From: simon <simon@cda61777-01e9-0310-a592-d414129be87e>
Date: Mon, 8 Sep 2008 18:26:51 +0000 (+0000)
Subject: Add a "-r" option to lns, which causes it to mirror an entire
X-Git-Url: https://git.distorted.org.uk/~mdw/sgt/utils/commitdiff_plain/53ddbe1a6b31f834ed918d41fcecbb76e08f87bd

Add a "-r" option to lns, which causes it to mirror an entire
directory tree in symlinks by constructing a parallel directory
hierarchy and populating it with symlinks to all the files in the
original one. I've wanted a tool that does that for ages, but never
got round to sitting down and writing it; today I suddenly realised
it's almost trivial to add as an extra option in lns, because nearly
all the required infrastructure is already present.


git-svn-id: svn://svn.tartarus.org/sgt/utils@8171 cda61777-01e9-0310-a592-d414129be87e
---

diff --git a/lns/lns b/lns/lns
index db35f9a..c8f20a7 100755
--- a/lns/lns
+++ b/lns/lns
@@ -25,6 +25,7 @@
 #      even if file2 is a link to a directory. This option implies -f.
 
 use Cwd;
+use POSIX; # for opendir and friends
 
 $usage =
   "usage: lns [flags] srcfile destfile\n".
@@ -32,6 +33,8 @@ $usage =
   "where: -a               create symlinks with absolute path names\n".
   "       -f               overwrite existing symlink at target location\n".
   "       -F               like -f, but works even if target is link to dir\n".
+  "       -r               recursively construct a directory tree which\n".
+  "                        mirrors the source, with symlinks to all files\n".
   "       -v               verbosely log activity (repeat for more verbosity)\n".
   "       -q               suppress error messages on failure\n".
   " also: lns --version    report version number\n" .
@@ -61,7 +64,7 @@ $licence =
   "CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE\n" .
   "SOFTWARE.\n";
 
-$abs=$force=$quiet=$verbose=$FILE=0;
+$abs=$force=$quiet=$verbose=$recurse=$FILE=0;
 while ($_=shift @ARGV) {
   last if /^--$/;
   unshift (@ARGV, $_), last unless /^-(.*)/;
@@ -83,6 +86,7 @@ while ($_=shift @ARGV) {
 	if ($opt eq "a") { $abs=1; }
 	elsif ($opt eq "f") { $force=1; }
 	elsif ($opt eq "q") { $quiet=1; }
+	elsif ($opt eq "r") { $recurse=1; }
 	elsif ($opt eq "v") { $verbose++; }
 	elsif ($opt eq "F") { $force=$FILE=1; }
 	else { die "lns: unrecognised option '-$1'\n"; }
@@ -130,10 +134,17 @@ sub makelink {
   # name we will reference the source by.
   $sourcename = $abs ? $source : &relname($source, $target);
 
+  my $donothing = 0;
+  my $recursing = $recurse && -d $source;
+  my $ok;
   # If the target exists...
   if (-e $target || readlink $target) {
-    # If it's a symlink and we're in Force mode, remove it and carry on.
-    if ($force && readlink $target) {
+    if ($recursing && -d $target) {
+      # If it's a directory and we're in recursive mode, just do nothing
+      # and work around it.
+      $donothing = 1;
+    } elsif ($force && readlink $target) {
+      # If it's a symlink and we're in Force mode, remove it and carry on.
       unlink $target || die "lns: unable to remove link $target\n";
       # Report that if in Verbose mode.
       warn "lns: removing existing target link $target\n" if $verbose;
@@ -145,9 +156,37 @@ sub makelink {
     }
   }
 
