(debian-policy.info)Scripts

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10.4 Scripts
============

All command scripts, including the package maintainer scripts inside the
package and used by ‘dpkg’, should have a ‘#!’ line naming the shell to
be used to interpret them.

In the case of Perl scripts this should be ‘#!/usr/bin/perl’.

When scripts are installed into a directory in the system PATH, the
script name should not include an extension such as ‘.sh’ or ‘.pl’ that
denotes the scripting language currently used to implement it.

Shell scripts (‘sh’ and ‘bash’) other than ‘init.d’ scripts should
almost certainly start with ‘set -e’ so that errors are detected.
‘init.d’ scripts are something of a special case, due to how frequently
they need to call commands that are allowed to fail, and it may instead
be easier to check the exit status of commands directly.  See Note:
Writing the scripts. for more information about writing ‘init.d’
scripts.

Every script should use ‘set -e’ or check the exit status of `every'
command.

Scripts may assume that ‘/bin/sh’ implements the POSIX.1-2017 Shell
Command Language (1) plus the following additional features not mandated
by POSIX.1-2017..  (2)

   - ‘echo -n’, if implemented as a shell built-in, must not generate a
     newline.

   - ‘test’, if implemented as a shell built-in, must support ‘-a’ and
     ‘-o’ as binary logical operators.

   - ‘local’ to create a scoped variable must be supported, including
     listing multiple variables in a single local command and assigning
     a value to a variable at the same time as localizing it.  ‘local’
     may or may not preserve the variable value from an outer scope if
     no assignment is present.  Uses such as:

          fname () {
              local a b c=delta d
              # ... use a, b, c, d ...
          }

     must be supported and must set the value of ‘c’ to ‘delta’.

   - The XSI extension to ‘kill’ allowing ‘kill -signal’, where signal
     is either the name of a signal or one of the numeric signals listed
     in the XSI extension (0, 1, 2, 3, 6, 9, 14, and 15), must be
     supported if ‘kill’ is implemented as a shell built-in.

   - The XSI extension to ‘trap’ allowing numeric signals must be
     supported.  In addition to the signal numbers listed in the
     extension, which are the same as for ‘kill’ above, 13 (SIGPIPE)
     must be allowed.

If a shell script requires non-POSIX.1-2017 features from the shell
interpreter other than those listed above, the appropriate shell must be
specified in the first line of the script (e.g., ‘#!/bin/bash’) and the
package must depend on the package providing the shell (unless the shell
package is marked “Essential”, as in the case of ‘bash’).

You may wish to restrict your script to POSIX.1-2017 features plus the
above set when possible so that it may use ‘/bin/sh’ as its interpreter.
Checking your script with ‘checkbashisms’ from the devscripts package or
running your script with an alternate shell such as ‘posh’ may help
uncover violations of the above requirements.  If in doubt whether a
script complies with these requirements, use ‘/bin/bash’.

Perl scripts should check for errors when making any system calls,
including ‘open’, ‘print’, ‘close’, ‘rename’ and ‘system’.

‘csh’ and ‘tcsh’ should be avoided as scripting languages.  See `Csh
Programming Considered Harmful', one of the ‘comp.unix.*’ FAQs, which
can be found at ‘http://www.faqs.org/faqs/unix-faq/shell/csh-whynot/’.
If an upstream package comes with ‘csh’ scripts then you must make sure
that they start with ‘#!/bin/csh’ and make your package depend on the
‘c-shell’ virtual package.

Any scripts which create files in world-writeable directories (e.g., in
‘/tmp’) must use a mechanism which will fail atomically if a file with
the same name already exists.

The Debian base system provides the ‘tempfile’ and ‘mktemp’ utilities
for use by scripts for this purpose.

   ---------- Footnotes ----------

   (1) The Open Group Base Specifications Issue 7, 2018 Edition, which
is also known as POSIX.1-2017 and as IEEE Std 1003.1-2017 and is
available on the World Wide Web from The Open Group
(http://pubs.opengroup.org/onlinepubs/9699919799/download/).

   (2) These features are in widespread use in the Linux community and
are implemented in all of bash, dash, and ksh, the most common shells
users may wish to use as ‘/bin/sh’.