Linux md5sum 命令

通过 Linux 的 md5sum 命令，可以对指定的文件，计算出唯一的一个MD5值(128bit)。

通过比较文件前后的MD5值，可以判断文件是否发生变化（是否被修改过）。

1、md5sum 命令使用方式（通过md5sum --help查看）：

Usage: md5sum [OPTION]... [FILE]...
Print or check MD5 (128-bit) checksums.
With no FILE, or when FILE is -, read standard input.

  -b, --binary         read in binary mode
  -c, --check          read MD5 sums from the FILEs and check them
      --tag            create a BSD-style checksum
  -t, --text           read in text mode (default)
  Note: There is no difference between binary and text mode option on GNU system.

The following four options are useful only when verifying checksums:
      --quiet          don't print OK for each successfully verified file
      --status         don't output anything, status code shows success
      --strict         exit non-zero for improperly formatted checksum lines
  -w, --warn           warn about improperly formatted checksum lines

      --help     display this help and exit
      --version  output version information and exit

The sums are computed as described in RFC 1321.  When checking, the input
should be a former output of this program.  The default mode is to print
a line with checksum, a character indicating input mode ('*' for binary,
space for text), and name for each FILE.

GNU coreutils online help: <http://www.gnu.org/software/coreutils/>
For complete documentation, run: info coreutils 'md5sum invocation'

2、计算一个文件的 MD5 值

1）新建一个文件test.txt，内容为 Hello, md5sum.

使用如下命令，生成改文件对应的 MD5 值：

md5sum test.txt > md5.out

2）此时会在当前目录下看到生成的 md5.out 文件

3）查看改文件对应的 MD5 值

cat md5.out

4）使用如下命令，比较文件的 MD5 值是否发生变化

md5sum -c md5.out

5）当修改test.txt 文件，增加 Modify 单词，再次对比 MD5 值，会发现不一致

6）BSD格式查看 MD5 值

md5sum --tag md5.out