What is Unix Shell Commands?
Unix shell commands is one of the four layers of Unix architecture which enables human interaction with the operating system intimating it to begin certain processes by giving commands through the interpreter. It consists a set of commands like cp, grep, cat, id, mv etc. that are pre-defined and stored in the libraries. Unix shell commands are of three types basic, intermediate and advanced consisting of various commands like ls, cat, rm, mv, mkdir, chmod, find, chown, chgrp, head, tail, grep, ln, cut etc.
UNIX Architecture
Here is the following UNIX architecture mention below
- Layer-1: Hardware
This layer consists of all the hardware resources being used.
- Layer-2: Kernel
The kernel is like the heart of the operating system. It is the mode of interaction between the hardware and operating system. It also manages tasks, resource using scheduling processes for the smooth functioning of the system.
- Layer-3: Shell commands
It is the way for a human to interact with the operating system and tell it to start certain processes. An interpreter is used where we give the command for the operating from the set of all commands for which the definition has been defined and stored in the libraries.
Some examples of commands are cp, mv, cat, grep, id, wc, nroff, a.out and more.
- Layer-4: Application Layer
It executes the given external applications. It is an outermost layer to execute the applications.
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List of Unix Shell Commands
Here is the list of following Unix Shell Commands mention below:
- Basic.
- Intermediate.
- Advanced.
Let us see above List of Unix Shell Commands in detail
1. Basic Unix Shell Commands
a. Listing files (ls) – ‘ls’ command is used to list out all the files in a directory.
Syntax- ls -<option> directory_name
Example- ls test
| option | Description |
| ls -a | To list all files with the hidden files starting with ‘.’ |
| ls –color | Shows colored list which can be [=always/never/auto] |
| ls -d | To list all directories |
| ls -F | To add one char of to the entries |
| ls -i | To list all files ignoring the case |
| ls -l | To list all the details of the file |
| ls -la | list long format including hidden files |
| ls -lh | list long format with the readable file size |
| ls -ls | list with the long format with the file size |
| ls -r | list in reverse order |
| ls -R | list recursively directory tree |
| ls -s | list file size |
| ls -S | sort by file size |
| ls -t | sort by time & date |
| ls -X | sort by extension name |
b. Creating & Viewing Files – ‘cat command can be used to create the file or view the contents of the file.
Syntax –cat >filename
Example – cat > test1.dat – will create a file and will wait for the input to be written into the file.
cat filname – will display the contents of the file on the screen.
c. Deleting Files – ’rm’ command is used to delete a file from the directory.
Syntax- rm filename
| Tag | Description |
| -f, –force | ignore nonexistent files, never prompt. |
| -i | prompt before every removal. |
d. Moving and Re-naming files – ‘mv’ command is used for moving a file from one location to another. This command can also be used for renaming the file as the source file gets deleted and a new file gets created.
Syntax– mv <source_file> <target_file>
Example – mv test1.dat test2.dat – here contents of test1 file gets copied to test2.dat in the same directory and test1.dat file gets deleted.
e. Making directories – Unix also provides us the command to make our own directory. It is just like making our own folder where all relevant files can be stored.
Syntax – mkdir <dir_name>
Example – mkdir /abi/sand/results – this command will create a directory at /abi/sand path . This command will not work if /abi/sand/ path doesnot exist.
2. Intermediate
a. Chmod – Sometimes when we need to write into the file that is write-protected we need to change the permissions given to a file or directory. Here ‘chmod’ command is used to give suitable permissions. But one should know the pattern for giving permissions.
Permissions are given as rwxrwxrwx
We must set permission to 1 if we need to enable it and to 0 if it needs to disable.
For eg- If one wants to give only read and execute permissions to users and others but all permissions to the group. Then we must set it as ‘101111101’. And that means ‘575’ if converted to decimal in triplets. Thus for giving permissions, we give the command as
Example – chmod 575 file1.dat
b. Find – This command is used for finding the files or directories in a particular directory as well as its subdirectories.
