lh
  • If you want to arrange the files in list and show their size

Listing Hidden Files and Normal Files

  • ls -al

  • If you want ot see the list of all files with all hidden files in the Current directory system in Linux

  • ls -lha

  • If you want to organize files in list format with their sizes and see hidden files

  • ls -l -Displays long list view with detailed file information including file type, permissions, link count, owner, group, size, date and time

  • ls -lt -Lists all files sorted by date and time with the newest file first.

  • ls -ltr -Lists all files sorted by date and time with the oldest file first.

  • ls -lh -Displays all files in current directory with file sizes in human readable format(eg:1.6K,328M,2.4G)

  • ls -a

  • If you want to list all files including hidden files that start with "."

  • ls -la

  • Lists all files,including hidden files,in the current directory with detailed information.

  • ls -F

  • Displays all files in current directory with their file types.

Creating & Viewing Files

  • To create a new file, use the command
    1. cat > filename
    2. Add content
    3. Press ctrl + d to return to command prompt.
  • Files can also be quickly created using
  • echo "Hello World" > ./fileName.txt
  • To view a file, use the command cat filename

Searching for content in a file/ files in a directory

  • To search for a word in a file using terminal,use the command grep
  • For eg., if you want to search for the word GNU in a file named GNU.txt,type grep GNU GNU.txt
  • This will show all lines which have the word GNU in that file
  • To search over multiple files in a directory, use grep with -r flag, like grep -r GNU .. Here -r stands for recursive search and . specifies the search root to be the current directory.

Searching for a file

  • find . -name ".js" -print: find all files with JS extnesion and print them on screen
  • find . -type d -name "*tmp" -print: print all directories ending with tmp
  • find . -type f -exec ls -l {} \;: use ls -l on all returned values

Exiting the shell

  • exit (also aliased to bye or quit in some shell flavors)

Get root access

  1. To log in as a super user use one of the following commands in the terminal. You can actually use this command to log in as any user on the machine, but when left blank it will attempt to log in as root.\
  • su -
  • sudo -i
  • These commands will ask you for password.
  1. To jump right into root, run\
  • sudo su -
  • Since there is no root password made after live install.
  1. To run other commands as root temporarily, use the command\
  • sudo command
  • Replace the command above with your command. Eg: sudo apt-get update

Updating, Installing and Listing packages

The commands for installing and updating applications depend on the version of Linux you are using, specifically whether it's Debian- or RPM-based.

  • Debian Based systems

    • Update the list of available packages and their versions, but doen not install or upgrade any packages. use the command sudo apt update
    • Install newer versions of installed packages,use the command sudo apt upgrade
    • Install any required package,use sudo apt install 'your package name'. Example: sudo apt install apache2

  • RPM-based systems

    • Update all or specified packages,use sudo yum update. Example for updating specific package: sudo yum update mysql
    • Lists known and installed packages,use sudo yum list sudo yum list --installed
    • Install requested package,use yum install. Example: sudo yum -y install firefox
    • On Fedora 22 and later, you can also use 'dnf' in place of 'yum'. dnf stands for dandified yum, and is just an updated version of yum. Syntax is very similar, Examples would be: sudo dnf update or sudo dnf install. More info on this here.
    • On openSUSE, use zypper install PACKAGE_NAME and zypper remove PACKAGE_NAME to install or remove packages. To add an repo in openSUSE, sudo zypper addrepo <URI> <ALIAS>. More info on this here

Download YouTube videos on Linux

For this you need to install an utility called youtube-dl, which is a command-line program to download videos from YouTube.com and a few more sites. It requires the Python interpreter, version 2.6, 2.7, or 3.2+, and it is not platform specific. It should work on your Unix box, on Windows or on macOS. It is open source project. You can contribute here.

To install it right away for all UNIX users (Linux, macOS, etc.), type:

sudo curl -L https://yt-dl.org/downloads/latest/youtube-dl -o /usr/local/bin/youtube-dl
sudo chmod a+rx /usr/local/bin/youtube-dl

If you do not have curl, you can alternatively use a recent wget:

sudo wget https://yt-dl.org/downloads/latest/youtube-dl -O /usr/local/bin/youtube-dl
sudo chmod a+rx /usr/local/bin/youtube-dl

It is very simple to use. Following are some example usages:

# simplest example
youtube-dl https://www.youtube.com/watch?v=Zuwa8zlfXSY

# customize the filename of the output
youtube-dl -o '%(title)s by %(uploader)s.%(ext)s' https://www.youtube.com/watch?v=Zuwa8zlfXSY

# download from multiple urls
youtube-dl <url1> <url2>

# or use a file with list of urls
youtube-dl -a url.txt

If you are more interested, there are plenty of other options to be explored from documentation page.

