Command Line Interface

The command line interface is one of the way to interact with the operating system using special keywords known as commands. By using the commands one can perform all the operations needed skipping the Graphical User Interface.

The interface is provided by a software known as shell. Most used shell is bash.

Common Commands to get started with CLI are as follows:

touch : This command creates new file in the current directory. Another way of creating file would be by specifying path "touch /folder1/folder2/fileName.txt".
```
 touch fileName.txt
```
mkdir : This command creates new directory or folder in the current path unless a specific path is followed by the command.
```
 mkdir newFolder
```

cd : This command stands for change directory. Used to navigate between the files and folders.

 # change to existing file
 cd Document

 # change directory to specfic path
 cd /path/to/directory

 # change directory to home directory
 cd

 #change directory to parent
 cd ..

mv : This command is used to move the files. This can be also used to rename files.
```
 mv fileName /destination/path
```
```
 mv fileName newFileName
```
cp : This command is used to copy files from source path to destination.
- -r : This option allows to copy recursively all the files.
- -a : This option allows to preserve the attributes and permissions of the copied files.
- -f : This option skips the prompts and overwrites the files if they exist.
- -i : This option prompts if the files exist.

    cp fileTobeCopied /destination/path

ls : This command is used to list directory content.
- -r : reverse sort files.
- -a : list all files including hidden files.cp fileTobeCopied /destination/path ls : This command is used to list directory content.
- -t : sort files.
- -h : human readable format.
- -l : long list format.

    ls -altrh # combination of all flags

pwd : It stands for print working directory.This command displays current path.
```
 pwd
```
rm : This command is used to remove files
- -r : recursively remove all the files.
- -f : forcefully remove all the files without prompting for permission of the owner.
- -i : prompts before removing each file.

    rm fileName

chmod : 'chmod' stands for change mode. This command is used to modify the file permissions. This can be done numerically or by specifying particular change of permission.
- Numeric : The sum of the values of permissions is the given to owner, others, group form a 3 digit numeric code.
  - Read (r) has value 4.
  - Write (w) has value 2.
  - Execute (x) has value 1.

        chmod 755 fileName
        # owner = 7 => r=4 + w=2 + x=1
        # group = 5 => r=4 + x=1
        # others = 5 => r=4 + x=1

Symbolic : Each symbol is specified with + or - symbols to add or remove the permissions.
```
  chmod u=rwx,g=r,o=r fileName
  chmod +r fileName
  chmod -x fileName
```

chown : This command is used to change the owner.
```
chown newOwnerName fileName
```
cat : This command is used to view the file contents.
```
cat fileName
```
grep: This command is used to search for patterns in a file.
- -o : displays pattern if found in new line each time.
- -w : finds the exact pattern.
- -i : ignores cases while searching.
- -c : counts number of lines containing the pattern.

    grep "patternToSearch" fileName

find : This command is used to find files in the directory specified or the working directory.

find /path/to/search/ -name "fileName.txt"  # search by name
find /path/to/search/ -name "*.txt"  # search by extension
find /path/to/search/ -type d # finds the directories only
find /path/to/search/ -type f # finds files only

wc : This command is used to count words. It stands for word count.

wc -l fileName # counts lines in the file
wc -c fileName # counts bytes in the file
wc -w fileName # counts words in the file

sort : This commands sorts alphabetically

sort fileName
sort -n fileName # sorts numerically
sort -r fileName # sorts in reverse

nano : This opens the specified file in the editor
```
nano fileName
```

awk : Used to manipulate and analyse data files. Usually piped with other command.

# This prints line if the line starts with the word Harry
awk '$1 == "Harry" { print $0 }' data.txt

sed : 'sed' stands for stream editor. This command is used as a text processing tool.

sed 's/old/new/' file.txt # substitutes old with new in file
sed -n '/pattern/p' file.txt # prints lines if pattern is found
sed '10,20d' file.txt # deletes lines from 10 to 20 from file
sed '10,20p' file.txt # prints lines from 10 to 20 from file

ping : Used to test reachability of the host.
```
ping -c 5 www.example.com
```

ps : 'ps' is a versatile tool for monitoring and managing processes.

ps # lists all the processes
ps -aux # full info about the process
ps -o pid,ppid,user,cmd # custom output of process list