Syntax – find <options> <paths>
Example –
| Option | Description |
| -atime n | Returns true if the file was accessed n days ago |
| -ctime n | Returns true if the file was changed n days ago |
| -mtime | Returns true if file contents were modified n days ago |
| -name | Return true if filename matching a particular pattern |
| -size | Returns true if the file size is n blocks. |
| -type c | Returns true if the file being searched is of type c( if c = ‘f’ means it is a file; if it ‘d’ means it is a directory) |
Example – If someone wants to search for file names ‘test1’ in the directory he should give a command like –
find –type f –name test1 /abi/sand – This command will give all test1 file in /abi/sand directory
c. chown – change ownership of the file. Sometimes someone wants to change the owner of the file so that someone who is currently working in that file has all access to that file. Only the owner of the file has the right to change the file ownership.
Syntax: chown [owner] [file]
Example: Change the owner of test1 to user name ‘aaggasa’ assuming that it is currently owned by the current user
> chown aaggasa test1
d. chgrp: change the group ownership of the file. This command is used to change the group to which the file belongs. Only the owner of the file has the right to change the file ownership.
Syntax: chgrp [group] [file]
Example: Change group of test1 to group2 assuming it is currently owned by the current user.
> chgrp group2 test1
e. Head: Unix gives us this command-line utility to extract the first part of the file. It writes the result on standard output.
Syntax – head <option> <filename>
| Option | Description |
| -n | Used to specify the number of lines to be fetched |
| –c | Used to specify the number of bytes to be fetched. |
| -q | Used to suppress the header line. |
Example – If someone wants to extract the first 5 lines of the file we must use
>head –n 5 /abi/sand/test1.dat
Note – By default UNIX will show 10 lines in case no option is specified with the head command.
f. Tail: Unix gives us this command-line utility to extract the first part of the file. It writes the result on standard output.
Syntax – tail <option> <filename>
| Option | Description |
| -n | Used to specify the number of lines to be fetched |
| –c | Used to specify the number of bytes to be fetched. |
| -q | Used to suppress the header line. |
Example – If someone wants to extract the first 5 lines of the file we must use
>head –n 5 /abi/sand/test1.dat
Note – By default UNIX will show 10 lines in case no option is specified with the head command.
3. Advanced Unix Shell Commands
a. Grep: This is command utility which helps to search a particular pattern or character in the file. It returns all the lines that match the pattern in that particular file.
Syntax- grep <options> <pattern> <files>
| Option | Description |
| -n | Display the matched lines and their line numbers. |
| -v | To print the lines that do not match the pattern. |
| -l | To display the list of filenames. |
| -c | Count of lines that matches the pattern can be extracted. |
| -h | Display the matched lines, but do not display the filename |
| -i | Ignores, the case for matching |
| -w | To Match whole word in the expression |
b. ln: make links and symlinks to files and directories. A symbolic link is made up of the special type of file which contains a reference to another file. This helps to create a link between files. There are 2 types of links”-
- Soft link – It refers to the abstract path to a file.
- Hard Link – It refers to the exact location of that file,
To create a soft link ‘ln’ command is used.
Syntax – ln -s {source_filename} {symbolic_filename}
Example – If we want to create a softlink link l1 to the path ‘/abi/sand/dir1’ then we must execute the following command:-
>ln link1 /abi/sand/dir1
This command will create a link to the directory in the current directory
To check the link execute-
ls –l
Output – lrwxrwxrwx 1 priya priya 16 2007-09-25 22:53 link1 -> /abi/sand/dir1
c. cut – This command utility is used to extract a particular column from a file. For extracting a column we need to specify the delimiter that will help to distinguish the columns in that file.
Syntax– cut <options> <file>
| Option | Description |
| -c | For fixed-width fields, the -c option is used. |
| -d | For specifying the delimiter. By default delimiter is tab. |
| -b | For specifying the number of bytes to be extracted |
| -f | For specifying the field number that needs to be extracted. |
Example – If someone wants to extract the second field from ‘city.txt’ file where ‘|’ is treated as a delimiter for the columns.
cut –d “|” –f 2 city.txt
Conclusion
Unix Commands is a very powerful tool that helps the user execute the processes and do various tasks they want. Its inbuilt parser helps in development using various scripting languages. With its powerful set of commands utility, one can perform all the features even that need to read from registers.
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