ultramunge: POSIX shell function to manipulate PATH-like variables
idempotently.
The source file sh-ultramunge.sh provides a function ultramunge that does
one thing and hopefully does it well: to add an element to a PATH-like,
colon-separated sequence of path prefixes in an idempotent manner, refraining
from doing anything if the element is already present in the sequence.
It can be used to manipulate the PATH environment variable, or other
variables following its convention.
You MUST read the section "Security" before using the function.
Do not install. The script is meant to be sourced by your own shell program.
. sh-ultramunge.shAfter sourcing the script, a function ultramunge is then available.
The function ultramunge manipulates a PATH-like variable.
ultramunge element variable_name [after]
The effect is to add element to the variable variable_name from the head
(default), or from the tail (if the string after appears as the 3rd
argument), if element is not already present in the variable. If element
is already present (no matter its location in the sequence), do nothing.
PATH=/sbin:/bin:/usr/bin
ultramunge /usr/local/bin PATH afterThe resulting PATH variable has the value
/sbin:/bin:/usr/bin:/usr/local/bin.
ultramunge "${HOME}/perl5/lib/perl5" PERL5LIBThe expansion result of "${HOME}/perl5/lib/perl5" is added to the variable
PERL5LIB from the head.
To manipulate the variable referred to by the name variable_name, a level of
indirection is necessary. Due to the limitation of the POSIX shell, eval is
used. In order to mitigate the possible damage that can be done by executing
arbitrary code in eval statements (a vulnerability known as code
injection), the following limitation is imposed, namely, that the
name of the identifier (variable_name) must "consis[t] solely of letters,
numbers, and underscores, and beginning with a letter or underscore." This
restriction makes it compatible with the identifier syntax of
Bash, which is also a superset of the Open Group POSIX
definition for environment variables:
Environment variable names used by the utilities in the Shell and Utilities volume of POSIX.1-2017 consist solely of uppercase letters, digits, and the
<underscore>( '_' ) from the characters defined in Portable Character Set and do not begin with a digit.
If the limitation is not satisfied, the function will return with status 1,
and no variable manipulation is performed.
You MUST use the function with caution, and never use it when you cannot control the content of the element or the value of the variable being manipulated.
You should consider unsetting the ultramunge function after you are done
with using it.
The name "ultramunge" is derived from an internal shell function "pathmunge"
found on Red Hat, CentOS, or Fedora systems during shell setup. Compared with
pathmunge, the major difference is that it munges on any PATH-like
variable.
The function relies on external commands like grep, in the Unix fashion. In
particular, base64 is used in order to prevent code-injection attack.
It is intended for use in POSIX shell scripts, but should be compatible with Bash as well.