Difference between sh and bash
When writing shell programs, we often use /bin/sh and /bin/bash . I usually use bash , but I don't know what's the difference between them.
What's main difference between bash and sh ?
What do we need to be aware of when programming in bash and sh ?
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:
sh to dash by default; initramfs . It uses the ash shell implementation. 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.
Eg
#!/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, eg
#!/bin/dash
Which one to use
For my own scripts, I prefer sh for the following reasons:
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.
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