dash does not refresh its hash table when PATH is re-declared

[agarzola]

The following line recommends re-declaring PATH to ensure the new location of Node.js is loaded in the current session.

n/bin/n

Line 744 in d569e33

printf 'To reset the command location hash either start a new shell, or execute PATH="$PATH"\n'

However, in some environments that might not suffice. I ran into this issue on Platform.sh, which uses dash. Interestingly enough, after installing Node with n auto (our project has an .nvmrc file specifying v14), running which node points to the location where we told n to install Node: /app/.global/bin/node. However, running node --version yielded v12. Simply invoking export PATH="$PATH" did not solve this issue.

As it turns out, dash’s implementation of the hash built-in command does not refresh the hash table where executables are stored for easy access. The fix suggested to us by Platform.sh support was to invoke hash -r instead. This worked: after invoking that, node --version yielded the version number we expect (the one installed via n).

I will submit a PR shortly that updates the line linked above to include a note about how to solve this issue in dash.

[shadowspawn]

I remember considering hash -r vs PATH=$PATH but can't find any notes I made at the time.

Related issues mentioned in the CHANGELOG: advice to reset command hash when node location changes (#170 #381 #451 #588)

[shadowspawn]

More documentation:

• https://hyperpolyglot.org/unix-shells#cmd-resolution
• https://man7.org/linux/man-pages/man1/dash.1.html
• https://pubs.opengroup.org/onlinepubs/9699919799/utilities/hash.html

The Open Group docs for hash say this, which is probably why I went for PATH over hash originally:

The effects of hash -r can also be achieved portably by resetting the value of PATH ; in the simplest form, this can be:

PATH="$PATH"

[agarzola]

That is true of Bash’s implementation of hash. Alas, that appears to not be the case in Dash’s implementation, judging by the experience I detailed above, wherein PATH="$PATH" did not have the desired effect, but hash -r did.

[Edited to add more context.]

[shadowspawn]

Yes "portably" is not universally. 🙊

[shadowspawn]

After some experimentation...

bash: PATH=$PATH or hash -r
zsh: PATH=$PATH or hash -r
fish: no action required
csh: rehash
tcsh: rehash
ash: hash -r
dash: hash -r
ksh: hash -r

[agarzola]

Oh, wow. So how about I modify my PR thusly:

To reset the command location hash, start a new shell. If starting a new shell is not an option, here's what you can do:

• For bash and zsh, execute PATH="$PATH" or hash -r.
• For csh and tcsh, execute rehash.
• For ash, dash, and ksh, execute hash -r.

[shadowspawn]

I am disappointed with PATH="$PATH" since it only works in some shells and they both support hash -r anyway!

Moving the command to the front to make it easier to spot, how about:

Note: the node command changed location and the old location may be remembered in your current shell.
         old : /usr/local/bin/node
         new : /root/.n/bin/node
If "node --version" shows the old version then start a new shell, or reset the location hash with:
hash -r  (for bash, zsh, ash, dash, and ksh)
rehash   (for csh and tcsh)

[agarzola]

Agreed, that’s much better. I’ll update my PR with that here shortly!

[agarzola]

@shadowspawn All done! PR is ready for review at your leisure.

[shadowspawn]

Improvements included in n v8.0.2