true命令啥都不做，只设置退出码为0。 在linux底下，每个程序执行完毕都会返回一个退出码给调用者，一般情况下0表示成功，其他值表明有问题，当然某些程序的退出码有特殊含义。有些shell下true可能是个内建命令，至少bash下是。（do nothing, successfully。Exit with a status code indicating success. NOTE: your shell may have its own version of true, which usually supersedes the version described here. Please refer to your shell’s documentation for details about the options it supports.）
