Unix/Linux Terminal

Table of Contents

• Opening the Terminal
• Keyboard Shortcuts
• File System
• Basics of Linux/Unix Commands
• Basic Commands
• Package Manager
• Run files on cmd
• Listing directory contents & directory structure
• Changing Permissions
• Creating Directories
• Creating Files
• Redirecting Output
• Editing a File
  • Vi
  • Vim
  • Nano
• Terminal Multiplexers
  • tmux
• Reading File Content
• Copying and Moving Files
• Renaming Files
• Deleting Files and Directories
• Chaining Commands
• Gathering System Information
• Finding Strings and Counting Occurrences
  • grep
• AWK
• Disks, USBs and Files
• Prompt customization
• Periodically Running Tasks
  • CRON Jobs
• Making Requests from the Command Line
  • Curl

Opening the Terminal

Linux

Search for “terminal” in the apps menu in most cases. You might have to look up specific instructions for your distro. Default Keyboard shortcut: Ctrl + Alt + T

MacOS

Search for “terminal” in Spotlight. Keyboard shortcut: Control + Option + Shift + T

Windows

The in-built terminals on Windows (cmd and PowerShell) are not POSIX compliant, so you will need to do one of the following:

• Install a POSIX-compatible runtime like MinGW or Cygwin.
  Note that git-for-windows includes a MinGW installation called git-bash by default, and is usually located in C:\Program Files\Git\git-bash.exe
• Enable the Windows Subsystem for Linux (WSL). WSL is a system developed by Microsoft that allows a full Linux OS to run alongside Windows.
  Learn More about WSL | Install WSL
• Use Docker containers. Docker allows programs to run in isolated environments called containers. Each container is like a fresh install of the respective OS. There are pre-built containers for many popular Linux distros like Ubuntu, Debian, CentOS, etc.
  Get started with Docker

Keyboard shortcuts

Shortcut	Function
Tab	Complete file & directory names and to further iterate over files
Shift + Tab	Go back to the previous names
↑ or Ctrl + P	Go to all the commands previously executed in that instance of the cmd, one by one
↓ or Ctrl + N	Go towards the recent commands
Ctrl + R	Search your command history
Ctrl + ←/→	Jump words
Home or Ctrl	Jump to the beginning and end of the line
End or Ctrl + E	Jump to the end of the line
Ctrl + C	Stop running processes in your terminal and bring back the prompt

File System

Let’s get a brief on the Linux file system. If you’ve used Windows, you would be familiar with C:, D: etc.
In Linux, directory structure starts with / symbol, which is referred as the root directory The ~(tilde) is shorthand for your home directory. Within your home directory, you will find common directories, such as Documents, Music, Video, etc.,

A Gist on the file system folders and their general functions.

Absolute and Relative Paths

An absolute or full path points to the same location in a file system regardless of the current working directory. To do that, it must start from the root directory.

An absolute path always starts with a forward slash (/), which represents the start of the filesystem.

Eg: /home/username is an absolute path.

By contrast, a relative path starts from some given working directory, avoiding the need to provide the full absolute path. A filename can be considered as a relative path based at the current working directory. If the working directory is not the file’s parent directory, a file not found error will result if the file is addressed by its name.

A relative path never starts with a forward slash (/).

A relative path can start with a

• .
  • Represents the CWD (Current Working Directory).
• ..
  • Represents the parent directory of the CWD.
• ~
  • Represents the home directory of the logged in user.

Eg: ../docs is a relative path

Basics of Linux/Unix Commands

Command Structure

The components of the command line are:

• the command
• any options required by the command
• the command’s arguments (if required)

The general form of a UNIX command is: command [-option(s)] [argument(s)]

#only the command
clear

#command with option
ls -l

#command with argument
man pwd

#command with option and argument
rm -r project

Command Help

• man <command> is used to display the user manual of any command that we can run on the terminal. It provides a detailed view of the command. eg: man ls.
• help command provides documentation for builtin commands.

$ help pwd
pwd: pwd [-LP]
  Print the name of the current working directory.

  Options:
    -L  print the value of $PWD if it names the current working directory
    -P  print the physical directory, without any symbolic links

  By default, `pwd' behaves as if `-L' were specified.

