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Subject: hash -l with empty hash table prints to stdout
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Xref: csiph.com gnu.bash.bug:16382

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -g -O2 -fdebug-prefix-map=3D/build/bash-2bxm7h/bash-5.=
0=3D. -fstack-protector-strong -Wformat -Werror=3Dformat-security -Wall -W=
no-parentheses -Wno-format-security
uname output: Linux XXX 4.19.0-9-amd64 #1 SMP Debian 4.19.118-2+deb10u1 (2=
020-06-07) x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.0
Patch Level: 3
Release Status: release

Description:
	(I used `bashbug' from my distributor's pre-compiled package, but,
	looking at the current bash source code, I strongly believe this
	bug still exists, and you can completely ignore the "Configuration
	Information" section above.)

	`hash -l' is supposed to generate output that is reusable as input
	(`help hash' says so, at least). In case the hash table is empty, a
	string not re-usable as input is output to stdout. The exit code
	indicates success in this case. The string being output is locale-
	dependant (and therefore is hard to filter out in a work-around).

	$ hash -r
	$ hash -l
	hash: hash table empty
	$ echo $?
	0
	$ hash -l > /dev/null # no output, the message is printed to stdout
	$ hash -l 2> /dev/null # the message is not printed to stderr
	hash: hash table empty

	A work-around bash function to save the current hash table (in a re-
	usable format) would need to do something along these lines:

	saved_hash_table() {
		local hash_table=3D"$( LANG=3DC hash -l )"
		# Let's hope, the string for an empty hash table never changes...
		[[ "$hash_table" =3D~ "hash table empty"$ ]] && hash_table=3D
		printf %s "$hash_table"
	}

Repeat-By:
	$ LANG=3DC bash
	$ hash -r
	$ $( hash -l ) # should always succeed, I think
	bash: hash:: command not found
	$ echo $?
	127

	Even weirder after creating an actual script named "hash:" (note
	trailing colon):

	$ cat /usr/local/bin/hash:
	#!/bin/sh
	echo do harm
	$ hash -r
	$ $( hash -l )
	do harm

Fix:
	I've never looked at the bash sources before, but the conundrum
	appears to be in line 138 in "bash-5.0/builtins/hash.def":

		printf (_("%s: hash table empty\n"), this_command_name);

	A bash script should be able to get a really re-usable output from
	`hash -l', at least if the return code of `hash' indicates success.

	So, either of the following changes should be applied:
	1. (preferable, in my opinion) Output the message about the empty hash
	table to stderr instead of stdout, that is, change the afore-mentioned
	line to
		fprintf (stderr, _("%s: hash table empty\n"), this_command_name);
	Saving the hash table in a script would be something along
		saved_hash_table=3D"$( hash -l 2> /dev/null )" || handle_error
	(not sure if any errors (needed to be handled in some cases) can
	happen at all here).
	2. Don't output anything (or a single newline-character only) in case
	the hash table is empty. Saving the hash table would be simply:
		saved_hash_table=3D"$( hash -l )" || handle_error
	with the draw-back of less informative output for interactive users.
	3. Consider an empty hash table an error so at least something along
	the following would work:
		saved_hash_table=3D"$( hash -l )" || saved_hash_table=3D
	This possibly makes other errors hard to detect, though
	`$saved_hash_table' would result in a loss of efficiency only, not in
	malfunctioning software (or a security risk, my "do harm"-example is a
	little lame, I confess, I currently don't quite see a real-life
	exploitable security bug here), so ignoring a possible error (whatever
	this should be) is probably a minor thing.