Fix for fnm (Fast Node Manager) issues with repath/hash -r and symlink accumulation (bash only, sorry)

fnm-repathing

#!/usr/bin/env bash
#
# This script fixes some outstanding issues with FNM (fast node manager's)
# "multishell" symlink system.  Specifically:
#
# 1) that FNM generates a symlink for every shell it's ever run in, and *never
#    cleans them up*, eventually leading to inode exhaustion.
#
# 2) that many shells (especially bash) do not change their cached PATH lookups
#    when a symlink on PATH is changed, rather than PATH itself being changed.
#
# So, this script fixes these issues (at least for bash) by
#
# 1) wrapping the `fnm` command to detect when it has been run, and update the
#    PATH if needed
#
# 2) reading the FNM-generated symlink to get the node version and update the
#    PATH with it
#
# 3) deleting the symlink after it's read
#
# In this manner, it prevents symlink accumulation because the symlink never
# exists for more than a fraction of a second after fnm creates it.  And the
# rehashing problem doesn't happen because the PATH is changed, which allows the
# shell to notice its cache is out of date.
#
# To use the script, add a line like this to your .bashrc:
#
#    source /wherever/you/put/this/script
#
# right after your `eval "$(fnm env)"` or equivalent, then restart your shell.
# (You can also just source the script in your current shell instead of
# restarting.)
#
# NOTE: Because this workaround deletes the fnm-generated symlink as soon as
# it's created, the `fnm current` command will NOT work properly in
# non-interactive shells (i.e., in scripts), unless you source this file there
# too.  (We recommend using `node --version` or similar instead of using `fnm
# current` in your scripts anyway.)
#
# Also, because the symlink isn't used, it's impossible for a script to change
# the active node version of a parent shell or script.  (This is actually a
# *feature*, for the most part, but if you currently have scripts that rely on
# being able to change a parent shell's node version, they won't work if you're
# using this.)
#
#EOF

if [[ $0 = $BASH_SOURCE ]]; then
    # Script must be sourced, not executed; output docs to stderr
    sed -n -e '2, /^#EOF/ { /^#EOF/d; s/^# \?//; p }' "$0" >&2
    exit 64
fi

# Wrap fnm as a function that does repathing after successful `fnm use`
# and supports `fnm current`
fnm() {
    case "${1-}" in
        current)
            # We replace the built-in `fnm current` because it relies on the symlink
            # we're deleting 
            fnm_repath
            case "$PATH" in
                *"$FNM_BASE/node-versions/"*)
                    : "${PATH#*"$FNM_BASE/node-versions/"}"; echo "${_%%/*}"
                    ;;
                *"$FNM_BASE/aliases/"*)
                    local alias="${PATH/*"$FNM_BASE/aliases/"/"$FNM_BASE/aliases/"}"
                    alias=${alias%%:*}
                    linkpath="$(readlink "$alias")"
                    if [[ $linkpath ]]; then
                        : "${linkpath#*"$FNM_BASE/node-versions/"}"; echo "${_%%/*}"
                    else
                        echo "none"
                    fi
                    ;;
                *) echo "none"
                    ;;
            esac
        ;;
        *)
            PATH="$PATH:$FNM_MULTISHELL_PATH" command fnm "$@" && fnm_repath
        ;;
    esac
}

# If FNM_MULTISHELL_PATH exists, read where it points to and put that on the
# PATH in the first spot where a previous entry pointing into FNM_BASE is found,
# or at the head of the PATH if no such entry exists.  This lets you put things
# in front of FNM on the PATH and keep them there even when swapping versions.
fnm_repath() {
    # Don't repath if symlink already removed
    [[ ! -d "$FNM_MULTISHELL_PATH" ]] || {
        # Get the new path
        local linkpath="$(readlink "$FNM_MULTISHELL_PATH")"
        [[ ! "$linkpath" ]] || {
            # Got a path; move to head of PATH and remove the symlink
            local f p="" parts
            IFS=: read -ra parts <<<"$PATH"  # split on ':'
            # Remove any path pointing into the FNM directory, replacing it
            # with the link path if found
            for f in "${parts[@]}"; do
                if [[ "$f" = "$FNM_BASE"/* ]]; then
                    # Replace old path w/new if this is the first one found
                    [[ ! $linkpath ]] || { p+="$linkpath:"; linkpath=; }
                else
                    p+="$f:"
                fi
            done
            # Put the link path at the start of PATH if there was no old path
            # to replace
            PATH="${linkpath:+"$linkpath":}${p%:}"
            # remove the symlink so there's no symlink buildup
            rm "$FNM_MULTISHELL_PATH"
        }
    }
}

# Remove the multishell path and set up the base
case ${OSTYPE-} in
    cygwin|msys)
        # Remove the multishell path from PATH
        PATH=${PATH#"$(cygpath "$FNM_MULTISHELL_PATH")":}
        # Prefix for PATH matching/updating
        FNM_BASE="$(cygpath "$FNM_DIR")"
    ;;
    *)
        # Remove the multishell path from PATH
        PATH=${PATH#"$FNM_MULTISHELL_PATH":}
        # Prefix for PATH matching/updating
        FNM_BASE="$FNM_DIR"
    ;;
esac

# Now initialize the PATH and get rid of the symlink
fnm_repath