对于非常大的文件或者很重要的文件，在不稳定的网络环境下，可能文件的某些字节会损坏。此时，对文件计算MD5即可以校验其完整性。比如本次的 OpenStreetMap 导出包，我的学弟反馈通过网盘下载无法解压，并建议我增加每个文件的MD5校验。

对于文件非常多的情况，需要批量计算。最简便的方法是使用git自带的md5sum进行计算。

1. 安装git并进入bash

到 https://git-scm.com/ 下载git，并安装。

安装后，右键单击网盘下载的文件夹，选择“git bash” 进入bash:

可以查看 md5sum的说明

$ md5sum --help
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 (default unless reading tty stdin)
  -c, --check          read MD5 sums from the FILEs and check them
      --tag            create a BSD-style checksum
  -t, --text           read in text mode (default if reading tty stdin)
  -z, --zero           end each output line with NUL, not newline,
                       and disable file name escaping

The following five options are useful only when verifying checksums:
      --ignore-missing  don't fail or report status for missing files
      --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 space, a character indicating input mode ('*' for binary,
' ' for text or where binary is insignificant), and name for each FILE.

Note: There is no difference between binary mode and text mode on GNU systems.

GNU coreutils online help: <https://www.gnu.org/software/coreutils/>
Report any translation bugs to <https://translationproject.org/team/>
Full documentation <https://www.gnu.org/software/coreutils/md5sum>
or available locally via: info '(coreutils) md5sum invocation'

2. 批量计算md5

Linux常见命令 find 能够枚举文件并批量执行指令。

执行下面的指令，可以在屏幕输出各个文件的md5：

$ find ./Arch*.* -exec md5sum {} \;
d060dd81785d957ae4e2bbd4f9ebeb4e *./ArchOSManjaro.7z.001
b7326e73452d3fbbc56a889f55aa9a14 *./ArchOSManjaro.7z.002
805c9ef68887953554c6c160c2a72eeb *./ArchOSManjaro.7z.003
#...
2cc5ab567abba1d7e3a284ec5c383d84 *./ArchOSManjaro.7z.059
$

执行下面的指令，可以在文件输出各个文件的md5：

$ find ./Arch*.* -exec md5sum {} >> md5.txt \;

3.比较两个文件是否一致

我们假设本地校验结果放在check.txt，标准校验结果放在 md5.txt，则使用下面指令比较：

$ diff --help
Usage: diff [OPTION]... FILES
Compare FILES line by line.

Mandatory arguments to long options are mandatory for short options too.
      --normal                  output a normal diff (the default)
  -q, --brief                   report only when files differ
  -s, --report-identical-files  report when two files are the same
  -c, -C NUM, --context[=NUM]   output NUM (default 3) lines of copied context
  -u, -U NUM, --unified[=NUM]   output NUM (default 3) lines of unified context
  -e, --ed                      output an ed script
  -n, --rcs                     output an RCS format diff
  -y, --side-by-side            output in two columns
  -W, --width=NUM               output at most NUM (default 130) print columns
      --left-column             output only the left column of common lines
      --suppress-common-lines   do not output common lines

  -p, --show-c-function         show which C function each change is in
  -F, --show-function-line=RE   show the most recent line matching RE
      --label LABEL             use LABEL instead of file name and timestamp
                                  (can be repeated)

  -t, --expand-tabs             expand tabs to spaces in output
  -T, --initial-tab             make tabs line up by prepending a tab
      --tabsize=NUM             tab stops every NUM (default 8) print columns
      --suppress-blank-empty    suppress space or tab before empty output lines
  -l, --paginate                pass output through 'pr' to paginate it

  -r, --recursive                 recursively compare any subdirectories found
      --no-dereference            don't follow symbolic links
  -N, --new-file                  treat absent files as empty
      --unidirectional-new-file   treat absent first files as empty
      --ignore-file-name-case     ignore case when comparing file names
      --no-ignore-file-name-case  consider case when comparing file names
  -x, --exclude=PAT               exclude files that match PAT
  -X, --exclude-from=FILE         exclude files that match any pattern in FILE
  -S, --starting-file=FILE        start with FILE when comparing directories
      --from-file=FILE1           compare FILE1 to all operands;
                                    FILE1 can be a directory
      --to-file=FILE2             compare all operands to FILE2;
                                    FILE2 can be a directory