  Exit Status:
  Returns 0 unless an invalid option is given or the current directory
  cannot be read.

Variables

Syntax: ${var_name} or $var_name

Pre-defined variables: $HOME, etc…

Custom variables:

$ foo=bar
$ echo ${foo}
bar

NOTE: Strings in bash can be defined with ' and " delimiters, but they are not equivalent. Strings delimited with ' are literal strings and will not substitute variable values whereas " delimited strings will.

$ name=selena

$ echo 'Hey ${name}!'
Hey ${name}!

$ echo "Hey ${name}!"
Hey Selena!

Basic Commands

Print name of cwd (current working directory)

Use pwd to print name of current/working directory

$ pwd
/home/username

Clear screen & command history

• Use clear or Ctrl + L to clear the screen of the cmd.
• Use history to view the last 500 commands executed on the terminal.

Changing directories

$ pwd
/home/username/

$ # providing an absolute path as argument
$ cd /etc
$ pwd
/etc

$ # to go back to previous working directory
$ # if there's a directory named '-', use './-' to go that directory
$ cd -
/home/username
$ pwd
/home/username

• Relative paths are well, relative to current working directory
• . refers to current directory
• .. refers to directory one hierarchy above

• ../.. refers to directory two hierarchies above and so on

  $ pwd
  /home/username
  
  $ # go to directory one hierarchy above
  $ cd ..
  $ pwd
  /home
  
  $ # go to directory 'username' present in current directory
  $ # './' is optional in this case
  $ cd ./username
  $ pwd
  /home/username
  
  $ # go to directory two hierarchies above
  $ cd ../..
  $ pwd
  /
  

NOTE:

• About the Use of Dot-Slash (./) in Commands
• Absolute vs Relative Path in Linux: What’s the Difference?

• cd ~/ or cd ~ or cd will go to directory specified by HOME shell variable (which is usually set to user’s home directory)

  $ pwd
  /
  $ echo "$HOME"
  /home/username
  
  $ cd
  $ pwd
  /home/username
  

• Note: Specifying / at end of path argument is optional

Package manager

• A package delivers and maintains new software for Linux-based computers. Just as Windows-based computers rely on executable installers, the Linux ecosystem depends on packages that are administered through software repositories.
• Different Linux distros use different package managers. Ubuntu uses APT (Advanced Package Tool) to manage its packages.

Check for package updates

$ # To check if any packages on the system have updates
$ sudo apt-get update

$ # To update all packages
$ sudo apt-get upgrade

NOTE: Always run the update command before the upgrade command.

Install a package

$ sudo apt-get install <package_name>
$ # This might prompt you with a Y (yes) or n (no) option.

Remove a package

$ sudo apt-get remove <package_name>
$ # This might prompt you with a Y (yes) or n (no) option.

Run files on cmd

• You can only run files if you have the permission to execute them. Refer Changing permissions section for more details.

$ ./file_to_run

Listing directory contents & directory structure

• ls to list all files and directories in the cwd (current working directory).

  $ ls
  projects  report.log
  

• ls path to view files in another directory without changing the cwd. The path can be absolute/relative.

  $ ls /var/
  backups  lib  lock  mail
  

• Use the -a option to list all the files, including the hidden files and folders, in the cwd (current working directory).

  $ ls
  CODE_OF_CONDUCT.md  CONTRIBUTING.md  LICENSE  README.md  build  build.sh  src  temp.md
  
  $ ls -a
  .  ..  .git  .github  .gitignore  CODE_OF_CONDUCT.md  CONTRIBUTING.md  LICENSE  README.md  build  build.sh  src  temp.md
  

• Use the -A option to print the same things as the -a option, but without printing . and ...

