*: replace expr commond to bc #725 (#729)

Author: acekingke

Dockerfile.sidecar

@@ -43,7 +43,7 @@ RUN set -ex; \
 ARG XTRABACKUP_PKG=percona-xtrabackup-24
 RUN set -ex; \
     apt-get update; \
-    apt-get install -y --no-install-recommends gnupg2 wget lsb-release curl; \
+    apt-get install -y --no-install-recommends gnupg2 wget lsb-release curl bc; \
     wget -P /tmp --no-check-certificate https://repo.percona.com/apt/percona-release_latest.$(lsb_release -sc)_all.deb; \
     dpkg -i /tmp/percona-release_latest.$(lsb_release -sc)_all.deb; \
     apt-get update; \

backup/syncer/job.go

@@ -160,7 +160,7 @@ func (s *jobSyncer) ensurePodSpec(in corev1.PodSpec) corev1.PodSpec {
 			s.backup.Namespace, s.backup.GetNameForJob(), backupToDir, DateTime)
 		// Add the check DiskUsage
 		// use expr because shell cannot compare float number
-		checkUsage := `[ $(expr $(df /backup|awk 'NR>1 {print $4}') \> $(du  /backup |awk 'END {if (NR > 1) {print $1 /(NR-1)} else print 0}')) -eq '1' ] || { echo disk available may be too small; exit 1;};`
+		checkUsage := `[ $(echo "$(df /backup|awk 'NR>1 {print $4}') > $(du  /backup |awk 'END {if (NR > 1) {print $1 /(NR-1)} else print 0}')"|bc) -eq '1' ] || { echo disk available may be too small; exit 1;};`
 		in.Containers[0].Args = []string{
 			checkUsage + fmt.Sprintf("mkdir -p /backup/%s;"+
 				"curl --user $BACKUP_USER:$BACKUP_PASSWORD %s/download|xbstream -x -C /backup/%s; err1=${PIPESTATUS[0]};"+