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This article explains what a shell is, find out which shell you are currently using, check a list of all available shells, and the difference between shand .bash

What is a Shell

A shell is a computer program that accepts commands. It also interprets the commands and passes them to the operating system for processing. It acts as an interface between the user and the operating system through which the user can interact with the operating system.

Most Linux operating systems come with at least one shell program. The shell program will be Bash, , Dashor both.

How to Find the Current Shell

To know the shell currently used on the Linux system, we can read /etc/passwdthe ./shell file as it stores the user account information. Let us run the following command to know the current shell.

grepThe search command is used to search for a string in the specified files. The following command searches for users in the file /etclocated at . When the search command finds a match, it prints a line with the result.passwdfumbagrep

$ grep fumba /etc/passwd

As you can see from the output below, fumbathe default shell for user W is bash.

fumba:x:1000:1000:,,,:/home/fumba:/bin/bash

How to find available shells

All the shells available in Linux system are listed in /etc/shellsthe file. We use the following catcommand to display /etc/shellsthe contents of the file.

cat /etc/shells

The output below shows a list of all available shells on our system.

# /etc/shells: valid login shells
/bin/sh
/bin/bash
/usr/bin/bash
/bin/rbash
/usr/bin/rbash
/bin/dash
/usr/bin/dash
/usr/bin/tmux
/usr/bin/screen

shCommands and their usage

shIs the command name of the Bourne shell. It is the standard command language interpreter for Unix-like operating systems. It executes commands from a command line string, standard input, or a specified file. It shcomplies with the POSIX standard.

Portable Operating System Interface (POSIX) is a set of standards specified by the IEEE Computer Society to maintain compatibility between operating systems. Once adhered to, these standards facilitate the development of cross-platform software for multiple operating systems.

Most systems have /bin/shthe file, but it is a symbolic link and is not called sh. In Ubuntu, /bin/shit is Dasha symbolic link to the shell. We can check this by running the command below.

$ file -h /bin/sh

The output below shows /bin/shthat is dasha symbolic link to .

/bin/sh: symbolic link to dash

Let's #!/bin/shadd to our script.

#!/bin/sh

printf "sh points to dash!\n"

The script above specifies /bin/shas the interpreter. However, since /bin/shpoints to dash, dashthe shell will execute the script as the interpreter.

Bashand its use

Bashstands for Bourne Again shell. shLike , it is a command language processor and a shell. However, bash is sha superset of . It supports shthe features of and provides more extensions and features. Bashis the default shell for the Linux operating system.

In the beginning, Bashit was POSIX compliant, but over time, as more features and extensions were added, it no longer followed the POSIX standard. However, bash can still -posixbe used in POSIX mode by setting the flag as shown below.

$ bash --posix

Let's write a POSIX-compliant bash script.

#!/bin/bash

set -o posix
printf "we are using bash shell!\n"

bashThe interpreter will execute the above script as it has been #!/bin/bashspecified in the first line.

setThe command enables options in the script. In our case, it enables the bash shell to run the script in POSIX mode.