  $ ls -a
  .  ..  .git  .github  .gitignore  CODE_OF_CONDUCT.md  CONTRIBUTING.md  LICENSE  README.md  build  build.sh  src  temp.md
  
  $ ls -A
  .git  .github  .gitignore  CODE_OF_CONDUCT.md  CONTRIBUTING.md  LICENSE  README.md  build  build.sh  src  temp.md
  

• Use the -l option for the long listing format. This format shows details like file permissions, ownership, size, timestamp, etc. (See changing permissions section for details on permissions, groups, etc)

  $ ls -l
  total 52
  -rwxrwxrwx 1 harshkapadia harshkapadia  3431 Apr 21 23:09 CODE_OF_CONDUCT.md
  -rwxrwxrwx 1 harshkapadia harshkapadia 20566 Apr 21 23:09 LICENSE
  -rwxrwxrwx 1 harshkapadia harshkapadia  1253 Apr 21 23:09 README.md
  drwxrwxrwx 1 harshkapadia harshkapadia  4096 Apr 22 00:48 build
  ...
  

• tree to view directory structure.

• It is not part of a standard Linux distribution, so you will need to install it.

  $ # Install tree
  $ sudo apt install tree
  
  $ sudo tree
  .
  ├── projects
  │  ├── a.txt
  │  ├── b.txt
  │  └── backup
  │    ├── a.txt
  │    └── b.txt
  └── report.log
  2 directories, 5 files
  

  • Use which or command -v to find the location of a file.
    • ‘which’ vs ‘command -v’ in Bash

  $ which bash
  /usr/bin/bash
  

Changing Permissions

• In the output of ls -l command, the first 10 characters displayed are related to type of file and its permissions.

  $ ls -l
  total 84
  lrwxrwxrwx 1 username eg  12 Mar 21 12:08 projects -> ../projects/
  -rw-rw-r-- 1 username eg 39120 Feb 14  2019 report.log
  

First character indicates the file type The most common are

• - regular file
• d directory
• l symbolic link
• for complete list, see -l option in info ls

The other 9 characters represent three sets of file permissions for ‘user’, ‘group’ and ‘others’ - in that order

• user file properties for owner of file G - u
• group file properties for the group the file belongs to - g

• others file properties for everyone else - o

  Permission characters and values

Character	Meaning	Value	File	Directory
r	read	4	file can be read	can see contents of directory
w	write	2	file can be modified	can add/remove files in directory
x	execute	1	file can be run as a program	can access contents of directory
-	no permission	0	permission is disabled	permission is disabled

$ chmod 664 sample.txt

This means it will change the permission of sample.txt to rw-rw-r– i.e, user and group only have read and write permissions and others only have read permission.

Further Reading:

• File Permissions

Creating directories

• mkdir dir_name will create a folder in the cwd, if the dir_name doesn’t already exist in the cwd.
• Use "" if the dir_name has spaces, eg: mkdir "dir name".
• Multiple folders can be created using mkdir {dir1,dir2,dir3}.
  • Do not add any spaces in the curly brackets for the directory names.
• Use the flag -p to create intermediate directories if they do not exist and not cause an error if they already exist.

# mkdir [option] dir_name
$ mkdir new_dir

# Spaces in directory name
$ mkdir "New Folder"

# Creating multiple directories
$ mkdir {dir1,dir2,dir3}

# Creating intermediate directories without causing errors if they already exist
$ mkdir -p public/css/summary public/html

Creating files

touch command

• This is the easiest way to create a file in Linux.

  $ touch sample.txt
  

Redirect operator (>)

• This operator tells the system to output results into whatever you specify next. The target is usually a filename. You can use this symbol by itself to create a new file.

  $ > sample.txt
  

cat command

• The cat command is short for concatenate. It can be used to output the contents of several files, one file, or even part of a file. If the file doesn’t exist, the cat command will create it.

  $ cat > sample.txt
  

• Te command tac (reverse of cat) can be used to print file contents backwards.

echo command

• The echo command will print out anything written after it.

  $ echo Print this text
  Print this text
  

• It will print out environment variables like $HOME, $USER, $SHELL, $HOSTNAME, etc as well.

  $ echo $SHELL
  /bin/bash
  

• It can be used to add text to a file using the output redirection operator. (It overwrites the contents of the entire file.)

  $ cat sample.txt
  Original text
  
  $ echo New text! > sample.txt
  
  $ cat sample.txt
  New text!
  

• It can be used to append text to a file using the output redirection operator.

  $ cat sample.txt
  Original text
  
  $ echo New text! >> sample.txt
  
  $ cat sample.txt
  Original text
  New text!
  

