编写shell 脚本时遇见 syntax error in conditional expression 错误， #!/bin/bash # cleanup /var/log/message LOG_DIR=/var/log ROOT_DID=0 LINES=50 E_XCD=66 E_NOTROOT=67 if [[ "$UID" -ne "$ROOT_UID"]] then  echo "Must be root to run this script."  exit $E_NOTROOT fi if [ -n "$1"] then     lines=$1 else    lines=$LINES fi ....... root@client.example.com # sh test.sh test.sh: line 12: syntax error in conditional expression test.sh: line 13: syntax error near `then' test.sh: line 13: `then' 仔细查看是由于 if 条件中的中括号[ ]与变量之间必须有空格 root@client.example.com # vi test.sh #!/bin/bash # cleanup /var/log/message LOG_DIR=/var/log ROOT_DID=0 LINES=50 E_XCD=66 E_NOTROOT=67 if [[ "$UID" -ne "$ROOT_UID" ]] then  echo "Must be root to run this script."  exit $E_NOTROOT fi if [ -n "$1"] then     lines=$1 else    lines=$LINES fi ..... "test.sh" 60L, 793C written 修改以后，再次执行成功 root@client.example.com # sh test.sh 20 Logs cleaned up