On Unix how to get the output from the diff command to look like a man page? Where I can press ‘b’ or ‘f’ to read further or back

diff()man

You know how when you use the man command to read the manual (e.g. man ls) and you get output that is paginated that you can read from top to bottom by pressing 'f' to go read further down and 'b' to read further up (assuming you're reading from top to bottom left to right) and you press 'q' to quit. I want the output from diff to behave the same way because sometimes I have a long list of differences to read but I find myself having to scroll all the way up in the terminal to read the beginning of the output.

The Git diff command does this and I've come to like it (e.g. git diff ) but I don't always have the luxury of having all my files under version control.

The best I can think off is piping the out put of the diff command to some other command but I'm not sure which. Can anyone help? Does this sort of output behavior provided (like man pages) have a name for future reference?

Best Answer

It's called "paging output" or (somewhat erroneously) "pagination" …

… and man does it by invoking your preferred pager shell command, named by your PAGER environment variable, upon the output of whatever pipeline was used to generate the output form of the manual page. It falls back to a default if you haven't specified a pager command. On early Unices the default pager program, that man invokes as the default pager command with known hardwired options, was pg. On your system it is probably more or less. Some man commands look at other environment variables for pager commands, as well.

You do it by doing what man is doing: pipe the output of what you want to see into the standard input of a pager program.

You should get yourself a good book on Unix. There are a fair few that explain this, which is a very basic feature of the system. A simple Google Books search for pager unix more by me today turned up over a hundred books that discuss this. (I stopped counting at a hundred.)

Example

$ diff afpuri.c afpuri1.c | diffstat
 unknown |   53 ++++++++++++++++++++---------------------------------
 1 file changed, 20 insertions(+), 33 deletions(-)

This can be used for diff output which includes multiple files in a tree as well.

References

How to get diff to report summary of new, changed and deleted lines

How does `man git init` get the right man page

Some implementations of man, including the one used by Ubuntu, replace spaces in its search terms with hyphens and attempt to find a manual page under that name. So man git init looks for the same thing as man git-init. Similarly, man run parts and man ntfs 3g work (if you have run-parts and ntfs-3g on your system).

It only does this with word pairs, though, so man git annex sync does not work (though man git-annex sync does, as that's again a word pair).

In fact, when you ask for two manual pages (e.g., man git bash to see both the git and bash manpages), man actually first tries to look for a git-bash manpage. You can see this in the debug output if you enable it with -d.

This man feature is called "subpages" you can read the source code implementing subpages in man-db (thanks, Stephen Kitt). Searching the man(1) manpage for "subpages" will also lead you to the description of this behavior under the --no-subpages option:

--no-subpages
      By default, man will try to interpret pairs of manual page
      names given on the command line as equivalent to a single
      manual page name containing a hyphen or an underscore.  This
      supports the common pattern of programs that implement a
      number of subcommands, allowing them to provide manual pages
      for each that can be accessed using similar syntax as would be
      used to invoke the subcommands themselves.  For example:

        $ man -aw git diff
        /usr/share/man/man1/git-diff.1.gz

      To disable this behaviour, use the --no-subpages option.

        $ man -aw --no-subpages git diff
        /usr/share/man/man1/git.1.gz
        /usr/share/man/man3/Git.3pm.gz
        /usr/share/man/man1/diff.1.gz

Best Answer

It's called "paging output" or (somewhat erroneously) "pagination" …

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Example

References

How does `man git init` get the right man page

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