Using text editors

• Vi Text Editor

  $ vi sample.txt
  

• Vim Text Editor

  $ vim sample.txt
  

• Nano Text Editor

  $ nano sample.txt
  

  Jump to Editing a File for more details.

Symbolic Links

• What is Symbolic Links in Linux? How to Create Symbolic Links?
• What is the difference between a symbolic link and a hard link?

$ ln --symbolic /actual/file/location.ext /symlink/location.ext

Redirecting output

Redirect to a file

• To redirect the output of a command to a file, use the > symbol.

Eg: ls -al > listings.txt will save the output of command ls -al is re-directed to listings.txt instead of your screen.

Note that if listings.txt already had some data, then this will overwrite the old data.

• To append to an existing file, use the >> symbol.

Eg: ls -al >> listings.txt will append the output of command ls -al is re-directed to listings.txt.

Redirect to another command

• Sometimes, it can be useful to use the output of one command as the input to another command. This can be achieved using pipes.

• A pipe is created using the pip character (|).

• Eg: To display output of a directory listing one page at a time, use ls -la | less.

Editing a file

Vi

• Vi is the oldest text editor in Linux. It was created alongside the Linux operating system for directly editing text files.

  $ vi sample.txt
  

  This will start the text editor in your terminal.

  • Press the letter i to switch to insert mode, then type a few words to try it out.
  • To save and exit press Esc :x and hit Enter.

Vim

• Vi editor wasn’t very user-friendly. Vim is a newer version, which stands for Vi editor, Modified.

  $ vim sample.txt
  

• Basic guide to Vim
  • Dotfiles (.vimrc)
  • Vim documentation
• Find and Replace in Vim / Vi
  • More on Regular Expression
  • NOTE: Vim will only replace the first occurrence of a string in a sentence by default. To change all occurrences, add the g option at the end. Eg: :0,$s/old-text/new-text/g
• Getting Started with Vim: An Interactive Guide
• Tips on Window Management in Vim
• How to add text at the end of each line in Vim?
• Plugins
  • A brief history of plugin management using Vim
  • Choices between various plugin systems/managers
    • How do I install a plugin in Vim/vi?
    • What are the differences between the Vim plugin managers?
  • Dotfiles (.vimrc) with plugins

Nano

• Nano is a newer and much easier text editor to navigate.

  $ nano sample.txt
  

  By default, Nano puts you directly into editing mode. It also displays a helpful list of commands at the bottom of the screen.

  • Enter some text, then press Ctrl+O to save the changes.
  • Press Ctrl+X to exit the editor.

Terminal Multiplexers

Terminal multiplexer introduction

tmux

• tmux introduction
• Cheat sheet

Reading file content

cat

• concatenate files and print on the standard output

  $ cat sample.text
  

less

• displays contents of a file, automatically fits to size of Terminal, allows scrolling in either direction and other options for effective viewing

  $ less sample.txt
  

head

• displays the first 10 lines of a text file by default

  $ head sample.txt
  

tail

• head command displays the last 10 lines of a text file by default

  $ tail sample.txt
  

Copying and moving files

• The cp command is used to copy files and directories
• To copy a single file or directory, specify the source as first argument and destination as second argument

• Similar to rm command, use -r for directories

  $ # Two file names
  $ # cp src_file dest_file
  $ cp a.txt b.txt
  
  $ # One or more arguments
  $ # cp src_file1 src_file2 src_file3 dest_dir
  $ cp a.txt b.txt new_folder
  
  $ # Two directory names
  $ # cp -r src_dir dest_dir
  $ cp -r old_folder new_folder
  

• The Secure Copy (scp) command can be used to transfer files from one machine to another. (It uses SSH under the hood.)

  $ scp file_to_be_transferred.txt [email protected]:/remote/directory/new_file_name.txt
  

More details

Renaming files

mv command

• mv command is used to rename or move files from one location to another.

  $ # mv [OPTIONS] source destination
  $ mv file1.txt file2.txt
  

rename command

• rename command in Linux is used to rename the named files according to the regular expression perlexpr.

• It is not part of a standard Linux distribution, so you will need to install it.

