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shell 变量间接引用

c++ 学习(f��n) — Mon, 02 Aug 2010 08:46:00 GMT

�q�种使用间接引用的方法是一个小技�? 如果�W�二个变量更改了(ji��n)它的�? 那么�W�一个变�?/div>

必须被适当的解除引�?��像上边的例子一�?. �q�运的是, 在Bash版本2中引�?/div>

�?{!variable}形式使得使用间接引用更加直观�?

假设一个变量的值是�W�二个变量的名字. 那么我们如何从第一个变量中取得�W�二个变量的值呢? 比如,

如果a=letter_of_alphabet�q�且letter_of_alphabet=z, 那么我们能够通过引用变量a来获得z�? �q�确

实是可以做到�? 它被�U�Cؓ(f��)间接引用. 它��用eval var1=\$$var2�q�种不��^常的形式.

c++ 学习(f��n) 2010-08-02 16:46 发表评论

c++ 学习(f��n) — Wed, 30 Jun 2010 14:15:00 GMT

正则表达式就是由一�p�d��Ҏ(gu��)��字符�l�成的字�W�串, 其中每个�Ҏ(gu��)��字符都被�U�Cؓ(f��)元字�W?/em>, �q�些元字�W?/em>�q�不表示为它们字面上的含�? 而会(x��)被解释�ؓ(f��)一些特定的含义. 具个例子, 比如引用�W�号, 可能��是表示某�h的演讲内�? 同上, 也可能表�C�Zؓ(f��)我们下面��要讲到的符��L(f��ng)��?含义. 正则表达式其实是由普通字�W�和元字�W�共同组成的集合, �q�个集合用来匚w��(或指�?模式.

一个正则表辑ּ��?x��)包含下列一��Ҏ(gu��)��多项:

一个字�W�集. �q�里所指的字符集只包含普通字�W? �q�些字符只表�C�它们的字面含义. 正则表达式的最��单�Ş式就�?em>�?/em>包含字符�? 而不包含元字�W?

�?/em>. �?/em>指定�?ji��n)正则表辑ּ�所要匹配的文本在文本行中所处的位置. 比如, ^, �?span class="TOKEN">$��是�?

修饰�W?/em>. 它们扩大或羃?y��u)?修改)�?ji��n)正则表辑ּ�匚w��文本的范�? 修饰�W�包含星�? 括号, 和反斜杠.

正则表达式最主要的目的就是用�?RE)文本搜烦(ch��)与字�W�串操作. (译者注: 以下正则表达式也�?x��)被��U�Cؓ(f��)RE.) RE能够匚w��单个字符或者一个字�W�集 -- �? 一个字�W�串, 或者一个字�W�串的一部分.

星号 -- * -- 用来匚w��它前面字�W�的��L��多次, 包括0��?/em>.

"1133*"匚w��11 + 一个或多个3 + 也允许后边还有其他字�W?/em>: 113, 1133, 111312, �{�等.

�?-- . -- 用于匚w��L��一个字�W? 除了(ji��n)换行�W? [1]

"13." 匚w��13 + 臛_��一个�Q意字�W?包括�I�格): 1133, 11333, 但不能匹�?tt class="REPLACEABLE">13 (因�ؓ(f��)�~�少"."所能匹配的臛_��一个�Q意字�W?.

脱字�W�号 -- ^ -- 匚w��行首, 但是某些时候需要依赖上下文环境, 在RE�? 有时候也表示对一个字�W�集取反.

��元�W?-- $ -- 在RE中用来匹配行��?

"XXX$" 匚w��行尾�?em>XXX.

"^$" 匚w��I��.

中括�?-- [...] -- 在RE�? ��匹配中括号字符集中的某一个字�W?

"[xyz]" ��会(x��)匚w��字符x, y, �?tt class="REPLACEABLE">z.

"[c-n]" 匚w��字符c到字�W?tt class="REPLACEABLE">n之间的�Q意一个字�W?

