The Ultimate Guide To Linux Commands - Zero to Hundred In Linux

Introduction

If you're a beginner who has just started out with Linux, this blog might be of great help to you. I have shown with examples how you can use the most commonly used Linux commands. It goes from basic level to intermediate level.

Coming to developers who are already familiar with Linux commands and are now moving towards more advanced topics like Bash Scripting, I will write separate blogs for such topics.

Most Commonly Used Linux Commands

pwd Command
Prints the current working directory.

cd Command
Change working directory. cd command requires you to provide at-most one argument or no argument. It cannot be greater than one.

Change directory from Home directory(represented by ~) to Downloads. When you are somewhere inside your Home directory, the terminal uses ~ as an abbreviation.
```
  cd Downloads
```
Go up to the parent directory. '..' means previous directory. '.' means current directory.
```
  cd ..
```
Move up through multiple levels of parent directories.
```
  cd ../..
```
Switch to the root directory, then follow the route from there towards the directory 'etc'
```
  cd /etc
```

whoami Command
Prints the Host username.

mkdir Command
Create a new directory. It requires you to provide at-least one argument.

Create a folder named 'abc' in the current directory.
```
  mkdir abc
```
Create a folder named 'gs' in the Downloads directory. It will first go to the Home directory(represented by ~), then go to the Downloads directory and create a directory named 'gs' in it.
```
  mkdir ~/Downloads/gs
```
Switch to root directory, then create tmp directory and src directory inside tmp.
```
  mkdir /tmp/src
```
Creates all folders in the current directory.
```
  mkdir dir1 dir2 dir3
```
Here, dir4 is created and dir5 is created inside dir4 and dir6 is further created in dir5. We use the -p switch which creates the parent directories as described in the argument.
```
  mkdir -p dir4/dir5/dir6
```

ls Command
ls stands for List. It displays files & directories in a directory. Use the -a switch to list the hidden files like the .bashrc file.

Capture the output of the command in a text file, we use the greater than(>) sign.
```
  ls > result.txt
```

echo Command
Prints the provided argument in the terminal. Use the -e switch to use newline characters(\n) or tab spaces(\t).

echo "Hello"

cat Command
Displays the content of the file in the terminal. If you pass more than one filename, it will output each one of them, one after the other. 'cat' comes from the word concatenate which means to link together.

Display the content of sample.txt stored in dir1.
```
  cat dir1/sample.txt
```

mv Command
mv command lets you move more than one file at a time. If you pass more than two arguments, the last one is taken to be the destination directory and the others are considered to be the files or directories to be moved.

Here, abc.txt and dir1 will be moved to dir2
```
  mv abc.txt dir1 dir2
```
Here, abc.txt(located in dir1) will be moved to dir4 which is inside dir3 which is further inside dir2.
```
  mv dir1/abc.txt dir2/dir3/dir4
```
To rename a file, use the mv command. Here, abc.txt is renamed to xyz.txt. Works on both files and directories.
```
  mv abc.txt xyz.txt
```

cp Command
Copy command to copy files & directories.

Here, sample.txt which is inside dir1 is copied to dir3.
```
  cp dir1/sample.txt dir3
```

rm Command
Remove file command

Remove sample.txt stored in dir2 which is further inside dir1/
```
  rm dir1/dir2/sample.txt
```
Remove all files starting with 'h' in the current directory
```
  rm h*
```
Remove everything in the current directory
```
  rm *
```
Remove dir1 and delete everything in it even if there were files in it. Use -r switch for this.
```
  rm -r dir1
```
Use -i switch with rm command which will prompt you to confirm the deletion of each file. Here, we are deleting all files(represented by * ) in the dir3
```
  rm -i dir3/*
```

rmdir Command
Remove directory command. If there are any files in the directories, then they will not be deleted.

Here, we are deleting multiple directories at the same time which are stored in a hierarchy. Use the -p switch with rmdir command for this purpose. It deletes dir3 first, then dir2 is deleted and finally dir1 is deleted. Files contained in these directories will not be deleted.
```
  rmdir -p dir1/dir2/dir3
```

wc Command
Word count command. Use the -l switch to know line count.

Here, ls ~ lists the content of the home directory and wc -l counts the lines. The vertical bar ' | ' is the piping operator.
```
  ls ~ | wc -l
```
Count the lines in the /etc directory.
```
  ls /etc | wc -l
```

less Command
Display file's content one screen at a time. This can be used if a file output is very large.

Display the content of sample.txt
```
  less sample.txt
```

uniq Command
Output unique lines in that file.

Display the content of sample.txt and count the number of unique lines.
```
  cat  sample.txt | uniq | wc -l
```

man Command
Instruction Manual command.

Open instruction manual for rmdir command.
```
  man rmdir
```

sort Command
Can be used to sort data in different kinds of ways

sort : Sort alphabetically
sort -r : Reverse alphabetical sorting
sort -f : Case insensitive sorting
sort -n : Sort numerically

touch Command
Create a file

Create a text file in dir2, which lies inside dir1.
```
  touch dir1/dir2/sample.txt
```

locate Command
Locate files

Locate all files that end with .txt
```
  locate *.txt
```

find Command
To find files & directories inside a directory. Shows hidden files as well. It is similar to ls but has got more functionality, which are shown in the examples.