  $ # Install rename
  sudo apt-get install rename
  
  $ # Syntax
  $ # rename [OPTIONS] perlexpr files
  $ rename 's/.html/.php/' \*.html
  

• The above example will change all files with the extension .html to .php

Further Reading:

• Rename command

Deleting files & directories

• This will permanently delete your files. They will not be found in the ‘Trash’ folder.

• To delete files

  $ rm samplefile.txt
  

• To delete directories, use -r option

  $ rm -r sampledir
  

• To delete empty directories

  $ rmdir sampledir
  

Chaining commands

You can chain multiple commands using &, &&, |, || and ; operators.

& Operator

$ command1 &

• This will run command_1 in the background so that other commands can be executed.

&& Operator

$ command1 && command2

• This will run will run command_1 and then run command_2 only if command_1 finished successfully.

| Operator

$ command1 | command2

• The output of the command1 acts as input to command2.

|| Operator

$ command1 || command2

• This will run will run command_1 and then run command_2 only if command_1 did not finish successfully.

; Operator

$ command1 ; command2

• This will run will run command_1 and then run command_2 even if command_1 did not finish successfully.

Gathering System Information

• CPU
  • No. of cores, no. of sockets, SMT on/off, CPU speed/frequency: lscpu
  • CPU topology and/or NUMA config
    • lscpu
    • lstopo (enable X11 Forwarding for GUI!)
      • Interpret the output of lstopo
    • numactl
• OS and Kernel versions
  • OS version
    • lsb_release -a (Will not work for all Linux distributions.)
    • cat /etc/os-release
  • Kernel version: uname -srm
  • 4 Useful Commands to Check Linux Version
• BIOS
  • sudo dmidecode -t bios (More details)
• Memory (RAM) config
  • Speed: sudo dmidecode -t memory
  • free -h
  • How to Determine the Number of RAM Slots in Use
  • DIMMS per channel (DPC): sudo dmidecode -t memory | grep "Bank Locator"
  • How do I tell if my Memory is ECC or Non-ECC?
• Storage (disk)
  • df -h
  • lsblk
  • fdisk -l
• Network
  • ip a
• Motherboard
  • sudo dmidecode -t baseboard
  • ipmitool
• Generic commands: htop, lstopo (enable X11 Forwarding!)
• CPU-Z (Windows) or CPU-X (Linux)

Finding Strings and Counting Occurrences

grep

• grep or g/re/p: Global Regular Expression Print

• Origin of grep

• To find the occurence of a string, use grep "<string>" file_name.ext

  $ grep "current" bank.lst
  101 Aditya 0 14/11/2000 current
                          ^^^^^^^
  103 Naman0 20/08/2009 current
                        ^^^^^^^
  

• To count the occurence(s) of a string, use grep -c "<string>" file_name.ext

  $ grep -c "current" bank.lst
  2
  

• To find the occurence(s) of a string with the line number, use grep -n "<string>" file_name.ext

  $ grep -n "current" bank.lst
  1:101 Aditya 0 14/11/2000 current
                            ^^^^^^^
  3:103 Naman0 20/08/2009 current
                          ^^^^^^^
  

• To find the occurence(s) of a line without the string, use grep -v "<string>" file_name.ext

  $ grep -v "current" bank.lst
  108 Chirag 0 15/12/2012 Current
  109 Arya 16000 14/12/2010 Current
  110 Priya 130 16/11/2009 Saving
  

• To find files with the occurence of a string, use grep -l "<string>" *.ext

  $ grep -l "current" *.lst
  bank.lst
  customers.lst
  

• To find files without the occurence of a string, use grep -L "<string>" *.ext

  $ grep -L "current" *.java
  bnk.java
  cust.java
  

• To find lines from different files with a string (including the file names), use grep -H "<string>" *.ext

  $ grep -H "current" *.lst
  bank.lst:101 Aditya 0 14/11/2000 current
                                   ^^^^^^^
  bank.lst:103 Naman0 20/08/2009 current
                                 ^^^^^^^
  clist.lst:101 Aditya 0 14/11/2000 current
                                    ^^^^^^^
  clist.lst:103 Naman0 20/08/2009 current
                                  ^^^^^^^
  newbank.lst:101 Aditya 0 14/11/2000 current
                                      ^^^^^^^
  newbank.lst:103 Naman0 20/08/2009 current
                                    ^^^^^^^
  

