[Bug-tar] [PATCH] Add optional numeric argument to -O / --to-stdout

[Alex Bligh]

If an optional numeric argument N is passed to -O / --to-stdout,
only extract the N'th file that would be sent to stdout, not
counting files that are excluded (e.g. with --exclude), directories
etc.

Signed-off-by: Alex Bligh <address@hidden>
---
 doc/tar.texi  |   21 ++++++++++++++-------
 src/common.h  |    2 ++
 src/extract.c |    4 ++++
 src/tar.c     |   14 ++++++++++++--
 4 files changed, 32 insertions(+), 9 deletions(-)

diff --git a/doc/tar.texi b/doc/tar.texi
index 5df61b1..1a529cd 100644
--- a/doc/tar.texi
+++ b/doc/tar.texi
@@ -3349,11 +3349,12 @@ During extraction @command{tar} will pipe extracted 
files to the
 standard input of @var{command}.  @xref{Writing to an External Program}.
 
 @opsummary{to-stdout}
address@hidden --to-stdout
address@hidden -O
address@hidden address@hidden
address@hidden address@hidden
 
 During extraction, @command{tar} will extract files to stdout rather
-than to the file system.  @xref{Writing to Standard Output}.
+than to the file system, or (if @var{number} is supplied) a specific
+file.  @xref{Writing to Standard Output}.
 
 @opsummary{totals}
 @item address@hidden
@@ -5483,16 +5484,22 @@ conjunction with @option{--extract} (@option{--get}, 
@option{-x}).  This option
 extracting files to send them through a pipe, and do not need to
 preserve them in the file system.  If you extract multiple members,
 they appear on standard output concatenated, in the order they are
-found in the archive.
+found in the archive, or a specific file if a numeric parameter
+is passed.
 
 @table @option
 @opindex to-stdout
address@hidden --to-stdout
address@hidden -O
address@hidden address@hidden
address@hidden address@hidden
 Writes files to the standard output.  Use only in conjunction with
 @option{--extract} (@option{--get}, @option{-x}).  When this option is
 used, instead of creating the files specified, @command{tar} writes
-the contents of the files extracted to its standard output.  This may
+the concatenated contents of the files extracted to its standard output.
+If @var{number} is specified, instead of concatenating, only the
+file with index @var{number} that would have been sent is used; in
+calculating the index, directories, excluded files and other files
+that would not have been extracted are ignored. Thus @option{--to-stdout=1}
+will always extract the first file that @option{--to-stdout} would 
concatenate. @option{--to-stdout} may
 be useful if you are only extracting the files in order to send them
 through a pipe.  This option is meaningless with @option{--list}
 (@option{-t}).
diff --git a/src/common.h b/src/common.h
index 9ac22b8..1753a3d 100644
--- a/src/common.h
+++ b/src/common.h
@@ -281,6 +281,8 @@ GLOBAL tarlong tape_length_option;
 
 GLOBAL bool to_stdout_option;
 
+GLOBAL uintmax_t to_stdout_index_option;
+
 GLOBAL bool totals_option;
 
 GLOBAL bool touch_option;
diff --git a/src/extract.c b/src/extract.c
index 319aaa8..b6f1fa3 100644
--- a/src/extract.c
+++ b/src/extract.c
@@ -1439,6 +1439,7 @@ static int
 prepare_to_extract (char const *file_name, int typeflag, tar_extractor_t *fun)
 {
   int rc = 1;
+  static uintmax_t extract_index = 0;
 
   if (EXTRACT_OVER_PIPE)
     rc = 0;
@@ -1529,6 +1530,9 @@ prepare_to_extract (char const *file_name, int typeflag, 
tar_extractor_t *fun)
   if (rc == 0)
     return 0;
 
+  if (to_stdout_index_option && (to_stdout_index_option != ++extract_index))
+    return 0;
+
   switch (old_files_option)
     {
     case UNLINK_FIRST_OLD_FILES:
diff --git a/src/tar.c b/src/tar.c
index 9111433..6fd5bb1 100644
--- a/src/tar.c
+++ b/src/tar.c
@@ -490,8 +490,9 @@ static struct argp_option options[] = {
   {NULL, 0, NULL, 0,
    N_("Select output stream:"), GRID },
 
-  {"to-stdout", 'O', 0, 0,
-   N_("extract files to standard output"), GRID+1 },
+  {"to-stdout", 'O', N_("NUMBER"), OPTION_ARG_OPTIONAL,
+   N_("extract files to standard output, or if NUMBER specified,"
+      " the file with index number NUMBER"), GRID+1 },
   {"to-command", TO_COMMAND_OPTION, N_("COMMAND"), 0,
    N_("pipe extracted files to another program"), GRID+1 },
   {"ignore-command-error", IGNORE_COMMAND_ERROR_OPTION, 0, 0,
@@ -1680,6 +1681,15 @@ parse_opt (int key, char *arg, struct argp_state *state)
 
     case 'O':
       to_stdout_option = true;
+      if (arg)
+       {
+         uintmax_t u;
+         if (xstrtoumax (arg, 0, 10, &u, "") == LONGINT_OK)
+           to_stdout_index_option = u;
+         else
+           FATAL_ERROR ((0, 0, "%s: %s", quotearg_colon (arg),
+                         _("Invalid number")));
+       }
       break;
 
     case 'p':
-- 
1.7.9.5