adding a
# missing arg
hg cat: invalid arguments
hg cat [OPTION]... FILE...

output the current or given revision of files

    Print the specified files as they were at the given revision. If no
    revision is given, the parent of the working directory is used, or tip if
    no revision is checked out.

    Output may be to a file, in which case the name of the file is given using
    a format string. The formatting rules are the same as for the export
    command, with the following additions:

    "%s"   basename of file being printed
    "%d"   dirname of file being printed, or '.' if in repository root
    "%p"   root-relative path name of file being printed

options:

 -o --output   print output to file with formatted name
 -r --rev      print the given revision
    --decode   apply any matching decode filter
 -I --include  include names matching the given patterns
 -X --exclude  exclude names matching the given patterns

use "hg -v help cat" to show global options
% [defaults]
a
a: No such file in rev 000000000000
% no repo
abort: There is no Mercurial repository here (.hg not found)!