haxx qua non

less, sudo, and /dev/null: Permission denied

by jja on
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Sigh. The ubiquitous utility less does a really bad and hard to understand thing. For some time now, it behaves not just as a reader but also a writer: it writes a history file (specified by the environment variable LESSHISTFILE which is $HOME/.lesshst by default). If you want less to be writing things, fine, but don't force it on me! This has caused some annoying problems that have been really hard to finally trace back to less.

To turn off this behavior, set the variable LESSHISTFILE (either in the environment or in your lesskey file) to '-'. The docs say you can use '/dev/null' too, but don't-- I'll explain below. Besides writing a file, less goes a step further and does a chmod on that file to 0600, which is actually a reasonable permission set to keep others from seeing your history. The problem is that sudo picks up the HOME variable but not LESSHISTFILE, so sudo less was writing a .lesshst file in my home directory.

Some time ago, I gakked and disgustedly symlinked .lesshst to /dev/null. But then when less does the chmod during sudo, it changes the permissions of /dev/null (which is why LESSHISTFILE set to /dev/null is bad-- less is supposed to ignore it in that case but still somehow chmods /dev/null). So I couldn't use /dev/null anymore (and quite a few scripts do so, it's a standard practice), getting messages like /dev/null: Permission denied.

The workaround to this bug 'feature' of less is to set the LESSHISTFILE variable in the LESSKEY file ($HOME/.less):

% lesskey -
#env
LESSHISTFILE = -
^D

then sudo will pick up $HOME, less will then read LESSHISTFILE from $HOME/.less and not create the stoopid .lesshst, so I don't need the symlink to /dev/null anymore.

There are other reports of changing permissions on /dev/null that are generally unexplained. Try to stay away from symlinks to /dev/null in your home directory, especially any that might get chmod applied by a program you run via sudo. (I still have .evolution and .recently-used symlinked to /dev/null. I don't think I sudo anything using those...)

Comments

Jesper commented at 2011-06-01 07:22:06

Interesting, thanks for the analysis. I just stumbled on this problem myself.

Scenario:

$ export LESSHISTFILE=/dev/null
$ sudo -s
# history|less (or any other invocation of less I guess)
*poof* /dev/null is 700, thus unusable for non-root users.

I guess the sudo -s carried over the LESSHISTFILE variable to the privileged shell, and less got a bit too eager about permissions on what it believed to be its history file.

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