Monitering the temperature of your system

"lm-sensors" is a set of tools to control and view sensors that may be present on your hardware. Included are several applications that run from the command line:

  • sensors: shows the current readings of all sensor chips.
  • sensord: daemon to periodically log sensor readings to syslog
  • fancontrol: calculates fan speeds from temperatures and sets the corresponding PWM outputs
  • pwmconfig: tests the PWM outputs of sensors and configures fancontrol
  • and more (see lm-sensors documentation for details)`

You can use it in the following way,

~$ sensors

k8temp-pci-00c3
Adapter: PCI adapter
Core0 Temp:  +59.0°C                                    
Core1 Temp:  +58.0°C                                    
Core2 Temp:  +60.0°C                                    
Core3 Temp:  +55.0°C                                    

it8718-isa-0228
Adapter: ISA adapter
[...] # shortened

###To Free up memory cache and buffer sync; echo 1 > /proc/sys/vm/drop_caches sync; echo 2 > /proc/sys/vm/drop_caches sync; echo 3 > /proc/sys/vm/drop_caches

Shutting down the machine

  • shutdown -h now -If you want to Shutdown the system and turn the power off immediately.
  • shutdown -h +10 -If you want to Shutdown the system after 10 minutes.
  • shutdown -r now -If you want to Reboot the system using shutdown command.

Moving between Directories

  • Exploring cd(change directory) command

  • cd [directory_name]

  • If you want to change directory to a particular directory,use this command. Eg: cd Documents

  • cd dir/subdir/subsubdir

  • If you want to move inside a directory from a directory,use this command. Eg: cd Documents/Hacktoberfest/linuxCommands

  • cd /

  • If you want to change directory to the root directory. Root directory is the first directory in your file system hierarchy.

  • cd ~ or cd

  • If you want to change directory to the home directory.Generally known as $HOME,located at path /home//. Also, cd command can be used to do the same above operation.

  • cd ..

  • It is used to go one level back directory(i.e to parent directory of current directory).

  • cd ../..

  • It is used to go two levels back directory(i.e to parent's parent directory of current directory).

  • cd -

  • It is used to change back to the previous working directory. Eg: If Documents is present working directory and Downloads is previous working directory, this command takes you back to Downloads directory.

  • cd "dir name" or cd dir\ name

  • This command is used to navigate to a directory with white spaces.Instead of using double quotes we can use single quotes then also this command will work.Eg: cd "linux commands" or cd linux\ commands will move to 'linux commands' directory.

Configure to mysql

  • 'mysql -u root -p'
  • enter the mysql password

Import and Export database using cli

Import

mysql -u root -p db-name < /path/to/databse/file.sql

Export

mysql -u root -p db-name > /path/to/new/database.sql

It will ask database password.

Creating and Removing Directories

  • mkdir command is used for creating directories.

    • Creating a directory is pretty straightforward. mkdir [dir-name]

    • Creating a complete directory structure. (Non of the directories should already exist) mkdir -p dir1/dir2/dir3

    • Make mkdir emmit the details of operation. mkdir -v [dir-name]

  • rmdir command is used for removing empty directories.

    • Removing a directory is also pretty straightforward. rmdir [dir-name]

    • Make rmdir ignore non empty directories. BY default, the rmdir command throws an error if you try deleting a non-empty directory. However, if you want, you can suppress this behavior of rmdir using the --ignore-fail-on-non-empty option.

      `rmdir --ignore-fail-on-non-empty [dir-name]`

    • Remove parent directories along with the directory. rmdir -p test/test-dir/

  • rm -r command is used for removing non empty directories.

    • Removing a non empty directory is also pretty straightforward. rm -r [dir-name] This will present a prompt for approval to delete each of the files.

    • If you don't want such a prompt, rm -rf [dir-name] This will not present a prompt for approval to delete each of the files.