"[B-Pk-y]" 匚w��?tt class="REPLACEABLE">B�?tt class="REPLACEABLE">P, 或者从k�?tt class="REPLACEABLE">y之间的�Q意一个字�W?

"[a-z0-9]" 匚w��L��写字母或数�?

"[^b-d]" ��会(x��)匚w��范围�?tt class="REPLACEABLE">b�?tt class="REPLACEABLE">d之外�?/em>��L��一个字�W? �q�就是��?span class="TOKEN">^对字�W�集取反的一个实�? (��好像在某些情况�? !所表达的含�?.

��多个中括号字符集组合��? 能够匚w��一般的单词或数�? "[Yy][Ee][Ss]"能够匚w��yes, Yes, YES, yEs, �{�等. "[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]" 可以匚w��C�保�?Social Security number).

反斜�?-- \ -- 用来转义某个�Ҏ(gu��)��含义的字�W? �q�意味着, �q�个�Ҏ(gu��)��字符��会(x��)被解释�ؓ(f��)字面含义.

"\$"��会(x��)被解释成字符"$", 而不是RE中匹配行��Ҏ(gu��)��字符. �怼��? "\\"��会(x��)被解释�ؓ(f��)字符"\".

转义�?span class="QUOTE">"��括�? -- \<...\> -- 用于匚w��单词边界.

��括号必��被转义才含有特�D�的含义, 否则它就表示��括��L(f��ng)��字面含义.

"\" 完整匚w��单词"the", 不会(x��)匚w��"them", "there", "other", �{�等.

bash$ cat textfile
This is line 1, of which there is only one instance. This is the only instance of line 2. This is line 3, another line. This is line 4.

bash$ grep 'the' textfile
This is line 1, of which there is only one instance. This is the only instance of line 2. This is line 3, another line.

bash$ grep '\' textfile
This is the only instance of line 2.

要想��定一个RE能否正常工作, 唯一的办法就是测试它.

1 TEST FILE: tstfile # 不匹�?
2 # 不匹�?
3 Run grep "1133*" on this file. # 匚w��.
4 # 不匹�?
5 # 不匹�?
6 This line contains the number 113. # 匚w��.
7 This line contains the number 13. # 不匹�?
8 This line contains the number 133. # 不匹�?
9 This line contains the number 1133. # 匚w��.
10 This line contains the number 113312. # 匚w��.
11 This line contains the number 1112. # 不匹�?
12 This line contains the number 113312312. # 匚w��.
13 This line contains no numbers at all. # 不匹�?

bash$ grep "1133*" tstfile
Run grep "1133*" on this file. # 匚w��. This line contains the number 113. # 匚w��. This line contains the number 1133. # 匚w��. This line contains the number 113312. # 匚w��. This line contains the number 113312312. # 匚w��.

扩展的正则表辑ּ�. ��d��?ji��n)一些额外的匚w��字符到基本集合中. 用于egrep, awk, �?a >Perl.

问号 -- ? -- 匚w��它前面的字符, 但是只能匚w��1�ơ或0��? 通常用来匚w��单个字符.

加号 -- + -- 匚w��它前面的字符, 能够匚w��一�ơ或多次. 与前面讲�?span class="TOKEN">*号作用类�? 但是不能匚w��0个字�W�的情况.

1 # GNU版本的sed和awk能够使用"+",
2 # 但是它需要被转义一�?
3
4 echo a111b | sed -ne '/a1\+b/p'
5 echo a111b | grep 'a1\+b'
6 echo a111b | gawk '/a1+b/'
7 # 上边3句的作用相同.
8
9 # 感谢, S.C.

转义"大括�? -- \{ \} -- 在�{义后的大括号中加上一个数�? �q�个数字��是它前面的RE所能匹配的�ơ数.

大括号必��ȝ��q��{�? 否则, 大括号仅仅表�C�字面含�? �q�种用法�q�不是基本RE集合中的一部分, 仅仅是个技巧而以.

