Recipe 7.3 Expanding Tildes in Filenames

7.3.1 Problem

You want to open filenames like ~username/blah or ~/.mailrc, but open doesn't interpret the tilde to mean a home directory.

7.3.2 Solution

Either use the glob function:

open(FH, glob("~joebob/somefile")) || die "Couldn't open file: $!";

or expand the filename manually with a substitution:

$filename =~ s{ ^ ~ ( [^/]* ) }
              { $1
                    ? (getpwnam($1))[7]
                    : ( $ENV{HOME} || $ENV{LOGDIR}
                         || (getpwuid($<))[7]
                       )
}ex;

7.3.3 Discussion

There is a useful convention, begun with the Unix csh shell and propagated widely by web addresses of the form http://www.example.com/~user/, that ~ in a filename represents a user's home directory. Thus:

    ~              # current user's home directory
    ~/blah         # file blah in current user's home directory
    ~user          # a particular user's home directory
    ~user/blah     # file blah in a particular user's home directory

Unfortunately, Perl's open function does not expand wildcards, including tildes. As of the v5.6 release, Perl internally uses the File::Glob module when you use the glob operator. So all you need to do is glob the result first.

open(MAILRC, "<", "~/.mailrc")           # WRONG: tilde is a shell thing
    or die "can't open ~/.mailrc: $!";

open(MAILRC, "<", glob("~/.mailrc"))     # so expand tilde first
    or die "can't open ~/.mailrc: $!";

The alternative solution, the substitution, uses /e to evaluate the replacement as Perl code. If a username follows the tilde, it's stored in $1, which getpwnam uses to extract the user's home directory out of the return list. This directory becomes the replacement string. If the tilde was not followed by a username, substitute in either the current HOME environment variable or the LOGDIR one. If neither of those is valid, look up the effective user ID's home directory.

You could spell glob('~gnat') as <~gnat>, but that would look too much like a read from a filehandle, so don't do that.