Unknown conditional expression and operators

Question

I needed bash to source my .bashrc upon login to a new terminal on my mac (because it doesn't do that upon login to a new terminal on macs, it only does .bash_profile). I found and successfully used this command and added it to my .bash_profile:

if [ -f ~/.bashrc ] && [ "${SHELL##*/}" == "bash" ]
then
  . ~/.bashrc
fi

There are three things about this command I don't understand.

1. What does the -f mean in the first condition?
2. What does the ##/ indicate in the second condition?
3. I understand that ". ~/.bashrc" is meant to have the shell source that rc, but how in the world does that command accomplish this? What is the "." supposed to indicate here?

Answer 1

1. -f is a predicate that tests to see if a particular file (~/.bashrc in this case) exists.

2. ##*/ as part of a variable substitution means remove something ending in "/" from the beginning of the variable's value -- for example, if SHELL were "/usr/local/bin/bash", it'd remove the "/usr/local/bin/" and just give "bash". In detail: ## means remove the longest match possible of what follows, */ matches anything ending in "/".

   Other related options: # means remove the shortest match (i.e. #*/ would remove through the first "/"), %% means remove the longest possible match from the end, and % means remove the shortest match from the end.

3. . is a shorter name for the source command -- a shell built-in command that executes the contents of a script (~/.bashrc in this case) in the current shell. Normally, if you run a shell script, it's run in a subshell and any variables, aliases, functions etc that it defines are lost when it exits; by sourceing it instead, things it defines are valid in the current shell.

Answer 2

1) checks to make sure the file ~/.bashrc exists

2) ${SHELL} is a variable that contains the full path of the current shell. If you type echo $SHELL or echo ${SHELL} at a terminal, it will print the path. ${SHELL##*/} removes the path and just prints the file name. Type echo ${SHELL##*/} in a terminal to see.

3) the period "." is a built-in bash command to run a script in the current process. I.E. it sources a file.

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So that line basically says, "if the file ~/.bashrc exists and the current shell is a bash shell, then source ~/.bashrc"

My ~/.bash_profile just contains:

source $HOME/.bashrc