How to make `man` work for shell builtin commands and keywords?

Question

I use the man command all the time when I want to get information about a specific command. But this doesn't help me too much when that specific command is a shell builtin. For example:

man cd

returns:

No manual entry for cd

My question is: it is possible to make man also work for all shell builtin commands (like cd, alias, history, etc.), and keywords (like if, while, [[, {, etc.)?

Answer 1

The help command when is used with -m option can display information about builtin commands in pseudo-manpage format. For example:

help -m cd | less

will display information about cd command in a format almost exactly like in a manual page.

Starting from this command you can wrap man command in one function in your .bashrc file as follow:

man () {
    case "$(type -t -- "$1")" in
    builtin|keyword)
        help -m "$1" | sensible-pager
        ;;
    *)
        command man "$@"
        ;;
    esac
}

After this man will work also for all shell builtin commands and keywords. For example:

man :

will display:

NAME
    : - Null command.

SYNOPSIS
    :

DESCRIPTION
    Null command.

    No effect; the command does nothing.

    Exit Status:
    Always succeeds.

SEE ALSO
    bash(1)

IMPLEMENTATION
    GNU bash, version 4.2.45(1)-release (x86_64-pc-linux-gnu)
    Copyright (C) 2011 Free Software Foundation, Inc.
    License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

Answer 2

man bash-builtins

This contains help snippets for the builtin commands, albeit in a slightly more condensed format than the help equivalent.

Answer 3

You can install manual pages about using a POSIX system for development as,

sudo apt-get install manpages-posix-dev

It will provide man pages for shell builtins.

$ type cd
cd is a shell builtin

Now try,

$ man cd
CD(P)                      POSIX Programmer's Manual                      CD(P)

NAME
   cd - change the working directory

SYNOPSIS
   cd [-L | -P] [directory]


...

Answer 4

This solution works perfectly well but is a bit of a joke as well because the first thing I thought when I read your question was 'Who still literally uses man from the command line? Doesn't everyone just Google the man page they want (so that they get fancy things like unlimited scrolling)?'. Then I realized that the sites I Google usually all have both types of commands so why not just use them to provide a uniform man page interface across all commands. Hence, this fun was born.

This requires an Internet connnection for any entries you haven't already looked up at least once. It also needs these two small apps which are missing in a default installation of Ubuntu:

 sudo apt-get install tidy html2text

These aren't absolutely needed but they do help make it look a little nicer. Tidy will clean the HTML and html2text will format that html as formatted text (which is usually pretty trivial since most of these sites are already text formatted and just wrapped in <pre> tags.

Now all you need to do is add this to the end of ~/.profile:

function iman() {
    if [ ! -d "/usr/share/iman" ]; then
        sudo mkdir -m a=rwx /usr/share/iman
    fi
    if [ ! -f "/usr/share/iman/$1.html" ]; then
        curl "http://unixhelp.ed.ac.uk/CGI/man-cgi?$1"| tidy -n -asxml 2>/dev/null| html2text > "/usr/share/iman/$1.html"; 
    fi
    if [ -f "/usr/share/iman/$1.html" ]; then
        cat "/usr/share/iman/$1.html";
    else
        echo "Entry not found."
    fi
}

After you logout and then back in you should be able to type this:

iman cd

and it'll display the man page for cd.

This uses a data directory (/usr/share/iman) in order to minimize our network requirements (so it'll work for entries you've already found before even without the connection; also to minimize load on this random linux man pages site I found with the system entries we want in it as well). If you don't use this anymore you'll want to remove that to recover disk space.

Hopefully, the rest is pretty straight forward.