What Is Bash (Bourne Again SHell)

Ubuntu Tutorials - Herong's Tutorial Examples

∟What Is Bash (Bourne Again SHell)

This section provides a quick introduction of Bash shell created by Brian Fox in 1989.

What Is Bash? - Bash (Bourne Again SHell) is an interactive command interpreter and programming language created by Brian Fox in 1989. Bash is a reimplementation and an extension of the Bourne shell for Unix/Linux-like operating systems. Bash also incorporates some useful features from the Korn and C shells.

Main features of Bash are:

Built-in Commands - The ability to use a few useful commands built into the shell.
Command Path Variable - The ability to define a list of paths where the shell can search for external program to execute.
Sub-command Substitution - The ability to execute extra commands in sub shells and uses their output in the command line.
Filename Substitution - The ability to expand filename wildcard characters with real file names.
Input and Output Redirection - The ability to change standard input and/or output from terminal console to files.
Job Control - The ability to suspend an interactive job and resume it at a later time, or send it into the "background".
Execution Control Commands - The ability to execute commands conditionally and repeatedly.
Execution Pipes - The ability to pipe output of one command into another command as input.
Quotation and Escape Sequences - The ability to protect special characters with single quotes and backslashes.
Shell Scripts - The ability to write shell scripts with execution flow control and variables.
Signal Handling - The ability to trap interruption signals (errors).
Variable Substitution - The ability to reference values of built-in and user defined shell variables.

The following Bash session shows you some of the above features:

# start a new Bash session
herong$ /bin/bash
herong@ubuntu$ 

# set the prompt to be 'bash$ ' by updating the built-in variable PS1 
herong@ubuntu$ PS1='bash$ '
bash$ 

# run the built-in command 'help'
bash$ help 
GNU bash, version 4.4.20(1)-release (x86_64-pc-linux-gnu)
These shell commands are defined internally.  Type `help' to see this list.
Type `help name' to find out more about the function `name'.
Use `info bash' to find out more about the shell in general.
Use `man -k' or `info' to find out more about commands not in this list.
...

# use sub-command substitution with '`' quotes 
bash$ DIR=`pwd`
bash$ echo $DIR 
/home/herong

# use filename substitution with wildcard characters
bash$ ls -ld D* 
drwxr-xr-x 2 herong herong 4096 Nov 11 10:20 Desktop
drwxr-xr-x 2 herong herong 4096 Jul  9  2023 Documents
drwxr-xr-x 2 herong herong 4096 Dec 15 08:06 Downloads

# use output redirection with '>'
bash$ date > output 
bash$ cat output 
Mon Jan 1 01:01:01 EST 2026

# use execution pipe to send output of one command to another
bash$ ls -d D* | wc 
      3       3      28

# end the shell session
bash$ exit
exit
herong$