Bash function that changes directory

Question

I have a common use case that I'd like to write a function for: I often want to cd to some directory relative to some file.

My current workflow looks like this:

$ gem which rspec/core | xargs echo -n | pbcopy
$ cd *paste and delete end until direcory looks right*

_{note: gem which rspec/core prints something like "/Users/joshcheek/.rvm/gems/ruby-1.9.3-p125/gems/rspec-core-2.10.0/lib/rspec/core.rb"}

I'd like it to look like this:

$ gem which rspec/core | 2dir 3

Which will cd me into "/Users/joshcheek/.rvm/gems/ruby-1.9.3-p125/gems/rspec-core-2.10.0" (passing the argument "3" tells it to remove "lib/rspec/core.rb" from the end)

This is the best I've gotten so far:

2dir() {
  read dir
  for i in $(seq 1 $1)
    do
      dir="${dir%/*}"
  done
  cd "$dir"
}

But the cd changes the function's directory, not mine. I've tried swapping it with an alias, but can't figure out how to make anonymous functions or pass the argument.

Answer 1

I'd use:

2dir()
{
    name=${2:?'Usage: 2dir count path'}
    count=$1
    while [[ $count -gt 0 ]]; do name=$(dirname "$name"); ((count--)); done
    cd "$name"
}

and use it as:

2dir 3 $(gem which rspec/core)

This works where your pipeline can't. The cd in the pipe process affects that (sub-)shell, but cannot affect the current directory of the parent process. This function can be made to work.

And you can use your dir="${dir%/*}" in place of my dirname if you prefer, except that you'll end up in your home directory instead of the current directory (or root directory, depending on whether you gave a relative or absolute path name) if you specify 10 when there are only 5 components.

Answer 2

Here's a variant of @Jonathan Leffler's suggestion to streamline usage a little -- it makes the count argument optional, and avoids the need for $( ) around the command:

2dir() {
# If first arg is a number, use it as a trim count; otherwise assume 2
if [[ "$1" =~ ^[0-9]+$ ]]; then
    count="$1"
    shift
else
    count=2
fi

if [[ $# -lt 1 ]]; then  # Make sure a command was specified
    echo "Usage: 2dir [count] command [commandargs ...]" >&2
    return 1
fi

name="$("$@")"  # Execute the remaining args as a command to get the target directory
while [[ $count -gt 0 ]]; do name=$(dirname "$name"); ((count--)); done
cd "$name"
}

Example uses:

2dir 3 gem which rspec/core
2dir gem which rspec/core