Compress files on linux

The following commands are used to compress files on linux. There are many ways to compress files in linux, the most common is tar

  • Compress
    • The following commands makes an archive named archive.tar containing file foo and bar present in the current directory.
    • tar -cf archive.tar foo bar
    • This command lists all the files in an archive verbosely.
    • tar -tvf archive.tar
    • The tar utility has many options which can be seen using
    • tar --help
  • Decompress
    • We will again use the tar utility to decompress files. The following command is used to decompress the file named yourfile.tar.gz. This command is for the .gz. extention.
    • tar -zxvf yourfile.tar.gz
    • If the file has the extention .tar, this command can be used.
    • tar -xvf yourfile.tar
    • The above commands will extract to the current folder, to extract to another folder use the following commands.
    • tar -C /myfolder -zxvf yourfile.tar.gz
    • tar -C /myfolder -xvf yourfile.tar

Creating users on linux

The following commands are used to create users on linux. One method allows you to create a user at a time while another allows you to create users in bulk

  • Single User
    • The following command creates a new user account without any additional actions
    • useradd joe
    • The following command creates a new user account and creates a home directory for that user based on the contents of /etc/skel
    • useradd -m joe
    • This command will create the user with the home directory you specify
    • useradd -d /path/to/home/dir joe
  • Multiple Users
    • The following command creates users based on a : delimited list of values. Each user is separated by a new line
    • The user is defined as follows username:encrypted-password:uid:gid:home-dir:shell
    • Only the first two fields are required for user creation and the password must be encrypted using the below command
    • mkpasswd --method=SHA-512
    • You can leave the user without a password by placing an x in place of the encrypted password.
    • Once you set up your file create your users by typing
    • newusers the-file

Modifying users on linux

  • usermod -L username - If you want to lockout a user
  • usermod -U username - If you need to unlock a users account locked with the above command
  • passwd username - If you need to change a users password
  • usermod -d /path/to/dir username - If you need to change a users home directory
  • usermod -s /path/to/shell username - If you need to change a users shell

Deleting a user on linux

  • userdel username - Deletes user
  • userdel -r username - Deletes user and their home directory and mail spool file

Basic Commands

htop

  • The htop command is used get information about the system such as CPU usage, RAM usage, Swap usage etc. It also give information regarding the uptime, average load and currently running tasks.
  • To use this utility, simply type htop and press enter.

Syntax:

$ htop

wget

  • The wget command is one of the simplist commands in linux. It allows the user to fetch internet resources without having to open a browser.
  • Basic Use:

Syntax:

$ wget url

Example:

$ wget https://example.com/cat.png
  • This will download and save the file to the current directory with the name cat.png.

wc

  • One of the most basic commands, wc allows the user to count the number of bytes, characters, words and lines of each given file or standard input and print the result.

  • Basic Use:

Syntax:

$ wc filename  # output: number_of_lines number_of_words number_of_characters /path/to/file

Example:

$ wc /proc/cpuinfo  # output: 208 1232 6336 /proc/cpuinfo

touch

  • touch is a simple command that allows the user to create an empty file. Note that with touch, you can only create the file and not edit it.

  • Basic Use:

Syntax:

$ touch filename
$ ls -l filename
-rw-rw-rw- 1 current_user users 0 Oct  9 22:03 filename

Example: Logged in as user linux_is_awesome

$ touch hello
$ ls -l hello
-rw-rw-rw- 1 linux_is_awesome users 0 Oct  9 22:03 hello

whoami

  • whoami command displays the username of the current user.

  • Basic Use:

Syntax:

$ whoami
current_user

Example: Logged in as user linux_is_awesome

$ whoami
linux_is_awesome

grep

  • grep is an extremely useful command to know in Linux.

  • It stands for “global regular expression print". Basically, it's used for pattern matching. grep processes text line by line and prints any lines which match a specified pattern.

  • Basic Use: Syntax:

$ grep [option(s)] pattern [file(s)]
 ``` Creating a test file for using `grep`:
 ```bash
$  cat > grep_test.txt
This is a test file. AAA, BBB, 123, CaSe ExAmPle.
    ```

