Fun With Bash Shell Parameter Expansion

Recently I switched back to bash from zsh for my shell environment. I needed a consistent shell on my local machines as well as on remote servers. One aspect of my bash environment that wasn’t working the way I wanted was displaying the current Git branch and Git status information when the current directory was Git controlled.

In my original attempt at building my prompt I combined PS1 and prompt_command. This worked on OS X machines, but not on Linux-based operating systems. After splitting apart the line of information I wished to display via the prompt_command from the actual prompt (controlled by PS1), none of the PS1 substituitions were working. Here’s the line before:

function prompt_command {
  export PS1="\n\u at \h in \w $(git_prompt_string)\n$ "
}

And here’s the code after:

function prompt_command {
  printf "\n$(id -un) at $(hostname) in ${PWD} $(git_prompt_string)"
}

The PROMPT_COMMAND is set to the function above, and the PS1 prompt has the $:

export PROMPT_COMMAND=prompt_command
export PS1="\n$ "

Instead of using \u for the current user, I’m using id -un. For the hostname, hostname rather than \h. And PWD displays the current working directory in place of \w.

The problem with PWD is that it displays the full path, and I wanted a ~ when in my $HOME directory. Fortunately Steve Losh has already solved this puzzle in his My Extravagent Zsh Prompt posting.

Here’s the solution:

${PWD/#$HOME/~}

It’s deceptively simple, and took me a few minutes to understand, with the help of the Shell Parameter Expansion section of the Bash Manual.

The pattern ${parameter/pattern/string} works in the following manner.

The pattern is expanded to produce a pattern just as in filename expansion. Parameter is expanded and the longest match of pattern against its value is replaced with string. If pattern begins with ‘/’, all matches of pattern are replaced with string. Normally only the first match is replaced. If pattern begins with ‘#’, it must match at the beginning of the expanded value of parameter. If pattern begins with ‘%’, it must match at the end of the expanded value of parameter. If string is null, matches of pattern are deleted and the / following pattern may be omitted. If parameter is ‘@’ or ‘’, the substitution operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with ‘@’ or ‘’, the substitution operation is applied to each member of the array in turn, and the expansion is the resultant list.

What all that means is $HOME is expanded and if it matches the expanded $PWD, starting at the beginning of the string, then the matching characters are replaced with a ~. The key is the # before $HOME.

Here’s the final printf line:

printf "\n$(id -un) at $(hostname) in ${PWD/#$HOME/~} $(git_prompt_string)"A

You can see the complete .bashrc file in my dotfiles repository.