What is sh

sh (or the Shell Command Language) is a programming language described by the POSIX
standard.
It has many implementations (ksh88, dash, ...). bash can also be
considered an implementation of sh (see below).

Because sh is a specification, not an implementation, /bin/sh is a symlink
(or a hard link) to an actual implementation on most POSIX systems.

What is bash

bash started as an sh-compatible implementation (although it predates the POSIX standard by a few years), but as time passed it has acquired many extensions. Many of these extensions may change the behavior of valid POSIX shell scripts, so by itself bash is not a valid POSIX shell. Rather, it is a dialect of the POSIX shell language.

bash supports a --posix switch, which makes it more POSIX-compliant. It also tries to mimic POSIX if invoked as sh.

sh = bash?

For a long time, /bin/sh used to point to /bin/bash on most GNU/Linux systems. As a result, it had almost become safe to ignore the difference between the two. But that started to change recently.

Some popular examples of systems where /bin/sh does not point to /bin/bash (and on some of which /bin/bash may not even exist) are:

1. Modern Debian and Ubuntu systems, which symlink sh to dash by default;


2. 
   Busybox, which is usually run during the Linux system boot time as part of initramfs. It uses the ash shell implementation.


3. BSDs, and in general any non-Linux systems. OpenBSD uses pdksh, a descendant of the Korn shell. FreeBSD's sh is a descendant of the original UNIX Bourne shell. Solaris has its own sh which for a long time was not POSIX-compliant; a free implementation is available from the Heirloom project.

How can you find out what /bin/sh points to on your system?

The complication is that /bin/sh could be a symbolic link or a hard link.
If it's a symbolic link, a portable way to resolve it is:

% file -h /bin/sh

/bin/sh: symbolic link to bash

If it's a hard link, try

% find -L /bin -samefile /bin/sh

/bin/sh

/bin/bash

In fact, the -L flag covers both symlinks and hardlinks,
but the disadvantage of this method is that it is not portable —
POSIX does not require find to support the -samefile option,
although both GNU find and FreeBSD find support it.

Shebang line

Ultimately, it's up to you to decide which one to use, by writing the «shebang» line.

E.g.

#!/bin/sh

will use sh (and whatever that happens to point to),

#!/bin/bash

will use /bin/bash if it's available (and fail with an error message if it's not). Of course, you can also specify another implementation, e.g.

#!/bin/dash

Which one to use

For my own scripts, I prefer sh for the following reasons:

• it is standardized


• it is much simpler and easier to learn


• it is portable across POSIX systems — even if they happen not to have bash, they are required to have sh
  

There are advantages to using bash as well. Its features make programming more convenient and similar to programming in other modern programming languages. These include things like scoped local variables and arrays. Plain sh is a very minimalistic programming language.

sh: http://man.cx/sh
bash: http://man.cx/bash

TL;DR: bash is a superset of sh with a more elegant syntax and more functionality. It is safe to use a bash shebang line in almost all cases as it's quite ubiquitous on modern platforms.

NB: in some environments, sh is bash. Check sh --version.

Post from UNIX.COM

Shell features

This table below lists most features that I think would make you choose one shell over another. It is not intended to be a definitive list and does not include every single possible feature for every single possible shell. A feature is only considered to be in a shell if in the version that comes with the operating system, or if it is available as compiled directly from the standard distribution. In particular the C shell specified below is that available on SUNOS 4.*, a considerable number of vendors now ship either tcsh or their own enhanced C shell instead (they don't always make it obvious that they are shipping tcsh.

