Should I use quotes in environment path names?

I'm in the process of cleaning up all my config files in an attempt to make them as readable as possible. I've been looking for a style guide on the use of quotes while exporting paths in, for example, a ~/.bashrc file:

export PATH="/users/me/path:$PATH"

vs

export PATH=/users/me/path:$PATH

The Google shell style guide suggests avoiding quotes for path names. In contrast, a lot of the popular dotfiles repos (such as Zach Holman's here) use quotes. Are there any situations when it is an advantage to use quotes in the path?

^{Tip of the hat to @gniourf_gniourf and @chepner for their help.}

tl;dr

To be safe, double-quote: it'll work in all cases, across all POSIX-like shells.

If you want to add a ~-based path, selectively leave the ~/ unquoted to ensure that ~ is expanded; e.g.: export PATH=~/"bin:$PATH". ^{See below for the rules of ~ expansion in variable assignments.}
Alternatively, simply use $HOME inside a single, double-quoted string:
export PATH="$HOME/bin:$PATH"

---

NOTE: The following applies to bash, ksh, and zsh, but NOT to (mostly) strictly POSIX compliant shells such as dash; thus, when you target /bin/sh, you MUST double-quote the RHS of export.^[1]

• Double-quotes are optional, ONLY IF the literal part of your RHS (the value to assign) contains neither whitespace nor other shell metacharacters.
• Whether the values of the variables referenced contain whitespace/metacharacters or not does not matter - see below.
  • Again: It does matter with sh, when export is used, so always double-quote there.

The reason you can get away without double-quoting in this case is that variable-assignment statements in POSIX-like shells interpret their RHS differently than arguments passed to commands, as described in section 2.9.1 of the POSIX spec:

• Specifically, even though initial word-splitting is performed, it is only applied to the unexpanded (raw) RHS (that's why you do need quoting with whitespace/metacharacters in literals), and not to its results.

• This only applies to genuine assignment statements of the form
  <name>=<value> in all POSIX-like shells, i.e., if there is no command name before the variable name; note that that includes assignments prepended to a command to define ad-hoc environment variables for it, e.g., foo=$bar cmd ....

• Assignments in the context of other commands should always be double-quoted, to be safe:

  • With sh (in a (mostly) strictly POSIX-compliant shell such as dash) an assignment with export is treated as a regular command, and the foo=$bar part is treated as the 1st argument to the export builtin and therefore treated as usual (subject to word-splitting of the result, too).
    (POSIX doesn't specify any other commands involving (explicit) variable-assignment; declare, typeset, and local are nonstandard extensions).

  • bash, ksh, zsh, in an understandable deviation from POSIX, extend the assignment logic to export foo=$bar and typeset/declare/local foo=$bar as well. In other words: in bash, ksh, zsh, export/typeset/declare/local commands are treated like assignments, so that quoting isn't strictly necessary.

    • Perhaps surprisingly, dash, which also chose to implement the non-POSIX local builtin^[2] , does NOT extend assignment logic to it; it is consistent with its export behavior, however.

  • Assignments passed to env (e.g., env foo=$bar cmd ...) are also subject to expansion as a command argument and therefore need double-quoting - except in zsh.

    • That env acts differently from export in ksh and bash in that regard is due to the fact that env is an external utility, whereas export is a shell builtin.
      (zsh's behavior fundamentally differs from that of the other shells when it comes to unquoted variable references).

• Tilde (~) expansion happens as follows in genuine assignment statements:

  • In addition to the ~ needing to be unquoted, as usual, it is also only applied:
    • If the entire RHS is ~; e.g.:
      • foo=~ # same as: foo="$HOME"
    • Otherwise: only if both of the following conditions are met:
      • if ~ starts the string or is preceded by an unquoted :
      • if ~ is followed by an unquoted /.
      • e.g.,
        foo=~/bin # same as foo="$HOME/bin"
foo=$foo:~/bin # same as foo="$foo:$HOME/bin"

Example

This example demonstrates that in bash, ksh, and zsh you can get away without double-quoting, even when using export, but I do not recommend it.

#!/usr/bin/env bash
# or ksh or zsh - but NOT /bin/sh!

# Create env. variable with whitespace and other shell metacharacters
export FOO="b:c &|<> d"

# Extend the value - the double quotes here are optional, but ONLY 
# because the literal part, 'a:`, contains no whitespace or other shell metacharacters.
# To be safe, DO double-quote the RHS.
export FOO=a:$foo # OK - $FOO now contains 'a:b:c &|<> d'

---

^{[1] As @gniourf_gniourf points out: Use of export to modify the value of PATH is optional, because once a variable is marked as exported, you can use a regular assignment (PATH=...) to change its value.
That said, you may still choose to use export, so as to make it explicit that the variable being modified is exported.}

^{[2] @gniourf_gniourf states that a future version of the POSIX standard may introduce the local builtin.}