"[0-9]\{5\}" �_��匚w��5个数�?(所匚w��的字�W�范围是0�?).

使用大括号�Ş式的RE是不能够�?span class="QUOTE">"�l�典"(非POSIX兼容)�?a >awk版本中正常运行的. 然�? gawk命��o(h��)中有一�?code class="OPTION">--re-interval选项, 使用�q�个选项��允�怋�用大括号形式的RE�?无需转义).

bash$ echo 2222 | gawk --re-interval '/2{3}/'
2222

Perl与某些版本的egrep不需要�{义大括号.

圆括�?-- ( ) -- 括�v一�l�正则表辑ּ�. 当你想��?a >expr�q�行子字�W�串提取(substring extraction)的时�? 圆括号就有用�? 如果和下面要讲的"|"操作�W�结合��? 也非常有�?

竖线 -- | -- ��是RE中的"�?操作�W? 使用它能够匹配一�l�可选字�W�中的�Q意一�?

bash$ egrep 're(a|e)d' misc.txt
People who read seem to be better informed than those who do not. The clarinet produces sound by the vibration of its reed.

与GNU工具一�? 某些版本�?strong class="COMMAND">sed, ed, �?strong class="COMMAND">ex一栯��够支持扩展正则表辑ּ�, 上边�q�部分就描述�?ji��n)扩展正则表辑ּ?

POSIX字符�c? [:class:]

�q�是另外一�U? 用于指定匚w��字符范围的方�?

[:alnum:] 匚w��字母和数�? �{��h(hu��n)�?kbd class="USERINPUT">A-Za-z0-9.

[:alpha:] 匚w��字母. �{��h(hu��n)�?kbd class="USERINPUT">A-Za-z.

[:blank:] 匚w��一个空格或是一个制表符(tab).

[:cntrl:] 匚w��控制字符.

[:digit:] 匚w��(十进�?数字. �{��h(hu��n)�?kbd class="USERINPUT">0-9.

[:graph:] (可打印的囑�Ş字符). 匚w��ASCII码��D��围在33 - 126之间的字�W? 与下面所提到�?kbd class="USERINPUT">[:print:]�c�M��, 但是不包括空格字�W?�I�格字符的ASCII码是32).

[:lower:] 匚w��写字母. �{��h(hu��n)�?kbd class="USERINPUT">a-z.

[:print:] (可打印的囑�Ş字符). 匚w��ASCII码��D��围在32 - 126之间的字�W? 与上边的[:graph:]�c�M��, 但是包含�I�格.

[:space:] 匚w��I�白字符(�I�格和水�q�_��表符).

[:upper:] 匚w��大写字母. �{��h(hu��n)�?kbd class="USERINPUT">A-Z.

[:xdigit:] 匚w��16�q�制数字. �{��h(hu��n)�?kbd class="USERINPUT">0-9A-Fa-f.

POSIX字符�c�通常都要用引��h��双中括号([[ ]])引�v�?

bash$ grep [[:digit:]] test.file
abc=723

如果在一个受限的范围�? �q�些字符�cȝ��臛_��以用�?a >通配(globbing)�?

bash$ ls -l ?[[:digit:]][[:digit:]]?
-rw-rw-r-- 1 bozo bozo 0 Aug 21 14:47 a33b

如果想了(ji��n)解POSIX字符�c�d��脚本中的使用情况, 请参�?a >例子 12-18�?a >例子 12-19.

Sed, awk, �?a >Perl在脚本中一般都被用作过滤器, �q�些�q��o(h��)器将�?x��)以RE为参�? �Ҏ(gu��)��件或者I/O��进�?�q��o(h��)"或�{�? 请参�?a >例子 A-12�?a >例子 A-17, 来详�l�了(ji��n)解这�U�用�?