Code:

                                     sh   csh  ksh  bash tcsh zsh  rc   es

Job control                          N    Y    Y    Y    Y    Y    N    N

Aliases                              N    Y    Y    Y    Y    Y    N    N

Shell functions                      Y(1) N    Y    Y    N    Y    Y    Y

"Sensible" Input/Output redirection  Y    N    Y    Y    N    Y    Y    Y

Directory stack                      N    Y    Y    Y    Y    Y    F    F

Command history                      N    Y    Y    Y    Y    Y    L    L

Command line editing                 N    N    Y    Y    Y    Y    L    L

Vi Command line editing              N    N    Y    Y    Y(3) Y    L    L

Emacs Command line editing           N    N    Y    Y    Y    Y    L    L

Rebindable Command line editing      N    N    N    Y    Y    Y    L    L

User name look up                    N    Y    Y    Y    Y    Y    L    L

Login/Logout watching                N    N    N    N    Y    Y    F    F

Filename completion                  N    Y(1) Y    Y    Y    Y    L    L

Username completion                  N    Y(2) Y    Y    Y    Y    L    L

Hostname completion                  N    Y(2) Y    Y    Y    Y    L    L

History completion                   N    N    N    Y    Y    Y    L    L

Fully programmable Completion        N    N    N    N    Y    Y    N    N

Mh Mailbox completion                N    N    N    N(4) N(6) N(6) N    N

Co Processes                         N    N    Y    N    N    Y    N    N

Builtin artithmetic evaluation       N    Y    Y    Y    Y    Y    N    N

Can follow symbolic links invisibly  N    N    Y    Y    Y    Y    N    N

Periodic command execution           N    N    N    N    Y    Y    N    N

Custom Prompt (easily)               N    N    Y    Y    Y    Y    Y    Y

Sun Keyboard Hack                    N    N    N    N    N    Y    N    N

Spelling Correction                  N    N    N    N    Y    Y    N    N

Process Substitution                 N    N    N    Y(2) N    Y    Y    Y

Underlying Syntax                    sh   csh  sh   sh   csh  sh   rc   rc

Freely Available                     N    N    N(5) Y    Y    Y    Y    Y

Checks Mailbox                       N    Y    Y    Y    Y    Y    F    F

Tty Sanity Checking                  N    N    N    N    Y    Y    N    N

Can cope with large argument lists   Y    N    Y    Y    Y    Y    Y    Y

Has non-interactive startup file     N    Y    Y(7) Y(7) Y    Y    N    N

Has non-login startup file           N    Y    Y(7) Y    Y    Y    N    N

Can avoid user startup files         N    Y    N    Y    N    Y    Y    Y

Can specify startup file             N    N    Y    Y    N    N    N    N

Low level command redefinition       N    N    N    N    N    N    N    Y

Has anonymous functions              N    N    N    N    N    N    Y    Y

List Variables                       N    Y    Y    N    Y    Y    Y    Y

Full signal trap handling            Y    N    Y    Y    N    Y    Y    Y

File no clobber ability              N    Y    Y    Y    Y    Y    N    F

Local variables                      N    N    Y    Y    N    Y    Y    Y

Lexically scoped variables           N    N    N    N    N    N    N    Y

Exceptions                           N    N    N    N    N    N    N    Y

Key to the table above.

Y Feature can be done using this shell.

N Feature is not present in the shell.

F Feature can only be done by using the shells function
mechanism.

L The readline library must be linked into the shell to enable
this Feature.

Notes to the table above

1. This feature was not in the original version, but has since become

   almost standard.

2. This feature is fairly new and so is often not found on many

   versions of the shell, it is gradually making its way into

   standard distribution.

3. The Vi emulation of this shell is thought by many to be

   incomplete.

4. This feature is not standard but unofficial patches exist to

   perform this.

5. A version called 'pdksh' is freely available, but does not have

   the full functionality of the AT&T version.

6. This can be done via the shells programmable completion mechanism.

7. Only by specifying a file via the ENV environment variable.

This question has frequently been nominated as a canonical for people who try to use sh and are surprised that it's not behaving the same as bash. Here's a quick rundown of common misunderstandings and pitfalls.

First off, you should understand what to expect.

• If you run your script with sh scriptname, or run it with scriptname and have #!/bin/sh in the shebang line, you should expect POSIX sh behavior.


