A shell is a special user program which provides an interface to the user to use operating system services. Shell accept human readable commands from the user and convert them into something which kernel can understand. It is a command language interpreter that execute commands read from input devices such as keyboards or from files. The shell gets started when the user logs in or start the terminal.
1). Displaying the file contents on the terminal:
- cat : It is generally used to concatenate the files. It gives the output on the standard output.
- more : It is a filter for paging through text one screenful at a time.
- less : It is used to viewing the files instead of opening the file.Similar to more command but it allows backward as well as forward movement.
- head : Used to print the first N lines of a file. It accepts N as input and the default value of N is 10.
- tail : Used to print the last N-1 lines of a file. It accepts N as input and the default value of N is 10.
2). File and Directory Manipulation Commands:
- mkdir : Used to create a directory if not already exist. It accepts directory name as input parameter.
- cp : This command will copy the files and directories from source path to destination path. It can copy a file/directory with new name to the destination path. It accepts source file/directory and destination file/directory.
- mv : Used to move the files or directories. This command’s working is almost similar to cp command but it deletes copy of file or directory from source path.
- rm : Used to remove files or directories.
- touch : Used to create or update a file.
3). Extract, sort and filter data Commands:
- grep : This command is used to search for the specified text in a file.
- grep with Regular Expressions : Used to search for text using specific regular expressions in file.
- sort : This commands is used to sort the contents of files.
- wc : Used to count the number of characters, words in a file.
- cut : Used to cut a specified part of a file.
4). Basic Terminal Navigation Commands:
- ls : To get the list of all the files or folders.
- cd : Used to change the directory.
- du : Show disk usage.
- pwd : Show the present working directory.
- man : Used to show the manual of any command present in Linux.
- rmdir : It is used to delete a directory if it is empty.
- ln file1 file2 : Creates physical link.
- ln -s file1 file2 : Creates symbolic link.
5). File Permissions Commands: The chmod and chown commands are used to control access to files in UNIX and Linux systems.
- chown : Used to change the owner of file.
- chgrp : Used to change the group owner of file.
- chmod : Used to modify the access/permission of a user.