Support logging to specified file

Currently, bats puts logs to a temporary file and removes the file
at the end of the test. This is not convenient for observing the logs.
This commit makes it possible for user to specify a log file and
observe the logs during test or for post test analyzing. A new option
named -l (or --logfile) is added for this function.

libexec/bats

@@ -7,7 +7,7 @@ version() {
 
 usage() {
   version
-  echo "Usage: bats [-c] [-p | -t] <test> [<test> ...]"
+  echo "Usage: bats [-c] [-p | -t] [-l <file>] <test> [<test> ...]"
 }
 
 help() {
@@ -21,6 +21,7 @@ help() {
   echo "  -p, --pretty   Show results in pretty format (default for terminals)"
   echo "  -t, --tap      Show results in TAP format"
   echo "  -v, --version  Display the version number"
+  echo "  -l, --logfile  Print logs to specified file"
   echo
   echo "  For more information, see https://github.com/sstephenson/bats"
   echo
@@ -57,7 +58,7 @@ export BATS_PREFIX="$(abs_dirname "$BATS_LIBEXEC")"
 export BATS_CWD="$(abs_dirname .)"
 export PATH="$BATS_LIBEXEC:$PATH"
 
-unset count_flag pretty
+unset count_flag pretty log_file
 [ -t 0 ] && [ -t 1 ] && pretty="1"
 [ -n "$CI" ] && pretty=""
 
@@ -106,6 +107,16 @@ while [[ $# -gt 0 ]]; do
       "-p" | "--pretty" )
         pretty="1"
         ;;
+      "-l" | "--logfile" )
+        # Bail out if the option is specified in a group or it's not
+        # given a value
+        if [ "$opt_grouped" = "true" ] || [ "$2" = "" ]; then
+            usage >&2
+            exit 1
+        fi
+        log_file="$2"
+        shift
+        ;;
       * )
         usage >&2
         exit 1
@@ -117,6 +128,12 @@ while [[ $# -gt 0 ]]; do
   continue
 done
 
+if [ -n "$log_file" ]; then
+    echo -n > $log_file
+fi
+
+export BATS_LOG_FILE="$log_file"
+
 if [ "${#arguments[@]}" -eq 0 ]; then
   usage >&2
   exit 1

libexec/bats-exec-test

@@ -263,7 +263,10 @@ bats_exit_trap() {
     status=0
   fi
 
-  rm -f "$BATS_OUT"
+  if [ -n "$BATS_OUT_TEMP" ]; then
+    rm -f $BATS_OUT_TEMP
+  fi
+
   exit "$status"
 }
 
@@ -309,7 +312,13 @@ fi
 
 BATS_TMPNAME="$BATS_TMPDIR/bats.$$"
 BATS_PARENT_TMPNAME="$BATS_TMPDIR/bats.$PPID"
-BATS_OUT="${BATS_TMPNAME}.out"
+
+if [ -z "$BATS_LOG_FILE" ]; then
+  BATS_OUT="${BATS_TMPNAME}.out"
+  BATS_OUT_TEMP=$BATS_OUT
+else
+  BATS_OUT=$BATS_LOG_FILE
+fi
 
 bats_preprocess_source() {
   BATS_TEST_SOURCE="${BATS_TMPNAME}.src"