This page describes the features of BSD Owl Scripts that are useful when developping shell scripts and shell routines. After reading this page, you will know:
- How to install a simple shell program.
- How to configure resource directories in shell programs.
- How to test your shell program without installing it.
- How to install shell libraries.
Minimal examples for a program and a library can be found under example/script/shell/showconfig. The Anvil project provides a more complete example.
As a shell program is nothing more than a regular text file coming with the whole sha-bang and installed somewhere in the path, there is little processing required before installing these files.
A simple Makefile to install a shell program looks like:
PROGRAM= simple-shell-program.sh
.include "shell.prog.mk"This will install simple-shell-program.sh as simple-shell-program
in ${BINDIR}. The PROGRAM variable can enumerate several programs
to install, ending with .sh, .bash, .ksh, .csh, .awk or
.sed. If a file simple-shell-program.1 or simple-shell-program.8
is found, it will also be installed as a compressed manual page.
There is one processing actually taking place before shell programs
are installed: variables like @prefix@ are expanded to their values
as known to make(1). These values are typically declared in the file
Makefile.config found at the root of the project, this file is expected
to be produced from a Makefile.config.in model by ./configure. The
variable REPLACESUBST contains the list of @-style variables which
should be replaced by their values as known by make(1). The variable
REPLACESUBST already contains the PACKAGE and VERSION variables, and
the standard directory variables. This mechanism can be used to
convey values found by the ./configure script to our program. An
important example can be the path to the interpreter, since shells
other than the Bourne shell do not need to have an installation path
which is consistent across all operating systems.
Note that this variable expansion also takes place in the prepraration of manual pages.
As a shell-script developer, we do not want to repeatedly install a
script in order to test it. Rather, we would like to test it without
installing it, and especially to run the script in a controlled
environment for automated testing. This problem can nicely be solved
by using the shell : operator and the := replacement modificator,
as in the following example:
: ${prefix:=@prefix@}
: ${exec_prefix:=@exec_prefix@}
: ${bindir:=@bindir@}
…With these declarations, the installed version of the script will use
the values supplied by the ./configure script while a special
environment can easily be crafted for test purpposes, so that running
the script does not require installation.
Note that allowing the user to supersede directory variables in the
environment might rise a security issue. The sudo(8) command can be
used to delegate privileges while maintaining a controlled
environment.
A simple Makefile requiring the installation of the shell routine
library simple-library.subr looks like:
LIBRARY= simple-library.subr
.include "shell.lib.mk"The files enumerated by LIBRARY are installed to ${SUBRDIR} which
defaults to ${SHAREDIR}. If a file simple-library.subr.3 or
simple-library.subr.8 is found, it is installed as a manual page.
The replacement of @-style variables does not happen when
libraries of shell routines are processed — but it still happens for
manual pages. The recommanded calling convention for libraries, is
that the library client prepares all variables required by the library
to run correctly. The library can use : constructs and :?
replacement modifiers to ensure it runs in a sane environment.
- UNIX Shell Programming, 2nd edition by Lowell Jay Arthur.
- Shellcheck, an online linter for shell scripts.
- A list of obsolete or deprecated shell constructs