1

I have trouble understanding:

  • How is .bashrc parsed
  • How are bash functions constructed

I try to customize my terminal prompt. My .bashrc make use of PROMPT_COMMAND like this (it's actually much more complex):

function prompt_command {
    PS1="-> "
}

PROMPT_COMMAND=prompt_command

I don't like this solution because it clutters my shell namespace with prompt_command, while it should be hidden from outside of this file (but I can't use unset as that would break the prompt).

So, I figured out I could also set the PROMPT_COMMAND in plain text like this:

PROMPT_COMMAND='PS1="-> "'

As I said, my prompt command is actually much more complex. So, I'm worried about the efficiency of this alternative.

Although the two methods are functionally equivalent, is there a fundamental difference in the way my .bashrc will be parsed and the prompt command will be constructed? Is Bash able to effectively "compile" and "cache" the prompt_command as a function object, or is it parsed for each new shell line the same way as if it would be a string?

  • I do the same thing: my function name is __bash_prompt and I live with it in the namespace. – glenn jackman May 1 at 13:35
0

The .bashrc file is parsed once each time a new terminal session is started.

During parsing, the function prompt_command is added to the environment variable, but there is no "compilation" step nor "object" constructed as Bash is a single-pass interpreter language.

When a new line is typed in my terminal, the PROMPT_COMMAND is executed, which effectively retrieve prompt_command from the environment variables (the same way I could type prompt_command in my shell) and execute it line by line. Consequently, the behavior is similar to directly assigning the command to PROMPT_COMMAND. Direct assignation should even be (imperceptibly) faster, as it avoid needing to fetch an environment variable.

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