Home / ubuntu / Questions / 1393543
Accepted
wjandrea
Asked: 2022-02-18 16:19:38 +0800 CST

Why does "less" accept an empty filename, and show some environment variables?

  • 772

I accidentally ran less with an empty filename, which I expected to fail, but to my surprise, it printed some environment variables:

$ less -XE ""
export LESSOPEN="| /usr/bin/lesspipe %s";
export LESSCLOSE="/usr/bin/lesspipe %s %s";

(-XE basically makes it behave like cat.)

cat, on the other hand, fails:

$ cat ""
cat: '': No such file or directory

And if you leave out the filename entirely, less does error:

$ less -XE
Missing filename ("less --help" for help)

Why does less do this? I can only imagine it being problematic, because if there's some bug in your code that causes an empty filename, you'll get bogus output. (Like, say, less "$(which nonexistent-script)".)

While we're here, why does it print those environment variables? Are they from my environment? They're identical:

$ declare -p LESSOPEN LESSCLOSE 
declare -x LESSOPEN="| /usr/bin/lesspipe %s"
declare -x LESSCLOSE="/usr/bin/lesspipe %s %s"

I checked man less but the filename parameter doesn't seem to be documented explicitly. Other mentions of it didn't seem to be related to this behaviour. I also checked the less FAQ.

command-line

1 Answers

  1. Best Answer

    The root problem is that less doesn't properly quote the filename it's passing to the LESSOPEN command: what it does is "Insert a backslash before each metacharacter", so that means it doesn't handle null strings at all. You can see it works as expected if you bypass LESSOPEN:

    $ less --no-lessopen ''
: No such file or directory

    On Ubuntu (plus Debian, I believe), by default, LESSOPEN is configured by running lesspipe with no arguments, which prints the necessary environment variables to set itself as the LESSOPEN handler. See Can you explain eval expression in the default .bashrc. So when running less with a null argument, the improper quoting deletes the argument being passed to lesspipe, and lesspipe prints its config.

    P.S.

    I considered opening a bug against less, but this hasn't been a big problem for me. If anyone wants to take the initiative, by all means, and let me know if you need anything. One possible solution (but not backwards compatible) is that less wouldn't try to quote the filename at all, instead the filename would need to be quoted in the config, like LESSOPEN='| /usr/bin/lesspipe "%s"'.

    It might also be a good idea to open a bug against lesspipe to recommend a more verbose configuration usage.

    • 4