Why does “less” sometimes forget its history

command historyless

I set and exported $LESSHISTFILE and $LESSHISTSIZE but if I perform a search in one less session, close, reopen it and try to repeat my last search I get "No previous regular expression (press RETURN)".

It works as expected in Ubuntu 14.04 but not in Fedora release 7. Any suggestion?

It's writing to $LESSHISTSIZE. I get the same text written to the history file on both Ubuntu and Fedora. Below I started with an empty history file and then executed cal | less. I searched for "xxx" while in less. But when I exit and rerun the less command it gives me the error message if try to repeat the search with just the / command.

$ cal | less
$ ls -l  $LESSHISTFILE
-rw------- 1 cs cs 40 Aug  4 14:23 .lesshist
$ cat  $LESSHISTFILE
.less-history-file:
.search
"xxx
.shell
$

Maybe it's because the version on Fedora is so old.

$ less --version
less 394
Copyright (C) 1984-2005 Mark Nudelman

Best Answer

Ubuntu 14.04 has less 458, and Fedora 7 has less 394. The NEWS file between those versions is the place to look:

            Major changes between "less" versions 394 and 406

    * Allow decimal point in number for % (percent) command.

    * Allow decimal point in number for -j option (fraction of screen height).

    * Make n command fetch previous pattern from history file on first search.

For that, you have to look at the sources (I'm looking at the file from less 451), because the online history doesn't go back that far. You can get older versions from ftp://ftp.gnu.org/gnu/less/

Since answering this in 2016, the RCS for less was exported to git here, apparently in mid-2017, and one can point more accurately to the change (early 2007). Exactly when the switch was done is unclear, since the repository commits are unverified.

Related Solutions

Why Does the Exclamation Mark `!` Sometimes Upset Bash?

The ! character invokes bash's history substitution (enabled by default in interactive shells). When followed by a string (as in your failing example) it tries to expand to the last history event that began with that string. Just like $var gets expanded to the value of that string, !echo would expand to the last echo command in your history.

Space is a breaking character in such expansions. First note how this would work with variables:

# var="like"
# echo "$var"
like
# echo "$"
$
# echo "Do you $var frogs?"
Do you like frogs?       <- as expected, variable name broken at space
# echo "Do you $varfrogs?"
Do you?                  <- $varfrogs not defined, replaced with blank
# echo "Do you $ var frogs?"
Do you $ var frogs?      <- $ not a valid variable name, ignored

The same thing will happen for history expansion. The bang character (!) starts off a history replacement sequence, but only if followed by a string. Following it with a space make it literal bang instead of part of a replace sequence.

You can avoid this kind of replacement for both variable and history expantion by using single quotes. Your first examples used single quotes and so ran fine. Your last examples are in double quotes and thus bash scanned them for expantion sequences before it did anything else. The only reason the first one didn't trip is that that the space is a break character as shown above.

Bash – sometimes history commands are not stored in .bash_history

The most likely cause for history items not to show up is not by setting HISTFILE to nothing or HISTSIZE to zero, it is by logging into the same machine twice and exiting with the second bash instance (in which you did little or nothing) after the one where you did a lot. By default Bash doesn't merge histories and the second Bash-exit overwrites the .bash_history that was so nicely update by the first Bash-exit.

To prevent this from happening you can append to the history file instead of overwriting, you can use the histappend shell option:

If the histappend shell option is enabled (see the description of shopt under SHELL BUILTIN COMMANDS below), the lines are appended to the history file, otherwise the history file is overwritten.

More details in this answer including how to use HISTSIZE, HISTFILESIZE and HISTCONTROL to control size, duplicates etc.

Best Answer

Related Solutions

Why Does the Exclamation Mark `!` Sometimes Upset Bash?

Bash – sometimes history commands are not stored in .bash_history

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