  • To find lines from different files with a string (without including the file names), use grep -h "<string>" *.ext

  $ grep -h "current" *.lst
  101 Aditya 0 14/11/2000 current
                          ^^^^^^^
  103 Naman0 20/08/2009 current
                        ^^^^^^^
  101 Aditya 0 14/11/2000 current
                          ^^^^^^^
  103 Naman0 20/08/2009 current
                        ^^^^^^^
  101 Aditya 0 14/11/2000 current
                          ^^^^^^^
  103 Naman0 20/08/2009 current
                        ^^^^^^^
  

• To find the occurence of a string while ignoring the case, use grep -i "<string>" file_name.ext

  $ grep -i "current" bank.lst
  101 Aditya 0 14/11/2000 current
                          ^^^^^^^
  103 Naman0 20/08/2009 current
                        ^^^^^^^
  106 Mukesh 14000 20/12/2009 Current
                              ^^^^^^^
  108 Chirag 0 15/12/2012 Current
                          ^^^^^^^
  

• To find the occurence of strings, use grep -e "<string_1>" -e "<string_2>" -e "<string_3>"... file_name.ext (Can be used to ignore case as well, by entering all the allowed strings.)

  $ grep -e "current" -e "Mu" bank.lst
  101 Aditya 0 14/11/2000 current
                          ^^^^^^^
  103 Naman0 20/08/2009 current
                        ^^^^^^^
  106 Mukesh 14000 20/12/2009 Current
      ^^
  

• To find all lines with a string ignoring the case of a few characters, use:

  $ grep "[Cc]urrent" bank.lst
  101 Aditya 0 14/11/2000 current
                          ^^^^^^^
  103 Naman0 20/08/2009 current
                        ^^^^^^^
  106 Mukesh 14000 20/12/2009 Current
                              ^^^^^^^
  108 Chirag 0 15/12/2012 Current
                          ^^^^^^^
  109 Arya 16000 14/12/2010 Current
                            ^^^^^^^
  

• To find a number of lines before and after a string, use grep -<number> "<string>" file_name.ext

  $ grep -1 "current" bank.lst
  101 Aditya 0 14/11/2000 current # The line above this is not printed as this is itself the first line in the file
                          ^^^^^^^
  102 Anil 10000 20/05/2011 saving
  103 Naman0 20/08/2009 current
                        ^^^^^^^
  104 Ram 10000 15/08/2010 saving
  

• To find all lines starting with a string, use grep "^<string>" file_name.ext

  $ grep "^11" bank.lst
  110 Priya 130 16/11/2009 Saving
  ^^
  

• To find all lines starting within a string range, use grep "^[<string>-<string>]" file_name.ext

  $ grep "^[1-2]" bank.lst
  101 Aditya 0 14/11/2000 current
  ^
  ...
  
  110 Priya 130 16/11/2009 Saving
  ^
  201 Bina 3000 11/03/2010 saving
  ^
  202 Diya 4000 13/04/2018 saving
  ^
  203 Gargi 2000 21/01/2015 saving
  ^
  

• To find all lines not starting with a string, use grep ^[^<string>] <file_name.ext> (The outer ^ is negating the pattern.)

  $ grep "^[^1]" bank.lst
  201 Bina 3000 11/03/2010 saving
  ^
  202 Diya 4000 13/04/2018 saving
  ^
  203 Gargi 2000 21/01/2015 saving
  ^
  

• To find all lines not starting with a string in a range of characters, use grep ^[^<string>-<string>] <file_name.ext> (The outer ^ is negating the pattern.)

