The LESS environment variable to man and how is it set up

Man is calling Less; the only control at the man level is choosing which options to call Less with.

Less's search case-sensitivity is controlled by two options.

• If -I is in effect, then searches are case-insensitive: either a or A can be used to match both a and A.
• If -i is in effect but not -I, then searches are case-insensitive, but only if the pattern contains no uppercase letter.

If you make -I a default option for Less, then all searches will be case-insensitive even in man pages.

Man-db passes extra options to the pager via the LESS environment variable, which Less interprets in the same way as command line options. The setting is hard-coded at compile time and starts with -i. (The value is "-ix8RmPm%s$PM%s$" as of Man-db 2.6.2; the P…$ part is the prompt string.)

If you don't want searches in man pages to be case-sensitive, or if you want them to be always case-insensitive, there is no way to configure this in man-db itself. You can make an alias for man or a wrapper script that manipulates the LESS enviroment variable, as Man-db prepends its content to the current value if present:

alias man='LESS="$LESS -I" man'

To turn off the -i option and thus make searches always case-sensitive by default in man pages:

alias man='LESS="$LESS -+i" man'

You can also hard-code a different value for LESS by setting the MANLESS environment variable, but if you do that, then man just sets LESS to the value of MANLESS, you lose the custom title line (“Manual page foo(42)”) and other goodies (in particular, make sure to include -R for bold and underline formatting).

If you're using Bash 4 or later, you can use coprocesses:

function f () { a=3; b=4 ; echo "`date`: $a $b"; }
coproc cat
f >&${COPROC[1]}
exec {COPROC[1]}>&-
read c <&${COPROC[0]}
echo a $a
echo b $b
echo c $c

will output

a 3
b 4
c Sun Jun 29 10:08:15 NZST 2014: 3 4

coproc creates a new process running a given command (here, cat). It saves the PID into COPROC_PID and standard output/input file descriptors into an array COPROC (just like pipe(2), or see here or here).

Here we run the function with standard output pointed at our coprocess running cat, and then read from it. Since cat just spits its input back out, we get the output of the function into our variable. exec {COPROC[1]}>&- just closes the file descriptor so that cat doesn't keep waiting forever.

Note that read takes only one line at a time. You can use mapfile to get an array of lines, or just use the file descriptor however you want to use it in a different way.

exec {COPROC[1]}>&- works in current versions of Bash, but earlier 4-series versions require you to save the file descriptor into a simple variable first: fd=${COPROC[1]}; exec {fd}>&-. If your variable is unset it will close standard output.

If you're using a 3-series version of Bash, you can get the same effect with mkfifo, but it's not much better than using an actual file at that point.