md5sum
md5sum用来计算一段内容的md5校验和，这段内容可以是从文件中读取的，也可以是从标准流输入的。甚至还可以将结果保存到文件，在以后使用这个文件来校验文件是否被改变。
需要注意的是md5sum只计算文件的内容，不关注文件的元信息。
简单使用
创建一个文件：
echo hello, world >> a.txt
计算它的md5检验和：
校验文件是否改变
可以把计算的结果存储到文件中，之后使用这个文件来校验文件是否发生了变化：
md5sum a.txt >> a.txt.md5sum
看下这个文件的内容：
使用这个文件来检查是否改变：
修改a.txt，追加一些内容：
再次检验：
从流读入
当不使用选项或者跟着-的时候，会从标准输入流stdin读取数据，可以使用流传递：
也可以手动输入，手动输入的时候使用Ctrl+D结束输入：
md5sum手册：
MD5SUM(1) User Commands MD5SUM(1)
NAME
md5sum - compute and check MD5 message digest
SYNOPSIS
md5sum [OPTION]... [FILE]...
DESCRIPTION
Print or check MD5 (128-bit) checksums. With no FILE, or when FILE is -, read standard input.
-b, --binary
read in binary mode
-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 charac‐
ter indicating input mode ('*' for binary, space for text), and name for each FILE.
GNU coreutils online help: <http://www.gnu.org/software/coreutils/> Report md5sum translation bugs to <http://translationproject.org/team/>
BUGS
The MD5 algorithm should not be used any more for security related purposes. Instead, better use an SHA-2 algorithm, implemented in the programs sha224sum(1), sha256sum(1),
sha384sum(1), sha512sum(1)
