mjhjmtu

I'm reading about the bash shell and I've read from many sources that to make bash POSIX compliant it needs "extensions". Which sounds to me like something I should be able to load or unload into bash. I've tried googling the subject but can't seem to find any detail about this subject. What little I can find basically says nothing more than "Bash needs extensions".

Is there a way I can view the extensions Bash is loading? Load or unload them? Configure them differently?

Bash has many features and behaviours beyond or contrary to what POSIX specifies. These are sometimes described as extensions to the standard; they are not loadable extensions in the sense found in some applications.

Those extensions include things like the [[ test syntax, a,b,c brace expansion, variable indirection with $!x, process substitution with <(...), and history expansion with !. The extensions are the non-POSIX parts of Bash, in this sense.

They also include various changes to the behaviour of commands and syntax. A POSIX script can behave differently when run with bash if, for example, it uses the ! character that will instead be interpreted as an attempted history expansion, or it relies on pre-command assignment statements sticking around after the command:

foo() ... ; 
x=1 foo
echo $x

In POSIX sh, that should output "1", but run with bash the assignment to x doesn't persist after the command, in order to avoid cluttering the variable namespace in the common case.

Bash has a POSIX mode that alters its behaviour to be more POSIX-like in the situations where a reasonable POSIX script would behave differently. One example of that other than the above is Bash's (very useful) extension of the time command to handle pipelines, which is by necessity built in. In POSIX mode, the system's time executable runs instead in some cases:

$ time --help
bash: --help: command not found
$ set -o posix
$ time --help
Usage: time [-apvV] [-f format] [-o file] [--append] [--verbose]

There is a list of currently-56 behavioural changes in POSIX mode in the documentation. Most other extensions, like [[ and $!x, still work even in POSIX mode, though process substitution doesn't. For the most part, only differences that might break a reasonable POSIX script are reverted. This is really the only "disabling" of the extensions that exists. In POSIX mode, a POSIX script ought to behave as expected (though it's still easily possible to construct cases where it doesn't). Even in POSIX mode, a Bash script can use some non-POSIX features provided by the shell that don't directly conflict with the standard.

There are other shells, notably dash, that attempt a more minimal POSIX compliance. The major way of eliminating non-POSIX extensions is to switch to using one of those shells.

There are three different ways to force bash to be POSIX compliant.

• Start bash with --posix option


• Set option posix while bash is already running: set -o posix
  


• 
  

  Use environment variable POSIXLY_CORRECT:

  
  
  

  
  

  If this variable is in the environment when bash starts, the shell
  enters posix mode before reading the startup files, as if the --posix invocation option had been supplied. If it is set while the shell is running, bash enables posix mode, as if the command set -o posix had been executed.

  
  

  
  

Using these trio one can write some extension (e.g. function loaded during bash start-up), and probably someone already did this, but none of them are bash standard.