losetup: add a new option -s

The losetup provides an -f option to discover an unused loop device.  As
implemented, it is racy. The problem is that -f prints the loop device
to stdout only when used standalone.  This means a script has to do:

 lodev=$(losetup -f)
 losetup $lodev $filename

which is racy if another script may be running.

This patch add a new option '-s'. The '-s' option prints device name
if the -f option and a file argument are present. For example:

 lodev=$(losetup -f -s $filename)

Signed-off-by: Karel Zak <kzak@redhat.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/mount/losetup.8 b/mount/losetup.8
index 1bfcf883c..5b44a4d8a 100644
--- a/mount/losetup.8
+++ b/mount/losetup.8
@@ -37,7 +37,7 @@ Setup loop device:
 .IR pfd ]
 .RB [ \-r ]
 .in +8
-.RB { \-f | \fIloop_device\fP }
+.RB { \-f [ \-s ] | \fIloop_device\fP }
 .I file
 .in -13
 .ad b
@@ -90,6 +90,13 @@ Read the passphrase from file descriptor with number
 instead of from the terminal.
 .IP \fB\-r\fP
 Setup read-only loop device.
+.IP \fB\-s\fP
+Print device name if the
+.I -f
+option and a
+.I file
+argument are present.
+
 .SH RETURN VALUE
 .B losetup
 returns 0 on success, nonzero on failure. When