Additional work on dealing with race condition.

1 files changed, 24 insertions, 15 deletions

diff --git a/examples/write_date.sh b/examples/write_date.sh
index 2fa6895..d9ae3ec 100755
--- a/examples/write_date.sh
+++ b/examples/write_date.sh
@@ -20,22 +20,15 @@
 # $ ./write_date.sh -f now.out
 # $ cat now.out
 
-# Prevent overwriting files with redirection operators.
-set -C >/dev/null 2>&1
-
 # Source shFlags.
-. ../shflags || ( echo "fatal: unable to source shflags" >&2; exit 64; )
+. ../shflags
 
 # Configure shFlags.
 DEFINE_boolean 'force' false 'force overwriting' 'f'
 FLAGS_HELP="USAGE: $0 [flags] filename"
 
-write_date() {
-	date >"$1"
-}
-
 die() {
-  [ $# -gt 0 ] && echo "error: $@" >&2
+  [ $# -gt 0 ] && echo "error: $@"
   flags_help
   exit 1
 }
@@ -44,11 +37,27 @@ die() {
 FLAGS "$@" || exit 1
 eval set -- "${FLAGS_ARGV}"
 
-# Check for filename.
-[ $# -gt 0 ] || die 'filename missing'
+# Check for filename on command-line.
+[ $# -gt 0 ] || die 'filename missing.'
 filename=$1
 
-[ -f "${filename}" -a ${FLAGS_force} -eq ${FLAGS_FALSE} ] && \
-    die "output file \"${filename}\" exists, not overwriting"
-
-write_date "${filename}"
+# Redirect STDOUT to the file ($1). This seemingly complicated method using exec
+# is used so that a potential race condition between checking for the presence
+# of the file and writing to the file is mitigated.
+if [ ${FLAGS_force} -eq ${FLAGS_FALSE} ] ; then
+  [ ! -f "${filename}" ] || die "file \"${filename}\" already exists."
+  # Set noclobber, redirect STDOUT to the file, first saving STDOUT to fd 4.
+  set -C
+  exec 4>&1 >"${filename}"  # This fails if the file exists.
+else
+  # Forcefully overwrite (clobber) the file.
+  exec 4>&1 >|"${filename}"
+fi
+
+# What time is it?
+date
+
+# Restore STDOUT from file descriptor 4, and close fd 4.
+exec 1>&4 4>&-
+
+echo "The current date was written to \"${filename}\"."