install-deps.sh: check if have access to stdout

$- is "hB" if launched via command line, and it is "himBH" only when
we are actually *in* an interactive shell. so checking "test -t 1" is
what we want.

Signed-off-by: Kefu Chai <kchai@redhat.com>

install-deps.sh

@@ -80,10 +80,9 @@ function ensure_decent_gcc_on_rh {
     local expected=5.1
     local dts_ver=$1
     if version_lt $old $expected; then
-	case $- in
-	    *i*)
-		# interactive shell
-		cat <<EOF
+	if test -t 1; then
+	    # interactive shell
+	    cat <<EOF
 Your GCC is too old. Please run following command to add DTS to your environment:
 
 scl enable devtoolset-7 bash
@@ -94,12 +93,10 @@ source scl_source enable devtoolset-7
 
 see https://www.softwarecollections.org/en/scls/rhscl/devtoolset-7/ for more details.
 EOF
-	    ;;
-	    *)
-		# non-interactive shell
-		source /opt/rh/devtoolset-$dts_ver/enable
-		;;
-	esac
+	else
+	    # non-interactive shell
+	    source /opt/rh/devtoolset-$dts_ver/enable
+	fi
     fi
 }