Re: [Help-bash] How to print a bash variable in the most succinct way?

[Greg Wooledge]

On Mon, Aug 15, 2016 at 10:36:23PM -0500, Peng Yu wrote:
> https://www.gnu.org/software/parallel/env_parallel.html

"env_parallel is a shell function that exports the current environment
to GNU parallel."

GRRAAAHHHHHHH!!!  NO!  STOP IT!  STOP IT!!!

Seriously.  WASN'T Node.js BAD ENOUGH?!

Why do people keep DOING THIS?!

> env_parallel uses something like `declare -p` to extract the current
> environment so that the variables and functions in the current bash

DO YOU KNOW HOW TO MAKE THE ENVIRONMENT AVAILABLE TO YOUR CHILD PROCESSES?

I WILL TELL YOU!

YOU DO NOTHING.

NOTHING!

YOU DO NOTHING.

THE ENVIRONMENT IS AUTOMATICALLY AVAILABLE TO CHILD PROCESSES.

THAT IS WHAT IT MEANS TO BE THE ENVIRONMENT.

THAT IS THE ENTIRE POINT OF HAVING AN ENVIRONMENT.




OK, now listen up.  I know you think I am an asshole now.  Maybe I am.
Too bad.  I am going to try to pound some sense into you anyway.
Because I'm too stupid to stop trying.

When you invoke a child process such as GNU parallel, it inherits
the environment from its parent.

That environment is, traditionally, a set of variables that you have
explicitly exported (marked as part of the environment by using the
"export" command).

In addition, bash has the ability to export FUNCTIONS through the
environment.  Again, you do this by marking them with the "export"
command.  Read "help export" to find out more.

When you (a bash script) invoke a child process (e.g. find or parallel)
which then invokes ANOTHER bash script as ITS child, then the
environment from the grandparent bash shell is all handed down
AUTOMATICALLY to the grandchild bash shell.

This includes every variable and every function that you have exported.

You don't need some stupid, awful, NODE.JS-LIKE (there are very few
insults stronger than this) HACK that tries to create a secondary
"environment" of some kind for you.

You already HAVE an environment.

Just use it.




But wait, there's more!  I know you won't believe me.  Nobody ever
believes anything I tell them until I reach physically through the
Internet and throttle them.  So here is an example.

First I will make up a function foo.  It will take one filename as
an argument.  It will do something VITALLY IMPORTANT to that filename.
So important that we had to do it in a bash function (because
obviously just making it a script and putting it in $PATH would have
been FAR TOO EASY, and easy solutions are not allowed).

address@hidden:~$ foo() { echo "<$1>"; }

OK, there is my super-important bash function that absolutely positively
had to be done as a function instead of a script.

Now I will export it.

address@hidden:~$ help export | grep function
      -f        refer to shell functions
      -p        display a list of all exported variables and functions
address@hidden:~$ export -f foo

Now I will use find to recurse through my file(s) and/or directory(ies)
and apply this function to each one, using the MAAAAGIC of bash's
environment.

address@hidden:~$ find .bashrc .profile -exec bash -c 'for f; do foo "$f"; 
done' _ {} +
<.bashrc>
<.profile>

Presto!  Abracadabra!  Expelliarmus!

Any more questions?  Read http://mywiki.wooledge.org/UsingFind for answers.

Wait, you've STILL got questions?  You want to know how to make bash
export fucking god-damned ALIASES through the environment or something?
You use the BASH_ENV variable to point to a file that contains your
alias definitions, so that the grandchild shell will re-invent them.


What, you're still here?  Now you want to export ARRAY VARIABLES through
the environment?  Well, you can't.  Just stop trying.  Work around this
limitation by redesigning your software.


If GNU env_parallel breaks in GNU bash because it tries to serialize an
array variable into an environment variable and gets it wrong, then file
a bug report against GNU env_parallel.  Meanwhile, STOP USING IT!

Seriously, any time you see "this software package modifies your shell
dot files, creating a series of new functions..." you should see HUGE
RED WARNING FLAGS.  This is bad.  You should not be using it.