android通过chmod命令实现文件权限修改

在LINUX下每个文件都有一个权限的属性，那么在Android中怎么用java改变某个文件的权限呢？

Android中有两种方法可以改变文件的权限

1. 用openFileOutput方法：

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01.FileOutputStream fos;
02.fos = openFileOutput("filename", MODE_WORLD_READABLE);
FileOutputStream fos;
fos = openFileOutput("filename", MODE_WORLD_READABLE);

FileOutputStream android.content.ContextWrapper .openFileOutput(String name, int mode) throws FileNotFoundException

Open a private file associated with this Context's application package for writing. Creates the file if it doesn't already exist.

可用的mode 参数如下：

/**
* File creation mode: the default mode, where the created file can only
* be accessed by the calling application (or all applications sharing the
* same user ID).
* @see #MODE_WORLD_READABLE
* @see #MODE_WORLD_WRITEABLE
*/
public static final int MODE_PRIVATE = 0x0000;
/**
* File creation mode: allow all other applications to have read access
* to the created file.
* @see #MODE_PRIVATE
* @see #MODE_WORLD_WRITEABLE
*/
public static final int MODE_WORLD_READABLE = 0x0001;
/**
* File creation mode: allow all other applications to have write access
* to the created file.
* @see #MODE_PRIVATE
* @see #MODE_WORLD_READABLE
*/
public static final int MODE_WORLD_WRITEABLE = 0x0002;
/**
* File creation mode: for use with {@link #openFileOutput}, if the file
* already exists then write data to the end of the existing file
* instead of erasing it.
* @see #openFileOutput
*/
public static final int MODE_APPEND = 0x8000;

其实该方法最终还是调用了系统的chmod来实现的改变文件权限的功能。

但是该方法有局限性，他创建的文件只能位于该程序的私有目录下，即/data/data/app-package/files/

2. 用Runtime.getRuntime().exec()

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01.Runtime.getRuntime().exec("chmod 644 " + filename);
Runtime.getRuntime().exec("chmod 644 " + filename);

在Unix和Linux的各种操作系统下，每个文件（文件夹也被看作是文件）都按读、写、运行设定权限。

读、写、运行三项权限可以用数字表示，就是r=4,w=2,x=1。所以，rw-r--r--用数字表示成644。
反过来说777就是rwxrwxrwx，意思是该登录用户（可以用命令id查看）、他所在的组和其他人都有最高权限。
Android中可用通过adb shell 方法修改文件的权限，有时候我们需要在代码中实现改功能，

该方法调用系统命令chmod来改变文件的权限，为了能判断命令的返回值，最好写成：

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01.Process p = Runtime.getRuntime().exec("chmod 644 " + filename);
02.int status = p.waitFor();
03.if (status == 0) {
04. //chmod succeed
05.} else {
06. //chmod failed
07.}

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