对于RE�q�个复杂的主�? 标准的参考材料是Friedl�?em class="CITETITLE">Mastering Regular Expressions. 由Dougherty和Robbins所�~�写�?em class="CITETITLE">Sed & Awk�q�本�? 也对RE�q�行�?ji��n)清晰的��? 如果惌��得这些书的更多信�? 请察�?a >参考文�?/em>.

注意事项

[1]
因�ؓ(f��)sed, awk, �?a >grep通常用于处理单行, 但是不能匚w��一个换行符. 如果你想处理多行输入的话, 那么你可以��?�?来匹配换行符.

1 #!/bin/bash
2
3 sed -e 'N;s/.*/[&]/' << EOF # Here Document
4 line1
5 line2
6 EOF
7 # 输出:
8 # [line1
9 # line2]
10
11
12
13 echo
14
15 awk '{ $0=$1 "\n" $2; if (/line.1/) {print}}' << EOF
16 line 1
17 line 2
18 EOF
19 # 输出:
20 # line
21 # 1
22
23
24 # 感谢, S.C.
25
26 exit 0

c++ 学习(f��n) 2010-06-30 22:15 发表评论

c++ 学习(f��n) — Mon, 28 Jun 2010 09:03:00 GMT

1.       expr expression

expr只能用于一元操作符,不支持二元操作符

1 x=1

2 x=$(expr $x + 1)

$x + 1之间必须有空�?/span>

2.       let expression

let 的��用方�?/span>

x=10

let x=$x+1

let x+=1

let x*=10

�{?/span>

Let没有�q�回�?/span>

3.       使用$((expression ))�?/span>((expression))形式

((expression))的��用方�?/span>

x=10

((x+=10))

(( expression)) 用法�?/span>let�c�M��

$(())的��用用�?/span>

�Q?/span> $((x=$x+10))

echo $x

y=$((x=$x-10))

echo $y

y=$(($x+1))

echo $y

echo $x

4.       使用$[ ]形式

例如�Q?/span>

n=1

: $[ n=$n+1 ](�Q�和$之间有空�?/span>)

y=$[ n = $n + 1 ]

echo $y

y=$[ $n+1 ]

echo $y

5.       使用decalare

例子�Q?/span>

decare –i num

num=$num+1

echo $num

c++ 学习(f��n) 2010-06-28 17:03 发表评论

c++ 学习(f��n) — Sun, 27 Jun 2010 09:15:00 GMT

Bash Process Substitution
May 22, 2008 By Mitch Frazier
in
HOW-TOs
In addition to the fairly common forms of input/output redirection the shell recognizes something called process substitution. Although not documented as a form of input/output redirection, its syntax and its effects are similar.
The syntax for process substitution is:
<(list) or >(list)
where each list is a command or a pipeline of commands. The effect of process substitution is to make each list act like a file. This is done by giving the list a name in the file system and then substituting that name in the command line. The list is given a name either by connecting the list to named pipe or by using a file in /dev/fd (if supported by the O/S). By doing this, the command simply sees a file name and is unaware that its reading from or writing to a command pipeline.

To substitute a command pipeline for an input file the syntax is:
command ... <(list) ...
To substitute a command pipeline for an output file the syntax is:
command ... >(list) ...

At first process substitution may seem rather pointless, for example you might imagine something simple like:
uniq <(sort a)
to sort a file and then find the unique lines in it, but this is more commonly (and more conveniently) written as:
sort a | uniq
The power of process substitution comes when you have multiple command pipelines that you want to connect to a single command.

