这又是一个文件比较命令，熟练掌握会节省你很多工作。还记得comm命令吗？diff比他更加复杂，好用，不仅能比较文件，还能比较目录。 当一个文件有多个版本时候，或者更多复杂的文件，目录比较时，你会期望有一个比comm更有用的命令，diff正是为此而生。
全称：different file
实例：
文件1： v1.txt
hello world v1_echo v1_diff v1_comm shell is easy!
文件2： v2.txt
hello world! v2_echo v1_diff easy shell!
1.普通比较：
$diff v1.txt v2.txt
1,2c1,2 < hello world < v1_echo --- > hello world! > v2_echo 4c4 < v1_comm --- > easy shell!
列出比较的区别，数字为行数，c是compare
2.一体化输出：
$diff -u v1.txt v2.txt > version.patch
-v1_echo +hello world! +v2_echo v1_diff -v1_comm +easy shell! shell is easy!
"+"表示补充的行，"-"表示删去的行
3. 更新覆盖v1.txt,注意这里借用了patch
$patch ?p1 v1.txt< version.patch
4. 比较v1,v2文件，仅输出v2的变化
$diff -e v1.txt v2.txt
4c easy shell! . 1,2c hello world! v2_echo .
5.比较目录
借用书上一段解释：
-N 将所有确实的文件视为空文件
-a 将所有文件市委文件文本
-u 生成一体化输出
-r 遍历目录下的所有文件
比较两个目录下所有文本信息
$diff ?Naur directory1 directory2
除非你确定这两个目录基本相同，不然不推荐这么用。
6.工作疲劳后，尤其同时做很多事情的时候，会忘了有没有做过上传，或者修改某个目录下的文件。可以这样：
$diff v1.txt /${dir}
直接比较同名文件，高强度下的工作者相信真有体会。
解读下—help
太长，截取少部分解读一下。
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 FILES 的格式可以是‘FILE1 FILE2'、‘DIR1 DIR2'、‘DIR FILE...'或 ‘FILE... DIR'。
解读：
1.-q ?s
-q: 只有两个不同是，提示“两个文件不同”
-s: 两个文件相同时，提示“两个文件相同”
$diff -qs v1.txt v2.txt
(有点无聊的命令 )
2. ?y
按照列的方式输出，增加了直观性
hello world | hello world! v1_echo | v2_echo v1_diff v1_diff v1_comm | easy shell! shell is easy! shell is easy!
3. file 后面的格式
可以是1.文件 文件 2.目录与目录 3.文件与目录 4. 目录与文件
解释一下，文件与目录的形式，比较的是该目录下 与 比较文件同名的文件比较
如：$ diff v1.txt diff/
则实际是v1.txt 与 diff/v1.txt的比较。这是个很常用的功能。
对比comm
就比较文件而言，comm功能比较简单，但是也很直观，方便
而diff 提供了更强大，更复杂的比较方式，还能对比目录，遍历目录。
用法：diff [选项]... 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
(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
-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 whose lines are all blank
-I, --ignore-matching-lines=RE ignore changes whose lines all match RE
-a, --text treat all files as text
--strip-trailing-cr strip trailing carriage return on input
-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 可以是‘old'、‘new'或‘unchanged'。GTYPE 可以是 LTYPE 的选择
或是‘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
--help display this help and exit
-v, --version output version information and exit
FILES 的格式可以是‘FILE1 FILE2'、‘DIR1 DIR2'、‘DIR FILE...'或
‘FILE... DIR'。
If --from-file or --to-file is given, there are no restrictions on FILE(s).
如果 FILE 是‘-'，则由标准输入读取内容。
如果输入相同，则退出状态为 0；1 表示输入不同；2 表示有错误产生。