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

    Returns 0 on success.

options:

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

[+] marked option can be specified multiple times

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