Linux – change the PATH dynamically based on the cwd

bashenvironment-variableslinuxpathshell

I would like to change my $PATH environment variable dependent on my current working directory.

Say I'm in /foo/bar/baz and I have the directories /foo/node_modules/.bin and /foo/bar/baz/node_modules/.bin. I would like to add every possible ./node_modules/.bin recursively to $PATH.

But when I cd into a different directory (like /foo/bar), I want my original, clean $PATH to be restored, and then to start to look for ./node_modules/.bin recursively again.

(I want to solve my own question from npm's issue tracker: Can we add locally installed packages to PATH, too?)

Note: I'm on a Mac, but interested in a general solution.

Best Answer

Introduction

If I understand you correctly, you want to add any directories "$X/node_modules/.bin" where $X is the $PWD or any of its ancestors.

The script at the end of this post should give the behaviour you want. You need to source it in every session where you want it. If you name the file augment_path.sh, then adding this line to your .bashrc should be sufficient:

source augment_path.sh

Discussion

I think garyjohn has the basic approach right, but he's searching all descendents rather than all ancestors.

The $PROMPT_COMMAND variable allows you to specify a command to be executed each time the prompt is displayed. I've added a $PROMPT_COMMAND_OLD variable to allow the original $PROMPT_COMMAND to be restored

It's probably not necessary, but for good form I add a $LAST_WD variable and test to avoid recomputing the path when the directory hasn't changed. You can remove all three lines containing LAST_WD if you like.

The augment_path function scans from $PWD upwards, looking for the target directories in each ancestor and adding any it finds to the path.

They are placed in the path in order, so the deepest such directory will take precedence if there are any conflicts. I assume this is the desired behaviour. If not, change
```
PATH_ADDITION="$PATH_ADDITION:$resolved_target"
```
to
```
PATH_ADDITION="$resolved_target:$PATH_ADDITION"
```
However, these directories will all take precedence over the rest of the path. If you want the rest of the path to take precedence, change:
```
PATH="$PATH_ADDITION:$RAW_PATH"
```
to
```
PATH="$RAW_PATH:$PATH_ADDITION"
```

Script

RAW_PATH="$PATH"
LAST_WD=`pwd`

augment_path() {
    target="node_modules/.bin"
    if [ "$PWD" = "$LAST_WD" ]; then return 0; fi;
    PATH_ADDITION=""
    scandir="$PWD"
    until [ "$scandir" = "" ]; do
    resolved_target="$scandir"/"$target"
    if [ -d "$resolved_target" ]; then
        PATH_ADDITION="$PATH_ADDITION:$resolved_target"
    fi
    scandir="${scandir%/*}"
    done 
    PATH="$PATH_ADDITION:$RAW_PATH"
    LAST_WD=`pwd`
}

PROMPT_COMMAND_OLD="${PROMPT_COMMAND%; augment_path}"
PROMPT_COMMAND="$PROMPT_COMMAND_OLD; augment_path"

Related Solutions

Linux – Change directory upwards to specified goal

Here's a function that does what you want:

cdto () { cd "${PWD%/$1/*}/$1"; }

Here's another handy one:

c2 () {
        local path num
    if (($# != 0))
    then
        path='./'
    fi
    if [[ -z ${1//.} ]]
    then
        num=${#1}
    elif [[ -z ${1//[[:digit:]]} ]]
    then
        num=$1
    else
        echo "Invalid argument"
        return 1
    fi
    for ((i=0; i<num-1; i++))
    do
        path+='../'
    done
    cd $path
}

Usage:

c2 .    # same as cd .
c2 ..   # same as cd ..
c2 ...  # same as cd ../..
c2 3    # also same as cd ../..
c2      # same as cd (which is the same as cd ~)

I thought one of the shells used to have the cumulative dot-dot-dot feature (I even checked Vista just now and it didn't have it although Google claims that some versions of Windows do).

Edit

An undocumented feature of Bash is that a lot of characters are acceptable in function names. As a result, you can do this:

.. () { cd ..; }
... () { cd ../..; }
.... () { cd ../../..; }
..... () { cd ../../../..; }

Windows – How to append to the user %PATH% from command line

It depends on what you're trying to do:

To update the PATH in your current commandline session only, use: set PATH=%PATH%;%FOO%.
To edit it for the current user only, use: setx PATH "%PATH%;%FOO%". Note that this change is not visible in your current command line session; you need to start a new command line.
To edit it for all users on the machine, use: setx /M PATH "%PATH%;%FOO%".

You can view the path by typing ECHO %PATH% in a command line or by checking it in the Windows environment settings.

Also, in Windows 7 and 8, the maximum environment variable string size is 32,767 characters. Although this is also valid for the PATH variable, a command in command console has a maximum length of 8191 characters, so you need to take that into consideration when using the PATH variable in command line commands. Anyhow, you have some more head-space above the 256 characters you have right now.