  $ grep "^[^0-1]" bank.lst
  201 Bina 3000 11/03/2010 saving
  ^
  202 Diya 4000 13/04/2018 saving
  ^
  203 Gargi 2000 21/01/2015 saving
  ^
  

• To find all lines ending with a string, use grep "<string>$" file_name.ext

  $ grep "nt$" bank.lst
  101 Aditya 0 14/11/2000 current
                               ^^
  103 Naman0 20/08/2009 current
                             ^^
  106 Mukesh 14000 20/12/2009 Current
                                   ^^
  108 Chirag 0 15/12/2012 Current
                               ^^
  109 Arya 16000 14/12/2010 Current
                                 ^^
  

• To find all lines starting or ending with a string anywhere in the line and having a fixed number of characters after or before it respectively, use grep "...<string>" file_name.ext or grep "<string>..." file_name.ext (Every . indicates one character.)

  $ grep "......nt" bank.lst
  101 Aditya 0 14/11/2000 current
                          ^^^^^^^
  103 Naman0 20/08/2009 current
                        ^^^^^^^
  106 Mukesh 14000 20/12/2009 Current
                              ^^^^^^^
  108 Chirag 0 15/12/2012 Current
                          ^^^^^^^
  109 Arya 16000 14/12/2010 Current
                            ^^^^^^^
  

• To find all lines starting and ending with a string and having a fixed number of characters in between, use grep "<string>...<string>" file_name.ext (Every . indicates one character.)

  $ grep "Cur..nt" bank.lst
  106 Mukesh 14000 20/12/2009 Current
                              ^^^^^^^
  108 Chirag 0 15/12/2012 Current
                          ^^^^^^^
  109 Arya 16000 14/12/2010 Current
                            ^^^^^^^
  

• To find all lines starting and ending with a string and having any number of characters in between, use grep "<string>.*<string>" file_name.ext

  $ grep "C.*nt" bank.lst
  106 Mukesh 14000 20/12/2009 Current
                              ^^^^^^^
  108 Chirag 0 15/12/2012 Current
                          ^^^^^^^
  109 Arya 16000 14/12/2010 Current
                            ^^^^^^^
  

• To find lines with strings stored in a file, use grep -f <pattern_file.ext> <target_file.ext>

  $ cat pattern.txt
  current
  Saving
  ^d
  
  $ grep -f pattern.txt bank.lst
  101 Aditya 0 14/11/2000 current
                          ^^^^^^^
  103 Naman0 20/08/2009 current
                        ^^^^^^^
  110 Priya 130 16/11/2009 Saving
                           ^^^^^^
  

• To search directories recursively, use grep -R

AWK

• AWK is a programming language executed by the AWK interpreter.
• The AWK language is a data-driven scripting language consisting of a set of actions to be taken against streams of textual data – either run directly on files or used as part of a pipeline – for purposes of extracting or transforming text, such as producing formatted reports.
• It designed for text processing and typically used as a data extraction and reporting tool.
• Like sed and grep, it is a filter, and is a standard feature of most Unix-like operating systems.
• It is named after the three people who developed it, namely Alfred Aho, Peter Weinberger and Brian Kernighan.

• General syntax for the AWK command: awk option '/pattern/ {action}' <file_name.ext>

  • An AWK command must have a pattern, an action or both.
  • Specifying an option is optional.
  • If a pattern is not specified, it will be entire line.
  • If an action is not specified, by default it is print.

• Print all records of file

  $ awk '{print}' bank.lst
  101 Aditya 0 14/11/2000 current
  102 Anil 10000 20/05/2011 saving
  ...
  110 Priya 130 16/11/2009 Saving
  201 Bina 3000 11/03/2010 saving
  202 Diya 4000 13/04/2018 saving
  203 Gargi 2000 21/01/2015 saving
  

• Print first 3 fields (Fields indicated by: $<field_no>)

  $ awk '{print $1 $2 $3}' bank.lst
  101Aditya0
  102Anil10000
  ...
  110Priya130
  201Bina3000
  202Diya4000
  203Gargi2000
  

• Sepearate fields by tabs.

  $ awk '{print $1 "\t" $2 "\t" $3}' bank.lst
  101     Aditya  0
  102     Anil    10000
  ...
  110     Priya   130
  201     Bina    3000
  202     Diya    4000
  203     Gargi   2000
  

• Format specifiers

  $ awk '{printf "%3d \t %7s \t %5d\n", $1, $2, $3}' bank.lst
  101       Aditya             0
  102         Anil         10000
  ...
  110        Priya           130
  201         Bina          3000
  202         Diya          4000
  203        Gargi          2000
  

• Print only those records having a pattern.

