02. Command Help which, man, apropos, whatis

Oftentimes on the command line, you'll need some help with a command.

We can try Googling our answers, but a lot of what we're looking for is right in the command line!

Checking which executable is run

To see where an executable resides in the file system, we use the which command. This displays the path of the executable that will be executed.

So for example, if you have git installed, try:

$ which git

Opening a command's manual page

If you need help understanding a command's function, we can use the man command, which displays a command's manual page. This is especially useful when we want to apply an option.

$ man ls

To move up/down, use j/k for line by line, or ^d/^u for pages at a time.

To quit the man page, press q.

Manual sections

Each manual page is divied up into sections. To jump to a particular section on a manual page, specify the number before the manual name.

$ man 3 ls
User commands
System calls
Higher-level unix programming library docs
Device and driver info
File descriptions
File formats, conventions and encodings
System commands and servers

Searching for a man page with a keyword

If you want to search for a manual page you may do so with the -k option.

$ man -k search
... apropos(1) - search the whatis database for strings lfind(3), lsearch(3) - linear search and append lkbib(1) - search bibliographic databases ...

Searching for a command with apropos

If you're looking for a command, but not sure where to start looking, we can use apropos, which displays the appropriate commands.

This searches all executables for your key term.

Let's say I want to move my directory, but I either forgot or don't know which command to use. I can try typing:

$ apropos move
... mv(1) - move files rm(1), unlink(1) - remove directory entries rmdir(1) - remove directories

Very conveniently, I'm presented with a variety of commands that have "move" in its title or description.

Checking command description with whatis

Lastly, if we have no idea what a command is, use:whatis, which displays a brief description of a command.

$ whatis ipconfig
ipconfig(8) - view and control IP configuration state

This will give you a brief description of the ls command.

info: an alternative to man

A similar command to man is info, which uses hypertext formatting instead that allows you to traverse from section to section. To see a list of navigation keys, type the 'h' key after opening up an info page.

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