Find all directories within the current directory. (.) the dot represents the current directory.
```
  find . -type d
```
Find all files in current directory
```
  find . -type f
```
Find file named sample.txt in the current directory
```
  find . -type f -name sample.txt
```
Find all files starting with abc in current directory.
```
  find . -type f -name abc*
```
Find all files ending with .txt in dir2, which lies inside dir1 which further lies inside Home directory
```
  find ~/dir1/dir2 -type f -name *.txt
```
Do the same but now it is case insensitive search
```
  find ~/dir1/dir2 -type f -iname *.txt
```
Find all files that were modified less than 20 minutes ago in the current directory
```
  find . -type f -mmin -20
```
Find all files modified more than 15 minutes ago in current directory
```
  find . -type f -mmin +15
```
Find all files modified less than 10 days ago in current directory
```
  find . -type f -mtime -10
```
Find folders with 1 file depth in current directory
```
  find . -type f -maxdepth 1
```
Find all files having size more than 1kB.
```
  find . -size +1k
```
Find all empty files and directories
```
  find .
```
Find files which have 777 permission in current directory
```
  find . -perm 777
```

chmod Command
Manage File Permissions

4 : Read
2 : Write
1 : Execute
0 : No permissions
Ex - If you want to give write and execute permissions, then 2 + 1 = 3
Give Read, Write and Execute permissions to User, Group and other in file named sample.txt stored in dir2
```
  chmod 777 dir2/sample.txt
```
Give Read, Write and Execute permissions to User, Read and Execute permissions to Group and only Read permission for others in the file abc.txt stored in dir3. 'u' stands for user, 'g' stands for group, 'o' stands for others
```
  chmod u=rwx, g=rx, o=r dir3/abc.txt
```

exec Command
Executes a terminal command without creating a new process. Instead, it replaces the currently open Shell operation. Can be used to rename a large number of files at once.

Find and delete all files ending with .txt

  find . -type f -name *.txt -exec rm -rf {} +

grep Command
Used to search for things written inside a file.

Search the word 'Garvit' inside names.txt
```
  grep Garvit names.txt
```
Display complete word
```
  grep -w Garvit names.txt
```
Case insensitive search
```
  grep -i garvit names.txt
```
Display the line number
```
  grep -n Garvit names.txt
```
Combine all three switches
```
  grep -win Garvit names.txt
```
Display 3 lines that come before the searched word
```
  grep -B 3 Garvit names.txt
```
Search for all files having 'Garvit' and ending with .txt in current directory.
```
  grep -win Garvit ./*.txt
```
List all files containing 'Garvit' in current directory
```
  grep -wirl Garvit .
```
Count how many files contain 'Garvit' in current directory
```
  grep -wirc Garvit .
```

history Command
Shows the history of all past commands executed.

Show history of all 'mkdir' commands using grep.
```
  history | grep "mkdir"
```

Some Other Commands

clear Command
Clear the terminal screen.

clear

jobs Command
Displays current jobs running in the shell.

jobs

ping Command
Displays connectivity status.

ping google.com

wget Command
Download files from internet.

wget https://example.com/file.txt

top Command
Display what all processes are running in the CPU.

top

kill Command
Kills a running process.

kill process_id

zip Command
Compress files and add to zip folder.

zip archive.zip abc.txt xyz.txt

hostname Command
Displays domain name, hostname, system name. Use -i switch to get IP Address.

Display Hostname.

hostname

Display the Host IP Address.

hostname -i

useradd Command
To add a new user

sudo useradd new_user

userdel Command
To remove an existing user.

sudo userdel existing_user

lscpu Command
Get CPU details.

lscpu

free Command
Check used and free memory.

free

vmstat Command
Virtual memory statistics.

vmstat

getent Command
To check if a user exists in a group or not.

getent group group_name
getent passwd user_name

lsof Command
List all open files.

lsof

nslookup Command
Find IP Addresses of a particular domain.

nslookup google.com

netstat Command
List active ports

To show both listening & non-listening ports

netstat -a

List all TCP Ports

netstat -at

List all UDP Ports

netstat -au

cut Command
Cut out some portion from a file.

cut -d ',' -f1 file.csv

su Command
Allows users to switch to the root account and perform administrative tasks when passed with no username as arguments. If given a username, then it switches to a specific user account.

su user_name

logout Command
Logout from superuser or root account back to normal

logout

sudo Command
It is used to run a command with superuser privileges. A configuration file is used to define which users can use sudo and which commands they can run. When running a command like this, user is prompted for their own password.

sudo apt update

reset Command
Clear the terminal

reset

whereis Command
To get the path or location of a file in the directories.

whereis rmdir

open Command
Open a directory or path

open /dir1/dir2/dir3

tr Command
Translate command to translate, squeeze and/or delete characters from standard input, writing to standard output.

echo "hello" | tr 'a-z' 'A-Z'

df Command
Display disk space usage and similar information. Use -m switch to display sizes in MBs instead of KBs.

df -m

du Command
Display disk usage statistics.

du -sh /dir1/dir2/dir3

head Command
Displays text starting from above.

head -n 10 file.txt

tail Command
Displays text starting from below.

tail -n 10 file.txt

diff Command
Compares content of two files and outputs every line that doesn't match.

diff abc.txt xyz.txt

Operators In Bash

& - Used to run commands in the background so that other commands can be run.
&& (AND) - Execute a command only if the command preceding to it is successfully executed.
| | (OR) - Execute only if the previous command fails
| (Pipe) - Combine multiple commands
'>>' (inside) - Transfer results inside the specified file without over-riding previously stored data in the file.
'>' - Completely overrides the content stored in the specified file.