Basic Shell commands and How do they execute?

Shell

A shell is a command line interface for a UNIX-like operating system. A UNIX system offers a variety of shell types like

sh or Bourne Shell
bash or Bourne Again shell : It is the standard GNU shell used commonly by Linux users.
csh or C Shell
ksh or Korn Shell
and many more . .

You can see what shell are available in your system by running cat /etc/shell

Since bash is the standard GNU shell, the focus of this blog will be that only.

Execution of shell commands

There are three ways a command can be executed by bash

Built-in
General
Shell Script

General

Bash determines the type of program that is to be executed. Normal system commands exist in compiled form in your system. When such a program is executed , bash uses the fork-and-exec mechanism. This mechanism is a commonly used technique in UNIX and is used to create all UNIX processes. First the fork() process happens, a new child process is created where bash makes an exact copy of itself. The child process environment is identical to that of the environment including the input and output devices except the process id. If you are wondering what process is , it is nothing but a special instance provided by the system that consists of all the services/resources that may be utilised by a program during its execution. After the forking process , the address space of the child process is overwritten with the new data using exec() scall to the system.

Built-In Commands

Bash built-in commands (also known as “internal command”) are part of the shell itself. Each built-in command is executed directly in the shell itself, instead of an external programme which the bash executes the general way.

How to differentiate between built-in and general commands

You can use the type command followed by a given command to find out the type of the command.

$ type cd
cd is a shell builtin
$ type type
type is a shell builtin              # type itself is a built in command
$ type ls
ls is /usr/bin/ls

As you can see ls is /usr/bin/ls so basically bash executes /usr/bin/ls the general way when ls command is called in shell.

Scripts

When a shell script is being executed, bash forks a new process. This process reads the lines from shell script one line at a time. Each line is read, interpreted and executed as if you were typing those commands from the keyboard.

A sample bash script :

#!/bin/bash

echo "Hello World"

pwd

cd ..

pwd

type cd

echo "Bye"

Output :

$ ./script.sh
Hello World
/home/daemon1024/test/test1
/home/daemon1024/test
cd is a shell builtin
Bye

Common shell commands

pwd - The print working directory command. As the name suggests, it prints out which directory you are in currently.
```
$ pwd
/home
```

ls - It is used to list the contents of a directory.

$ pwd
/
$ ls
bin   dev  home  lib32	libx32	    media  opt	 root  sbin  sys  usr
boot  etc  lib	 lib64	lost+found  mnt    proc  run   srv   tmp  var
$ ls home
daemon1024  lost+found
$ ls -l home
total 20
drwxr-xr-x 38 daemon1024 daemon1024  4096 Jul 24 14:45 daemon1024
drwx------  2 root       root       16384 May 23 16:58 lost+found

cd - The change directory command. You can change your working directory using this command.

$ pwd
/
$ cd home
$ pwd
/home
$ cd daemon1024
$ pwd
/home/daemon1024
$ cd ..                    # .. takes us one directory up.
$ pwd
/home
$ cd -                     # - takes us to the previous working directory.
/home/daemon1024

mkdir - The make directory command. You can create a new directory using this.

$ pwd
/home/daemon1024
$ cd test
cd: no such file or directory: test    # throws out an error
$ mkdir test
$ cd test
$ pwd
/home/daemon1024/test

cp and mv - The copy and move command. cp command simply copies files and directories and the mv command moves.

$ pwd
/home/daemon1024/test
$ ls
hello  hello1
$ mkdir test1            # make a new directory
$ ls
hello  hello1  test1
$ cd test1
$ ls                     # empty directory
$ cd ..
$ cp hello test1         # copy <source> <destination>
$ cd test1
$ ls                     # file copied
hello
$ cd ..
$ mv hello1 test1        # move <source> <destination>
$ ls
hello  test1             # hello1 is not present in current directory now
$ cd test1
$ ls
hello  hello1            # file moved
$ pwd
/home/daemon1024/test/test1

cat - It is used to read files sequentially and write them to standard output.

head and tail - The head command command prints lines from the beginning of a file, and the tail command prints lines from the end of files.

$ cat hello.txt
Lorem ipsum dolor sit amet,
consectetur adipiscing elit,
sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.
Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur.
Excepteur sint occaecat cupidatat non proident,
sunt in culpa qui officia deserunt mollit anim id est laborum.

$ head -n 3 hello.txt
Lorem ipsum dolor sit amet,
consectetur adipiscing elit,
sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
# Prints 3 lines from the beginning

$ tail -n 2 hello.txt
Excepteur sint occaecat cupidatat non proident,
sunt in culpa qui officia deserunt mollit anim id est laborum.
# Prints 2 lines from the end

$ tail -n +2 hello.txt
consectetur adipiscing elit,
sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.
Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur.
Excepteur sint occaecat cupidatat non proident,
sunt in culpa qui officia deserunt mollit anim id est laborum.
# Prints after skipping 2 lines from the beginning.

$ head -n -4 hello.txt
Lorem ipsum dolor sit amet,
consectetur adipiscing elit,
sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
# Prints after skipping 4 lines from the end.

man - It is used to display the user manual of any command that we run in the shell.

Syntax : $man [OPTION]... [COMMAND NAME]...

It provides a detailed view of the command which includes NAME, SYNOPSIS, DESCRIPTION, OPTIONS, EXIT STATUS, RETURN VALUES, ERRORS, FILES, VERSIONS, EXAMPLES, AUTHORS and SEE ALSO.

An example man page of ls.

man pages are the most important and reliable source of any UNIX related query. To know more about man pages read here.

Hope you got to learn some new stuff through this blog post.

Special thanks to folks at #dgplug for helping me learn new things in each session.

If you are interested in learning more about shell and command-line itself, here are some resources :