源起
从国内的第三方网站下载了Android sutiod的zip包下载地址，为了安全起见还是得跟Android官网上的对应的zip包的SHA值做下对比。以前是经常使用md5命令的，所以理论在命令行下应该是有对应的命令行工具可以计算SHA值的。后来搜索到可以用 shasum命令来完成我的需求。
我们man一下shasum命令，注：需要注意-a参数，即计算文件散列值时可以选择具体的算法
SHASUM(1) Perl Programmers Reference Guide SHASUM(1)
NAME
shasum - Print or Check SHA Checksums
SYNOPSIS
Usage: shasum [OPTION]... [FILE]...
Print or check SHA checksums.
With no FILE, or when FILE is -, read standard input.
-a, --algorithm 1 (default), 224, 256, 384, 512, 512224, 512256
-b, --binary read in binary mode
-c, --check read SHA sums from the FILEs and check them
--tag create a BSD-style checksum
-t, --text read in text mode (default)
-U, --UNIVERSAL read in Universal Newlines mode
produces same digest on Windows/Unix/Mac
-0, --01 read in BITS mode
ASCII '0' interpreted as 0-bit,
ASCII '1' interpreted as 1-bit,
all other characters ignored
The following five options are useful only when verifying checksums:
--ignore-missing don't fail or report status for missing files
-q, --quiet don't print OK for each successfully verified file
-s, --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
-h, --help display this help and exit
-v, --version output version information and exit
When verifying SHA-512/224 or SHA-512/256 checksums, indicate the
algorithm explicitly using the -a option, e.g.
实践
PS: sha256算法的散列值是256个byte，即256/8=32字节，1个字节用2位16制表示，即散列值的长度为64个字符
对比一下官网的信息，good，安全较验通过！
回顾
md5与sha256的区别
计算字符串长度
${#字符串变量}
