Why does this script work in the terminal but not from a file?
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:
- 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 theash
shell implementation. - BSDs, and in general any non-Linux systems. OpenBSD uses
pdksh
, a descendant of the Korn shell. FreeBSD'ssh
is a descendant of the original UNIX Bourne shell. Solaris has its ownsh
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 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.
Adding to the excellent answer from @Hunter.S.Thompson I'd like to point out that the non-portable part of the script is
pdf_file="${html_file/.html/.pdf}"
The ${variable/search/replace}
is a GNU extension. But you can easily avoid it with pure POSIX:
pdf_file="${html_file%.html}".pdf
Following Hunter, this is the better fix than changing the shebang to #! /bin/bash