Source: StackOverflow
By: roman-cheplyaka
Note: Full credit goes to the author and source website you see above.
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.
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
.
For a long time, /bin/sh
used to point to /bin/bas
h 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/bas
h (and on some of which /bin/bash may not even exist) are:
- Modern Debian and Ubuntu systems, which symlink
sh
todash
by default; - Busybox, which is usually run during the Linux system boot time as part of
initramfs
. It uses the ash shell implementation. - BSDs. OpenBSD uses
pdksh
, a descendant of the Korn shell. FreeBSD'ss
h is a descendant of the original UNIX Bourne shell.
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 havesh
.
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.