For example, given the two files:
# cat a e d c b a # cat b g f e d c b
To view the lines unique to each of these two unsorted files you might do something like this:
# sort a | uniq >tmp1 # sort b | uniq >tmp2 # comm -3 tmp1 tmp2 a f g # rm tmp1 tmp2
With process substitution we can do all this with one line:
# comm -3 <(sort a | uniq) <(sort b | uniq) a f g

Depending on your shell settings you may get an error message similar to:
syntax error near unexpected token `('
when you try to use process substitution, particularly if you try to use it within a shell script. Process substitution is not a POSIX compliant feature and so it may have to be enabled via:
set +o posix
Be careful not to try something like:
if [[ $use_process_substitution -eq 1 ]]; then set +o posix comm -3 <(sort a | uniq) <(sort b | uniq) fi
The command set +o posix enables not only the execution of process substitution but the recognition of the syntax. So, in the example above the shell tries to parse the process substitution syntax before the "set" command is executed and therefore still sees the process substitution syntax as illegal.

Of course, note that all shells may not support process substitution, these examples will work with bash.

�q�程替换与命令替换很�怼�. 命��o(h��)替换把一个命令的�l�果赋值给一个变�? 比如dir_contents=`ls -
al`或xref=$( grep word datafile). �q�程替换把一个进�E�的输出提供�l�另一个进�E?换句话说, 它把
一个命令的�l�果发给�?ji��n)另一个命�?.
命��o(h��)替换的模�?/p>
用圆括号扩�v来的命��o(h��)
>(command)
<(command)
启动�q�程替换. 它��?dev/fd/文�g��圆括号中的�q�程处理�l�果发送给另一个进�E? [1] (�?/p>
者注: 实际上现代的UNIX�c�L��作系�l�提供的/dev/fd/n文�g是与文�g描述�W�相关的, 整数n指的��?/p>
是进�E�运行时对应数字的文件描�q�符)
�?<"�?>"与圆括号之间是没有空格的. 如果加了(ji��n)�I�格, �?x��)��生错�?
bash$ echo >(true)
/dev/fd/63
bash$ echo <(true)
/dev/fd/63
Bash在两个文件描�q�符之间创徏�?ji��n)一个管�? --fIn和fOut--. true命��o(h��)的stdin被连接到fOut
(dup2(fOut, 0)), 然后Bash�?dev/fd/fIn作�ؓ(f��)参数传给echo. 如果�pȝ��~�Z��/dev/fd/文�g, Bash�?/p>
使用临时文�g. (感谢, S.C.)
�q�程替换可以比较两个不同命��o(h��)的输�? 甚至能够比较同一个命令不同选项情况下的输出.
bash$ comm <(ls -l) <(ls -al)
total 12
-rw-rw-r-- 1 bozo bozo 78 Mar 10 12:58 File0
-rw-rw-r-- 1 bozo bozo 42 Mar 10 12:58 File2
-rw-rw-r-- 1 bozo bozo 103 Mar 10 12:58 t2.sh
total 20
drwxrwxrwx 2 bozo bozo 4096 Mar 10 18:10 .
drwx------ 72 bozo bozo 4096 Mar 10 17:58 ..
-rw-rw-r-- 1 bozo bozo 78 Mar 10 12:58 File0