Example:
```bash
$ grep AAA grep_test.txt
This is a test file. AAA, BBB, 123, CaSe ExAmPle. # AAA will be highlighted in the output

$ grep -i case grep_test.txt  # ignore case while searching
This is a test file. AAA, BBB, 123, CaSe ExAmPle. # CaSe will be highlighted in the output
###  TO CHECK DISK SPACE USED AND AVAILAVBLE
- COMMANDS
$df     (Table that lists for each device name on the system)
$df -h  (To see output in Human Readable Format)
$df -m  (Output in one-megabyte)
$df -k  (Output in one-kilobyte blocks)

### TO CHECK DISK SPACE USED BY SPECIFIED FILES AND FOR EACH SUB-DIRECTORY
-COMMANDS
$du      (Names and space consumption of each of the directories including all subdirectories)
$du -h   (To see output in Human Readable Format)

To find out /etc/ directory space usage :
$du /etc/
$du -h /etc/

echo

  • echo display a line of text.

  • Basic Use: Syntax:

$ echo options string
$ echo "hello world"
hello worl

Example:

$ echo $USER
carlan
$ echo $HOME
/home/carlan
$ forename=carlan
$ echo "${forename}"
carlan
$ echo -e "hello\nworld"
hello
world
$ echo '${forename}'
${forename}
$ echo "This is my home folder ${HOME}" > /tmp/myhomefolder.txt
$ cat /tmp/myhomefolder.txt
This is my home folder /home/carlan

diff

  • diff is an extremely useful command to know in Linux and often used in GIT as well as git diff.

  • Compare FILES line by line

  • Basic Use: Syntax: diff [OPTION]... FILES

$  cat > diff_test_file1.txt
This is a test file 1
$  cat > diff_test_file2.txt
This is a test file 2

Example:

$ diff diff_test_file1.txt diff_test_file2.txt
1c1
< This is a test file 1
---
> This is a test file 2

printf

printf command in Linux is used to display the given string, number or any other format specifier on the terminal window.

It's just like echo, but on steroids.

Syntax:

printf [-v var] format [arguments]

Printf can have format specifiers, escape sequences or ordinary characters.

commonly used printf specifiers:

specifiers Description
%s a string specifier for string output
%d an integer specifier for showing the integral values

Example:

printf "%s" "Hello, World!"
# > Hello, World!
printf "%d\n" "213" "109"
# > 213
# > 109

tee

tee command reads the standard input and writes it to both the standard output and one or more files.

It basically breaks the output of a program so that it can be both displayed and saved in a file, simultaneously

Syntax:

tee [OPTION]... [FILE]...

The most useful option is probably the -a switch, which appends to the specified files rather than overwriting them. tee also creates the files if they don't exist.

Example:

printf "Hello World" >> a.txt
# Creates file a.txt (assuming it wasn't there) with content "Hello World"
touch b.txt
#Creates b.txt
wc -w a.txt|tee -a b.txt
# Output: 2 a.txt
cat b.txt
# Output: 2 a.txt
#Run it again
wc -w a.txt|tee -a b.txt
# Output: 2 a.txt
cat b.txt
# Output: 2 a.txt (on 2 lines)

Multiple file example:

# Follow steps of previous example first
touch c.txt
wc -w a.txt|tee -a b.txt c.txt
# Output: 2 a.txt
cat b.txt
# Output: 2 a.txt (on 3 lines)
cat c.txt
cat b.txt
# Output: 2 a.txt

ssh

ssh command can be used for accessing remote server (remote login program)

Syntax:

ssh username@serverip

This command connects you to a server which has an IP address as serverip and username as username.

🔥 For complete info hit man ssh in terminal

screen

screen -- a full-screen software program that can be used to multiplexes a physical console between several processes (typically interactive shells). It offers a user to open several separate terminal instances inside a one single terminal window manager.

  • Prerequisites
apt-get install screen (On Debian based Systems)

or

 yum install screen (On RedHat based Systems)

ex: Create a screen

$ screen -S [NAME_of_screen]

  • e.g screen -S frodo

ex: Detach a screen (when already inside screen interactive shell)

press Ctrl-A and 'D'

you will see the output like this:

[detached from 711.frodo]

ex: List of current screen sessions

$ screen -ls

@ubuntu ~ $ screen -ls
There are screens on:
        711.frodo (10/01/2016 01:15:45 AM)        (Detached)
        2231.pts-0.ubuntu (06/06/2015 08:12:05 AM)       (Detached)
2 Sockets in /var/run/screen/S-nightz

ex: restore screen (e.g 711.frodo) to continue your works

$ screen -r 711

ex: Kill screen (when already inside screen interactive shell)

press Ctrl-A and 'k'

then the terminated prompt will be appeared as:

Want to kill this window [Y/N] >

strace

strace -- powerful command line tool for debugging and trouble shooting programs in Unix-like operating systems such as Linux. It captures and records all system calls made by a process and the signals received by the process

  • strace will fill your screen with continues output that shows system calls being made by the process, to end it, press [Ctrl + C]