• If you run your script with bash scriptname, or run it with scriptname and have #!/bin/bash (or the local equivalent) in the shebang line, you should expect Bash behavior.

Having a correct shebang and running the script by typing just the script name (possibly with a relative or full path) is generally the preferred solution. In addition to a correct shebang, this requires the script file to have execute permission (chmod a+x scriptname).

So, how do they actually differ?

The Bash Reference manual has a section which attempts to enumerate the differences but some common sources of confusion include

• 
  [[ is not available in sh (only [ which is more clunky and limited).


• 
  sh does not have arrays.


• Some Bash keywords like local, function, and select are not portable to sh.


• Bash has many C-style syntax extensions like $'stringnwithtCaescapes' and the three-argument for((i=0;i<=3;i++)) loop, += increment assignment, etc.


• Bash supports <<<'here strings'.


• Bash has *.{png,jpg} and {0..9} brace expansion.


• 
  ~ refers to $HOME only in Bash (and more generally ~username to the home directory of username).This is in POSIX, but may be mising from some pre-POSIX /bin/sh implementations.


• Bash has process substitution with <(cmd) and >(cmd).


• Bash supports coprocesses with <> redirection.


• Bash has significantly extended facilities for shell arithmetic (though still no floating-point support) and variable substring manipulation with ${substring:1:2}, ${variable/pattern/replacement}, case conversion, etc.


• Many, many Bash-only extensions to enable or disable optional behavior and expose internal state of the shell.


• Many, many convenience features for interactive use which however do not affect script behavior.

Remember, this is an abridged listing. Refer to the reference manual for the full scoop, and http://mywiki.wooledge.org/Bashism for many good workarounds; and/or try http://shellcheck.net/ which warns for many Bash-only features.

A common error is to have a #!/bin/bash shebang line, but then nevertheless using sh scriptname to actually run the script. This basically disables any Bash-only functionality, so you get syntax errors e.g. for trying to use arrays.

Unfortunately, Bash will not warn when you try to use these constructs when it is invoked as sh. It doesn't completely disable all Bash-only functionality, either, so running Bash by invoking it as sh is not a good way to check if your script is properly portable to ash/dash/POSIX sh or variants like Heirloom sh

Shell is an interface between a user and OS to access to an operating system's services. It can be either GUI or CLI (Command Line interface).

sh (Bourne shell) is a shell command-line interpreter, for Unix/Unix-like operating systems. It provides some built-in commands. In scripting language we denote interpreter as #!/bin/sh. It was one most widely supported by other shells like bash (free/open), kash (not free).

Bash (Bourne again shell) is a shell replacement for the Bourne shell. Bash is superset of sh. Bash supports sh. POSIX is a set of standards defining how POSIX-compliant systems should work. Bash is not actually a POSIX compliant shell. In a scripting language we denote the interpreter as #!/bin/bash.

Analogy:

• Shell is like an interface or specifications or API.


• sh is a class which implements the Shell interface.


• Bash is a subclass of the sh.

TERMINAL

• program(s) that put a window up


• xterm, rxvt, konsole, kvt, gnome-terminal, nxterm, and eterm.

SHELL

• Is a program that runs in the terminal


• Shell is both a command interpreter and a programming language


• Shell is simply a macro processor that executes commands.


• Macro processor means functionality where text and symbols are expanded to create larger expressions.

SH Vs. BASH

SH

• (SHell)


• Is a specific shell


• a command interpreter and a programming language


• Predecessor of BASH

BASH

• (Bourne-Again SHell)


• Is a specific shell


• a command interpreter and a programming language


• Has sh functionality and more


• Successor of SH


• BASH is the default SHELL

REFERENCE MATERIAL:

SHELL
gnu.org:

At its base, a shell is simply a macro processor that executes
commands. The term macro processor means functionality where text and
symbols are expanded to create larger expressions.