-rw-rw-r-- 1 bozo bozo 42 Mar 10 12:58 File2
-rw-rw-r-- 1 bozo bozo 103 Mar 10 12:58 t2.sh
使用�q�程替换来比较两个不同目录的内容(可以查看哪些文�g名相�? 哪些文�g名不�?:
1 diff <(ls $first_directory) <(ls $second_directory)
一些进�E�替换的其他用法与技�?
1 cat <(ls -l)
2 # �{��h(hu��n)�?ls -l | cat
3
4 sort -k 9 <(ls -l /bin) <(ls -l /usr/bin) <(ls -l /usr/X11R6/bin)
5 # 列出�pȝ��3个主�?bin'目录中的所有文�? �q�且按文件名�q�行排序.
6 # 注意�?�?查一�? 上面��?个圆括号)明显不同的命令输��Z��递给'sort'.
7
8
9 diff <(command1) <(command2) # �l�出两个命��o(h��)输出的不同之�?
10
11 tar cf >(bzip2 -c > file.tar.bz2) $directory_name
12 # 调用"tar cf /dev/fd/?? $directory_name", �?bzip2 -c > file.tar.bz2".
13 #
14 # 因�ؓ(f��)/dev/fd/的系�l�属�?
15 # 所以两个命令之间的��道不必被命�?
16 #
17 # �q�种效果可以被模拟出�?
18 #
19 bzip2 -c < pipe > file.tar.bz2&
20 tar cf pipe $directory_name
21 rm pipe
22 # �?/p>
23 exec 3>&1
24 tar cf /dev/fd/4 $directory_name 4>&1 >&3 3>&- | bzip2 -c > file.tar.bz2 3>&-
25 exec 3>&-
26
27
28 # 感谢, Stephane Chazelas
一个读者给我发�?ji��n)一个有��的例子, 是关于进�E�替换的, 如下.
1 # 摘自SuSE发行版中的代码片�?
2
3 while read des what mask iface; do
4 # �q�里省略�?ji��n)一些命�?..
5 done < <(route -n)
6
7
8 # ��Z��(ji��n)��试�? 我们让它做点�?
9 while read des what mask iface; do
10 echo $des $what $mask $iface
11 done < <(route -n)
12
13 # 输出:
14 # Kernel IP routing table
15 # Destination Gateway Genmask Flags Metric Ref Use Iface
16 # 127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
17
18
19
20 # ��像Stephane Chazelas所�l�出的那�? 一个更�Ҏ(gu��)��理解的等价代码是:
21 route -n |
22 while read des what mask iface; do # ��道的输�?gu��)��赋值给�?ji��n)变�?
23 echo $des $what $mask $iface
24 done # �q�将产生��Z��上边相同的输�?
25 # 然�? Ulrich Gayer指出 . . .
26 #+ �q�个��单的�{��h(hu��n)版本在while循环中��用了(ji��n)一个子shell,
27 #+ 因此当管道结束后, 变量��消�׃��(ji��n).
28
29
30
31 # 更进一�? Filip Moritz解释�?ji��n)上面两个例子之间存在一个细微的不同之处,
32 #+ 如下所�C?
33
34 (
35 route -n | while read x; do ((y++)); done
36 echo $y # $y 仍然没有被声明或讄��
37
38 while read x; do ((y++)); done < <(route -n)
39 echo $y # $y 的��gؓ(f��)route -n的输�?gu��)��?
40 )
41
42 # 一般来�? (译者注: 原书作者在�q�里�q�未加注释符�?#", 应该是笔�?
43 (
44 : | x=x
45 # 看上��L��启动�?ji��n)一个子shell
46 : | ( x=x )
47 # �?/p>
48 x=x < <(:)
49 # 其实不是
50 )
51
52 # 当你要解析csv或类��g��西的时侯, �q�非常有�?
53 # 事实�? �q�就是SuSE的这个代码片断所要实现的功能.
注意事项
[1] �q�与命名��道(临时文�g)��h��相同的作�? �q�且, 事实�? 命名��道也被同时使用在进�E?/p>
替换�?