  -i, --ignore-case               ignore case differences in file contents
  -E, --ignore-tab-expansion      ignore changes due to tab expansion
  -Z, --ignore-trailing-space     ignore white space at line end
  -b, --ignore-space-change       ignore changes in the amount of white space
  -w, --ignore-all-space          ignore all white space
  -B, --ignore-blank-lines        ignore changes where lines are all blank
  -I, --ignore-matching-lines=RE  ignore changes where all lines match RE

  -a, --text                      treat all files as text
      --strip-trailing-cr         strip trailing carriage return on input
      --binary                    read and write data in binary mode

  -D, --ifdef=NAME                output merged file with '#ifdef NAME' diffs
      --GTYPE-group-format=GFMT   format GTYPE input groups with GFMT
      --line-format=LFMT          format all input lines with LFMT
      --LTYPE-line-format=LFMT    format LTYPE input lines with LFMT
    These format options provide fine-grained control over the output
      of diff, generalizing -D/--ifdef.
    LTYPE is 'old', 'new', or 'unchanged'.  GTYPE is LTYPE or 'changed'.
    GFMT (only) may contain:
      %<  lines from FILE1
      %>  lines from FILE2
      %=  lines common to FILE1 and FILE2
      %[-][WIDTH][.[PREC]]{doxX}LETTER  printf-style spec for LETTER
        LETTERs are as follows for new group, lower case for old group:
          F  first line number
          L  last line number
          N  number of lines = L-F+1
          E  F-1
          M  L+1
      %(A=B?T:E)  if A equals B then T else E
    LFMT (only) may contain:
      %L  contents of line
      %l  contents of line, excluding any trailing newline
      %[-][WIDTH][.[PREC]]{doxX}n  printf-style spec for input line number
    Both GFMT and LFMT may contain:
      %%  %
      %c'C'  the single character C
      %c'\OOO'  the character with octal code OOO
      C    the character C (other characters represent themselves)

  -d, --minimal            try hard to find a smaller set of changes
      --horizon-lines=NUM  keep NUM lines of the common prefix and suffix
      --speed-large-files  assume large files and many scattered small changes
      --color[=WHEN]       color output; WHEN is 'never', 'always', or 'auto';
                             plain --color means --color='auto'
      --palette=PALETTE    the colors to use when --color is active; PALETTE is
                             a colon-separated list of terminfo capabilities

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

FILES are 'FILE1 FILE2' or 'DIR1 DIR2' or 'DIR FILE' or 'FILE DIR'.
If --from-file or --to-file is given, there are no restrictions on FILE(s).
If a FILE is '-', read standard input.
Exit status is 0 if inputs are the same, 1 if different, 2 if trouble.

Report bugs to: bug-diffutils@gnu.org
GNU diffutils home page: <https://www.gnu.org/software/diffutils/>
General help using GNU software: <https://www.gnu.org/gethelp/>

执行指令：

$ diff  check.txt  md5.txt
36c36
< 23dfa036cd5d772f01173da67ebf2634 *./ArchOSManjaro.7z.029
---
> db2ce0bc5c39fd5d8f672f478788095c *./ArchOSManjaro.7z.029

可以看到第29号文件有问题。

使用 -y 选项，可以查看完整输出（左右两列）

$ diff -y check.txt  md5.txt
..
e155774f7dd158ded02d9a9aae68f5eb *./ArchOSManjaro.7z.028        e155774f7dd158ded02d9a9aae68f5eb *./ArchOSManjaro.7z.028
23dfa036cd5d772f01173da67ebf2634 *./ArchOSManjaro.7z.029      | db2ce0bc5c39fd5d8f672f478788095c *./ArchOSManjaro.7z.029
c8c32363ebd14a7eefce1cadaaa64def *./ArchOSManjaro.7z.030        c8c32363ebd14a7eefce1cadaaa64def *./ArchOSManjaro.7z.030
...

不一致的行，会用竖线“|”标记。