Search – Can I Force `man` to Do Lower Case Sensitive Matching?

A case-insensitive filesystem just means that whenever the filesystem has to ask "does A refer to the same file/directory as B?" it compares the names of files/directories ignoring differences in upper/lowercase (exactly what upper/lowercase differences count depends on the filesystem—it's non-obvious once you get beyond ASCII). A case-sensitive filesystem does not ignore those differences.

A case-preserving filesystem stores file names as given. A non-case-preserving filesystem does not; it'll typically convert all letters to uppercase before storing them (theoretically, it could use lowercase, or RaNsOm NoTe case, or whatever, but AFAIK all real-world ones used uppercase).

You can put those two attributes together in any combination. I'm not sure if you can find non-case-preserving case-sensitive filesystems, but you could certainly create one. All the other combinations exist or existed in real systems, though.

So a case-preserving, case-insensitive filesystem (the most common type of case-insensitive filesystem nowadays) will store and return file names in whatever capitalization you created them or last renamed them, but when comparing two file names (to check if one exists, to open one, to delete one, etc.) it'll ignore case differences.

When you use a case-insensitive filesystem on a Unix box, various utilities will do weird things because Unix traditionally uses case-sensitive filesystems—so they're not expecting Document1 and document1 to be the same file.

In the pwd case, what you're seeing is that it by default just outputs the path you actually used to get to the directory. So if you got there via cd DirName, it'll use DirName in the output. If you got there via DiRnAmE, you'll see DiRnAmE in the output. Bash does this by keeping track of how you got to your current directory in the $PWD environment variable. Mainly this is for symlinks (if you cd into a symlink, you'll see the symlink in your pwd, even though it's actually not part of the path to your current directory). But it also gives the somewhat weird behavior you observe on case-insensitive filesystems. I suspect that pwd -P will give you the directory name using the case stored on disk, but haven't tested.

The apropos command reads from a cache. It's possible that recently-installed man pages are not yet referenced in that cache. Generally the cache is updated either after each package installation or in a cron job, I don't know how it's set up on CentOS.

To update the cache, run mandb. There may be a prefered way to run it on your distribution (e.g. as a dedicated user), see if you have a system crontab entry (e.g. /etc/cron.daily/man-db or a line in /etc/crontab) and if you do, run that instead of calling mandb directly.