  $ awk '/current/ {print}' bank.lst
  101 Aditya 0 14/11/2000 current
  103 Naman 0 20/08/2009 current
  
  # OR
  
  $ awk '/current/ {print $0}' bank.lst  # '$0' means the entire line
  101 Aditya 0 14/11/2000 current
  103 Naman 0 20/08/2009 current
  

• Print records based on mathematical conditions

  $ awk '$3 < 5000 {print $1 "\t" $2 "\t" $3}' bank.lst
  101     Aditya  0
  103     Naman   0
  108     Chirag  0
  110     Priya   130
  201     Bina    3000
  202     Diya    4000
  203     Gargi   2000
  
  $ awk '$3 > 3000 && $3 < 8000 {print $1 "\t" $2 "\t" $3}' bank.lst  # '||' is logical OR
  105     Jyotsna 5000
  202     Diya    4000
  

• Print records if a field matches a certain string/pattern or not.

  $ awk '$5 == "current"' bank.lst  # OR awk '$5 ~/current/' bank.lst
  101 Aditya 0 14/11/2000 current
  103 Naman 0 20/08/2009 current
  
  $ awk '$5 != "current"' bank.lst  # OR awk '$5 !~/current/' bank.lst
  102 Anil 10000 20/05/2011 saving
  104 Ram 10000 15/08/2010 saving
  ...
  110 Priya 130 16/11/2009 Saving
  201 Bina 3000 11/03/2010 saving
  202 Diya 4000 13/04/2018 saving
  203 Gargi 2000 21/01/2015 saving
  

• Print records with a field ending or not ending with a string/pattern.

  $ awk '$5 ~/nt$/' bank.lst
  101 Aditya 0 14/11/2000 current
  103 Naman 0 20/08/2009 current
  106 Mukesh 14000 20/12/2009 Current
  108 Chirag 0 15/12/2012 Current
  109 Arya 16000 14/12/2010 Current
  
  $ awk '$5 !~/nt$/' bank.lst
  102 Anil 10000 20/05/2011 saving
  104 Ram 10000 15/08/2010 saving
  105 Jyotsna 5000 16/06/2012 saving
  107 Vishal 14500 30/11/2011 saving
  110 Priya 130 16/11/2009 Saving
  201 Bina 3000 11/03/2010 saving
  202 Diya 4000 13/04/2018 saving
  203 Gargi 2000 21/01/2015 saving
  

• Print records with a field value, while ignoring the case.

  $ awk '$5 ~/[Cc]urrent/' bank.lst
  101 Aditya 0 14/11/2000 current
  103 Naman 0 20/08/2009 current
  106 Mukesh 14000 20/12/2009 Current
  108 Chirag 0 15/12/2012 Current
  109 Arya 16000 14/12/2010 Current
  

• Built-in AWK variables

  • https://www.thegeekstuff.com/2010/01/8-powerful-awk-built-in-variables-fs-ofs-rs-ors-nr-nf-filename-fnr/
  • https://www.linuxnix.com/awk-scripting-learn-awk-built-in-variables-with-examples/

• Print record numbers 4 to 6.

  $ awk 'NR > 3 && NR < 7 {print $1 "\t" $2}' bank.lst
  104     Ram
  105     Jyotsna
  106     Mukesh
  
  $ awk 'NR == 4, NR == 6 {print $1 "\t" $2}' bank.lst
  104     Ram
  105     Jyotsna
  106     Mukesh
  

Disks, USBs and Files

• How to list disks, partitions and filesystems in Linux?
• 4 Ways to Check File Size in Linux
• How to access a usb flash drive from the terminal?

Prompt Customization

• Why is bash’s prompt variable called PS1?
• Bash/Prompt Customization
• Controlling the Prompt
• Why do commands starting with a space, not show up in bash history?

Periodically Running Tasks

CRON Jobs

• Scheduling Cron Jobs with Crontab
• How to Run Cron Jobs Every 5, 10, or 15 Minutes
• How to view a cron job running currently?
• Run Crontab in directory

Making Requests from the Command Line

Curl

• Mastering the Curl Command Line