Why can't I access my bash prompt in a Ruby script?

Question

In my .bash_profile I have:

if [ -f $(brew --prefix)/etc/bash_completion ]; then
  source $(brew --prefix)/etc/bash_completion
  GIT_PS1_SHOWDIRTYSTATE=1
  git_prompt='$(__git_ps1)'
fi
export PS1="\[\e[33m\]  \d \t \w$git_prompt\n\[\e[m\]\\$ "
export MYTESTVAR="hello world"

I have a Ruby script foo.rb that contains:

`source ~/.bash_profile`
puts `printenv`
puts `echo $PS1`
puts `echo $MYTESTVAR`

When I run ruby foo.rb the printenv command lists all environment variables, and the $MYTESTVAR line correctly returns hello world.

However, $PS1 does not show up. Why is this?

that other guy · Accepted Answer · 2016-05-23 23:43:05Z

3

$MYTESTVAR line correctly returns hello world. $PS1 does not show up.

Indeed, you're right. Most variables work fine, but "PS1" in particular fails:

$ PS1=ps1 TEST1=test1 ruby -e 'puts `echo $PS1 and $TEST1`'
and test1

Why is this?

In Ruby, as you're probably aware, `somestring` executes somestring in a shell, as if you in C had opened a pipe to execl("/bin/sh", "-c", somestring, NULL).

On many systems, /bin/sh is provided by bash. Bash has this snippet in its initialization code:

  /* Execute the start-up scripts. */

  if (interactive_shell == 0)
    {
      unbind_variable ("PS1");
      unbind_variable ("PS2");
      interactive = 0;

In other words, it explicitly removes "PS1" and "PS2" because these variables are only intended for use with interactive shells.

On other systems where /bin/bash is provided by dash, as many modern Debian-based distros default to, you instead get the result you expected:

$ PS1=ps1 TEST1=test1 ruby -e 'puts `echo $PS1 $TEST1`'
ps1 and test1

The reason why your `source ~/.bash_profile` doesn't fix this is that each `backtick command` runs in a separate shell. Variables set in one is not reflected in the other.

If you just want the variable as exported by your shell, you should do what @anujm suggests and get it directly with ENV['PS1']. Not only is this easier and more robust, it's also a thousand times faster.

If you want the variable as set in your .bash_profile, regardless of how you ran your script, you can do both sourcing and printing in the same shell:

puts `bash -c 'source ~/.bash_profile; printf "%s" "$PS1"'`

edited May 23, 2016 at 23:43

answered May 23, 2016 at 22:43

that other guy

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2 Comments

mu is too short Over a year ago

But it is more complicated than that because there five separate processes involved and each has its own environment.

that other guy Over a year ago

I didn't catch that part. Updated.

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