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Install Missing Man Pages Of Commands On Ubuntu

How To Install Missing Man Pages Of Commands On Ubuntu

By sk
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The other day I was testing alias and unalias commands on my Ubuntu 18.04 desktop. I wanted to refer all available arguments, flags and options of the alias command, so I opened the man page. But, I got the following message when I tried to open the man pages of alias or unalias commands.

$ man alias
No manual entry for alias
$ man unalias 
No manual entry for unalias

It turns out that man pages are missing for some other programs, such as export and eval.

Man Pages Are Missing For Some Programs

There is no other way except googling to learn about these programs.

On RPM-based systems(E.g. CentOS), if you run "man alias" command, it will open man pages for BASH BUILTINS. You may need to go through the whole man page to find details of the respective command.

After a bit of Google search, I learned that there is no dedicated manual pages for shell builtins, like alias, export, eval in Linux Programmer's manual. If you're not sure whether the given command is an alias, shell built-in, file, function, or keyword, use type command to find out.

$ type alias
alias is a shell builtin

The documentation for these builtins are available in the related shell man page. As you can see in the above output, alias is a shell builtin, so you can view the documentation in the associated shell man page. In my case, it is BASH.

Let us open the man pages of BASH:

$ man bash

Search for alias or unalias entries in the Bash man page.

Here is the information of alias and unalias:

alias man page


Tip: Having trouble at finding something in the man pages? Refer the following guide.


View information about shell builtins using "help" command

Alternatively, you can get the information of shell builtins using help command like below.

$ help alias

Sample output:

alias: alias [-p] [name[=value] ... ]
    Define or display aliases.
    
    Without arguments, `alias' prints the list of aliases in the reusable
    form `alias NAME=VALUE' on standard output.
    
    Otherwise, an alias is defined for each NAME whose VALUE is given.
    A trailing space in VALUE causes the next word to be checked for
    alias substitution when the alias is expanded.
    
    Options:
      -p	print all defined aliases in a reusable format
    
    Exit Status:
    alias returns true unless a NAME is supplied for which no alias has been
    defined.

If you prefer man page format, simply use -m flag with help command below.

$ help -m alias

Sample output:

alias help section in man page format

All bash builtins have help pages. Even help command itself has a help page.

$ help help
help: help [-dms] [pattern ...]
    Display information about builtin commands.
    
    Displays brief summaries of builtin commands.  If PATTERN is
    specified, gives detailed help on all commands matching PATTERN,
    otherwise the list of help topics is printed.
    
    Options:
      -d	output short description for each topic
      -m	display usage in pseudo-manpage format
      -s	output only a short usage synopsis for each topic matching
    		PATTERN
    
    Arguments:
      PATTERN	Pattern specifiying a help topic
    
    Exit Status:
    Returns success unless PATTERN is not found or an invalid option is given.

This is how you can find the information of shell builtins.

Now, let us get back to the topic. Is there any way to install missing man pages of commands? That's what we are going to do now.

Install Missing Man Pages Of Commands On Ubuntu

As stated earlier, Builtins are part of the shell. Each shell has its own set of builtins. They are not independent commands and they don't have separate man pages. Luckily, the man pages of shell builtins are available in POSIX Programmer's Manual. You need to install it to access those man pages.

On Debian, Ubuntu and other DEB-based systems, simply run the following command to install POSIX Programmer's Manual:

$ sudo apt install manpages-posix

Now, you can access the man pages of a shell builtin (E.g. alias) using command:

$ man alias

POSIX Programmer's Manual


Suggested read:


Hope this helps.

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1 comment

Alexander Sofianos March 14, 2020 - 3:16 am

Really helpful article for newbies like myself.
Thanks a lot!

Reply

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