A Unix shell is both a command interpreter and a programming language.
As a command interpreter, the shell provides the user interface to the
rich set of GNU utilities. The programming language features allow
these utilities to be combined. Files containing commands can be
created, and become commands themselves. These new commands have the
same status as system commands in directories such as /bin, allowing
users or groups to establish custom environments to automate their
common tasks.

Shells may be used interactively or non-interactively. In interactive
mode, they accept input typed from the keyboard. When executing
non-interactively, shells execute commands read from a file.

A shell allows execution of GNU commands, both synchronously and
asynchronously. The shell waits for synchronous commands to complete
before accepting more input; asynchronous commands continue to execute
in parallel with the shell while it reads and executes additional
commands. The redirection constructs permit fine-grained control of
the input and output of those commands. Moreover, the shell allows
control over the contents of commands’ environments.

Shells also provide a small set of built-in commands (builtins)
implementing functionality impossible or inconvenient to obtain via
separate utilities. For example, cd, break, continue, and exec cannot
be implemented outside of the shell because they directly manipulate
the shell itself. The history, getopts, kill, or pwd builtins, among
others, could be implemented in separate utilities, but they are more
convenient to use as builtin commands. All of the shell builtins are
described in subsequent sections.

While executing commands is essential, most of the power (and
complexity) of shells is due to their embedded programming languages.
Like any high-level language, the shell provides variables, flow
control constructs, quoting, and functions.

Shells offer features geared specifically for interactive use rather
than to augment the programming language. These interactive features
include job control, command line editing, command history and
aliases. Each of these features is described in this manual.

BASH gnu.org:

Bash is the shell, or command language interpreter, for the GNU
operating system. The name is an acronym for the ‘Bourne-Again SHell’,
a pun on Stephen Bourne, the author of the direct ancestor of the
current Unix shell sh, which appeared in the Seventh Edition Bell Labs
Research version of Unix.

Bash is largely compatible with sh and incorporates useful features
from the Korn shell ksh and the C shell csh. It is intended to be a
conformant implementation of the IEEE POSIX Shell and Tools portion of
the IEEE POSIX specification (IEEE Standard 1003.1). It offers
functional improvements over sh for both interactive and programming
use.

While the GNU operating system provides other shells, including a
version of csh, Bash is the default shell. Like other GNU software,
Bash is quite portable. It currently runs on nearly every version of
Unix and a few other operating systems - independently-supported ports
exist for MS-DOS, OS/2, and Windows platforms.

Other answers generally pointed out the difference between Bash and a POSIX shell standard. However, when writing portable shell scripts and being used to Bash syntax, a list of typical bashisms and corresponding pure POSIX solutions is very handy. Such list has been compiled when Ubuntu switched from Bash to Dash as default system shell and can be found here:
https://wiki.ubuntu.com/DashAsBinSh

Moreover, there is a great tool called checkbashisms that checks for bashisms in your script and comes handy when you want to make sure that your script is portable.

/bin/sh may or may not invoke the same program as /bin/bash.

sh supports at least the features required by POSIX (assuming a correct implementation). It may support extensions as well.

bash, the "Bourne Again Shell", implements the features required for sh plus bash-specific extensions. The full set of extensions is too long to describe here, and it varies with new releases. The differences are documented in the bash manual. Type info bash and read the "Bash Features" section (section 6 in the current version), or read the current documentation online.

Linux operating system offers different types of shell. Though shells have many commands in common, each type has unique features.
Let’s study different kind of mostly used shells.

Sh shell:

Sh shell is also known as Bourne Shell. Sh shell is the first shell developed for Unix computers by Stephen Bourne at AT&T's Bell Labs in 1977. It include many scripting tools.

Bash shell :

Bash shell stands for Bourne Again Shell. Bash shell is the default shell in most linux distribution and substitute for Sh Shell (Sh shell will also run in the Bash shell) . Bash Shell can execute the vast majority of Sh shell scripts without modification and provide commands line editing feature also.