shell脚本中[0: command not found

在写 Redis 主从复制和哨兵模式部署脚本时，代码执行报错。
代码如下：

if [$REDIS_SENTINEL_ENABLE -eq 1]; then
    cp /etc/redis-sentinel.conf /etc/redis-sentinel.conf.backup
    sed -i "s/^port.*/port ${REDIS_SENTINEL_PORT}/g" /etc/redis-sentinel.conf
    sed -i "s/^sentinel monitor.*/sentinel monitor mymaster ${REDIS_MASTER_IP} ${REDIS_MASTER_PORT} ${REDIS_SLAVE_NUM}/g" /etc/redis-sentinel.conf
    if [ `grep "^sentinel auth-pass mymaster" /etc/redis-sentinel.conf |wc -l` -eq 0 ]; then
        sed -i "/^sentinel monitor mymaster/asentinel auth-pass mymaster ${REDIS_PASSWD}" /etc/redis-sentinel.conf
    else 
        sed -i "s/^sentinel auth-pass mymaster/#sentinel auth-pass mymaster/g" /etc/redis-sentinel.conf
        sed -i "/^sentinel monitor mymaster/asentinel auth-pass mymaster ${REDIS_PASSWD}" /etc/redis-sentinel.conf
    fi
    # redis-sentinel /etc/redis-sentinel.conf
    # service 默认使用/etc/redis-sentinel.conf启动
    service redis-sentinel restart
    if [ $? -ne 0 ];then
        echo "redis-sentinel restart failed!"
        return 1
    fi 
fi

错误：

line 2: [0: command not found

看到错误时有点奇怪，这个是什么错误? 看一下报错的行数，感觉没写错啊，为什么执行不了，然后笔者问一下组长，才发现 if 判断少了空格。
if [$REDIS_SENTINEL_ENABLE -eq 1]; then
应该写成
if [ $REDIS_SENTINEL_ENABLE -eq 1 ]; then

结论

在if判断中[]这个符号有特殊要求。里面与代码之间要有空格隔开的。