大家在工作的时候可能经常会遇到这样的需求，在类 Unix 系统的 bash 环境下，怎样检查文件是否存在呢？既然有需求，当然就有解决的办法了。

Shell 中的 test 命令可以用来检测文件的类型或者比较数值是否相等，该命令也能用来检查文件是否存在，具体如下所示：

test -e filename
[ -e filename ]
  
test -f filename
[ -f filename ]

更常见的用法则是将上述命令放置在 if … else … fi 条件判断的条件表达式中，然后在其中写上不同的分支逻辑：

#!/bin/bash
file="/etc/hosts"
if [ -f "$file" ]
then
    echo "$file found."
else
    echo "$file not found."
fi

下面的命令则使用 Shell 的条件表达式，判断 /etc/hosts 文件是否存在：

[ -f /etc/hosts ] && echo "Found" || echo "Not found"

该组合命令会输出以下内容：

Found

检测文件属性的相关操作符：

如果文件存在，并且具有相应的属性，如下的操作符都会返回 true ：

       -b FILE
              FILE exists and is block special

       -c FILE
              FILE exists and is character special

       -d FILE
              FILE exists and is a directory

       -e FILE
              FILE exists

       -f FILE
              FILE exists and is a regular file

       -g FILE
              FILE exists and is set-group-ID

       -G FILE
              FILE exists and is owned by the effective group ID

       -h FILE
              FILE exists and is a symbolic link (same as -L)

       -k FILE
              FILE exists and has its sticky bit set

       -L FILE
              FILE exists and is a symbolic link (same as -h)

       -O FILE
              FILE exists and is owned by the effective user ID

       -p FILE
              FILE exists and is a named pipe

       -r FILE
              FILE exists and read permission is granted

       -s FILE
              FILE exists and has a size greater than zero

       -S FILE
              FILE exists and is a socket

       -t FD  file descriptor FD is opened on a terminal

       -u FILE
              FILE exists and its set-user-ID bit is set

       -w FILE
              FILE exists and write permission is granted

       -x FILE
              FILE exists and execute (or search) permission is granted

以上命令从 man test 复制而来。

使用上述符号的方法一模一样：

if [ operator FileName ]
then
    echo "FileName - Found, take some action here"
else
    echo "FileName - Not found, take some action here"
fi

还有两个文件之间相互比较的三个表达式：

       FILE1 -ef FILE2
              FILE1 and FILE2 have the same device and inode numbers

       FILE1 -nt FILE2
              FILE1 is newer (modification date) than FILE2

       FILE1 -ot FILE2
              FILE1 is older than FILE2