ex: trace Linux Command System Calls

  • e.g: tracing of all system calls made by the df command
$ strace df -h


execve("/bin/df", ["df", "-h"], [/* 50 vars */]) = 0
brk(NULL)                               = 0x136e000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f82f78fd000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=147662, ...}) = 0
mmap(NULL, 147662, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f82f78d8000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)

Where:

  • access -- one of types of system call
  • ("/etc/ld.so.preload", R_OK) -- system call argument
  • -1 ENOENT -- system call return value

ex: trace Linux Process PID

$ sudo strace -p [PID]

example  

$ sudo strace -p 3569

strace: Process 3569 attached
restart_syscall(<... resuming interrupted poll ...>) = 1
recvmsg(4, {msg_name(0)=NULL, msg_iov(1)=[{"U0!70

Network

ifconfig

  • Used to configure network interface parameters and displays information about all network interfaces currently in operation.

  • ifconfig -a

  • Print out the configuration of all network interfaces on the system.

  • ifconfig interface

  • Print out the configuration of a specific interface.

ex: ifconfig interface


$ ifconfig eth0
  • sudo ifconfig interface up
  • Enable an interface if it is disabled.

ex: Enabling an interface

example  

$ sudo ifconfig eth0 up
  • sudo ifconfig interface down
  • Disable an interface if it is enabled.

ex: Disabling an interface

example  

$ sudo ifconfig eth0 down

ping

  • It is used to test the connectivity status between a source and destination computer/device over an IP network.

ex: use ping

$ ping ip

example  

$ ping 192.168.1.254

whois

  • whois is a simple tool that allows you to see information about a specific domain name or IP address. You can use it to see things like contact information, nameservers, and domain registration information.

ex: install whois

$ sudo apt install whois

ex: use whois

$ whois domain

example  

$ whois www.github.com

netstat

  • Netstat command allows you a simple way to review each of your network connections and open sockets.

  • netstat -a

  • List out all the current connections

  • netstat -at

  • List only TCP connections

  • netstat -au

  • List only UDP connections

  • netstat -an

  • Disable reverse dns lookup for faster output if you do not need to know the host name and just the ip address.

  • netstat -s

  • Print out network statistics like total number of packets received and transmitted by protocol type and so on.

  • netstat -i

  • Print out the information about the network interfaces.

  • netstat -g

  • Display the multicast group information for IPv4 and IPv6 protocols.

w

  • w provides a quick summary of every user logged into a computer, what each user is currently doing, and what load all the activity is imposing on the computer itself.

ex: using w

example  

$ w

ssh

  • SSH stands for secure shell. To login in to a system with user name user and ip-10.85.85.85,type: ssh [email protected]
  • This shall allow you to access the terminal of the remote PC with the user account of user

ssh-keygen(Generation and Usage)

  • Inorder to avoid entering the password to remote servers/applications.
  • While git push, we can avoid this scenario by adding a key to our GitHub.
  • Steps: Scenario 1 using the existing key
  1. Check for existing keys. ls ~/.ssh eg: default names:id_rsa.pub id_rsa `` Here id_rsa is the private key. Files ending with the .pub are the public key which is to be added to theauthorized_keys`` on remote servers. For GitHub, go to Settings click on SSH and GPG keys
  2. Choose SSH-keys, give it a name and the .pub file. cat ~/.ssh/id_rsa.pub copy and paste this to the key section.

Scenario 2 creating a new key

  1. Using ssh-keygen we will create keys. ssh-keygen -t rsa -b 4096 t-flag: for choosing encryption methods such as RSA,DSA,ECDSA. b-flag: for choosing no. of bits. with help of man command you can know more about ssh-keygen.
  2. It will prompt for the name and location for the keys: default: /home/[user]/.ssh/id_rsa You can choose this, if there is an existing key it will overwrite it.
  3. Pick a relevant name: /home/[user]/.ssh/id_rsa_[github|Server A]
  4. It will ask for passphrase, you can avoid this or enter passphrase which will be asked whenever your using this key. Enter the same passphrase again to confirm it.
  5. Thus you have created the keys(public and private). Sometimes you have to add the generated private key to cache by using ssh-add ~/.ssh/[filename]
  6. Copy the public key using cat command. eg:cat ~/.ssh/[file_name.pub]
  7. Add this key to GitHub, by going to Settings and then click on SSH and GPG keys, give a name and paste it.