-  # Make the link.
-  warn "lns: linking $source: $target -> $sourcename\n" if $verbose;
-  symlink($sourcename, $target) || die "lns: unable to make link to $target\n";
+  if ($recursing) {
+    # Make the directory.
+    if ($donothing) {
+      warn "lns: directory $target already exists, no need to create it\n"
+          if $verbose;
+      $ok = 1;
+    } else {
+      warn "lns: making directory $target\n"
+          if $verbose;
+      if (mkdir $target) {
+        $ok = 1;
+      } else {
+        warn "lns: unable to make directory '$target': $!\n";
+      }
+    }
+    # Now recurse into it.
+    if ($ok) {
+      my $dh = POSIX::opendir($source);
+      my @files = POSIX::readdir($dh);
+      my $f;
+      POSIX::closedir($dh);
+      foreach $f (@files) {
+        next if $f eq "." or $f eq "..";
+        &makelink("$source/$f", "$target/$f");
+      }
+    }
+  } else {
+    # Make the link.
+    warn "lns: linking $source: $target -> $sourcename\n" if $verbose;
+    symlink($sourcename, $target) || die "lns: unable to make link to $target\n";
+  }
 }
 
 sub normalise {
diff --git a/lns/lns.but b/lns/lns.but
index 8bfd953..5caab26 100644
--- a/lns/lns.but
+++ b/lns/lns.but
@@ -36,6 +36,17 @@ programs}, exactly as you would have done if the command had been
 \cw{cp}; and \cw{lns} will figure out for itself that the literal
 text of the symlink needs to be \c{../hello.c}.
 
+\cw{lns} also has a mode in which it will create a symlink mirror of
+an entire directory tree: that is, instead of creating a single
+symlink to the root of the tree, it will create \e{directories} in
+the same structure as the whole of the original tree, and fill them
+with individual symlinks to the files. This is occasionally handy if
+you want to work with a slightly modified version of a large file
+hierarchy but you don't want to waste the disk space needed to
+create an entirely separate copy: you can symlink-mirror the whole
+tree, and then just replace one or two of the symlinks with modified
+versions of the files they point to.
+
 \U ARGUMENTS
 
 If you provide precisely two arguments to \cw{lns}, and the second
@@ -93,6 +104,23 @@ second argument as a destination \e{file} name rather than a
 destination directory. This option is useful for overriding an
 existing link to one directory with a link to a different one.
 
+\dt \cw{-r}
+
+\dd Enables recursive link-tree construction. If the source pathname
+exists and is a directory, then instead of creating a symlink to it
+at the target site, \cw{lns} will create a fresh directory, and then
+recursively attempt to link every file inside the source directory
+to the inside of the new target directory.
+
+\lcont{
+
+If a directory already
+exists at the target site, \cw{lns} will recurse into it; so you
+can, for instance, use \cw{lns -r -f} to refresh an existing link
+tree.
+
+}
+
 \dt \cw{-v}
 
 \dd Verbose mode: makes \cw{lns} talk about what it is doing. You
@@ -191,6 +219,26 @@ In order to overwrite the directory symlink correctly, you need the
 overwriting. So now you get what you wanted: the previous symlink
 \cw{subdir3} is replaced with one whose link text reads \cq{subdir}.
 
+Next, a couple of examples with \cw{-r}. Suppose you have your
+subdirectory \cw{subdir}. Then running
+
+\c $ lns -r subdir subdir-mirror
+\e   bbbbbbbbbbbbbbbbbbbbbbbbbbb
+
+will create a new subdirectory called \c{subdir-mirror}, containing
+symlinks to everything in \c{subdir}.
+
+If the directory \c{subdir-mirror} already existed, however,
+\cw{lns}'s command-line processing will notice that it's a
+directory, and will assume things are supposed to be copied \e{into}
+it, so that your mirror of \c{subdir} will end up at
+\c{subdir-mirror/subdir}. To fix this, you can again use \cw{-F}, to
+tell \cw{lns} to literally create its output at the precise location
+you specify rather than inside it:
+
+\c $ lns -rF subdir subdir-mirror
+\e   bbbbbbbbbbbbbbbbbbbbbbbbbbbb
+
 \U LICENCE
 
 \cw{lns} is free software, distributed under the MIT licence. Type