crontab 定时执行 bash 脚本报错：[: -gt: unary operator expected

Question

#!/bin/shfilesize=`ls-lnohup.out|awk'{print$5}'`echo"`date+%Y-%m-%d_%H:%M:%S`fileSize:$filesizebit">>clearNohup.logif[$filesize-gt1073741824]thenecho"`date+%Y-%m-%d_%H:%M:%S`$filesizebiggerthan1G,wewillbackupandclear...">>clearNohup.logcp-fnohup.outnohup.out.bak>nohup.outecho"`date+%Y-%m-%d_%H:%M:%S`filebackupandclaerover!">>clearNohup.logelseecho"`date+%Y-%m-%d_%H:%M:%S`$filesizelessthan1G,donothing!">>clearNohup.logfi手动执行时候没有报错，正常执行；在crontab执行时候一直在报错.系统是centOS6报错:clearNohup.sh:line4:[:-gt:unaryoperatorexpected求大神指点！