c++ 学习(f��n) 2010-06-27 17:15 发表评论

关于bash的管道与�q�程的几个知识点

c++ 学习(f��n) — Fri, 25 Jun 2010 08:35:00 GMT

几个知识�?br style="word-wrap: break-word; line-height: normal; ">1.Bash在实现pipeline(��道|)时会(x��)发�v两个subshell(子shell)来运行|两边�?span href="tag.php?name=%C3%FC%C1%EE" class="t_tag" style="word-wrap: break-word; line-height: normal; cursor: pointer; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(255, 0, 0); white-space: nowrap; ">命��o(h��)�Q�对�?span href="tag.php?name=%CF%B5%CD%B3" class="t_tag" style="word-wrap: break-word; line-height: normal; cursor: pointer; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(255, 0, 0); white-space: nowrap; ">�pȝ��来说��是发�v两个childprocess(子进�E�）(j��)

2.fork是��生process的唯一途径�Q�exec*是执行程序的唯一途径

3.子进�E�会(x��)完全复制父进�E�，除了(ji��n)$PID�?PPID

4.fork子进�E�时�l�承父进�E�的�q�程名，在exec*执行命��o(h��)时才由exec*替换为子�q�程对应的命令，同一�q�程的命令名可以�׃��个个exec*��L��多次的改�?/strong>

[注]对于linux�q�_��Q�JB上就是这��L(f��ng)��Q�其它��^��C��好发表意见，当然对于2中的两个唯一有一个例外，��是在kenerl  init的初�?
暂时找不到相兛_��考，也没有功力读源码�Q�所以此论是道听途说�U�别�Q�错误之处请指出�Ҏ(gu��)��Q�如果没有改正的价值可一�W�而过

我觉得要先弄清楚sub shell的定义�?br style="word-wrap: break-word; line-height: normal; ">
查了(ji��n)些资料，发现subshell的定义有些�؜乱�?br style="word-wrap: break-word; line-height: normal; ">bashMan:

QUOTE:
Each command in a pipeline is executed as a separate process (i.e., in a subshell).

QUOTE:
When a simple command other than a builtin or shell function is to be executed, it is invoked in a
separate execution environment that consists of the following. Unless otherwise noted, the values are
inherited from the shell.

�q�个separate execution是subshell吗？
A. 在当前shell执行外部命��o(h��)�Q�如shell> date, 是fork+exec, ��不是subshell?
B. ()是fork一个child shell�Q�该child再fork+exec来执行命令。这个subshell和A中的"subshell"昄��是不同的�?br style="word-wrap: break-word; line-height: normal; ">
UNIX: The Textbook, by Syed Mansoor Sarvar, Robert Koretsky and Syed Aqeel Sarvar中提刎ͼ�(x��)

QUOTE:
A child shell is also called subshell

问题是fork+exec是fork一个child shell�Q�然后在该child shell中exec.
而执行脚�?shell>scriptname)�Ӟ��是fort一个child shell A�Q�该child shell A再fork一个child shell B, 在B中再exec.

那么child shell是指哪种情况�Q?br style="word-wrap: break-word; line-height: normal; ">

UNIX at Fermilab中的Important UNIX Concepts:

QUOTE:
When you give the shell a command associated with a compiled executable or shell script, the shell
creates, or forks, a new process called a subshell.

外部命��o(h��)也在subshell中执行�?br style="word-wrap: break-word; line-height: normal; ">

QUOTE:
To execute most built-in commands, the shell forks a subshell which executes the command directly (no
exec system call). For the built-in commands cd, set, alias and source, the current shell executes the
command; no subshell is forked.

shell> built-inCommands�q�样执行�Ӟ��大部分内部命令也是在subshell中执行�?br style="word-wrap: break-word; line-height: normal; ">可见�Q�UNIX at Fermilab认�ؓ(f��)fork 一个child shell��是subshell, 不管是fork-fork+exec, �q�是 fork+exec�?br>
我刚才又�ȝ��?ji��n)下ABS�Q�定义与我的理解不一�?br style="word-wrap: break-word; line-height: normal; ">

QUOTE:
A subshell is a separate instance of the command processor -- the shell that gives you the prompt at the
console or in an xterm window. Just as your commands are interpreted at the command line prompt, similarly
does a script batch-process a list of commands. Each shell script running is, in effect, a subprocess (child
process) of the parent shell.
./script也是external_cmd的一�U�啊�Q�都是fork-exec�Q�至于external-cmd再作什么动作完全是external-cmd�?/td>

c++ 学习(f��n) 2010-06-25 16:35 发表评论

shell metacharacters 与IFS讨论

c++ 学习(f��n) — Fri, 25 Jun 2010 08:29:00 GMT
good job!
�^�算有�h看得懂了(ji��n)�?/span>

不過�Q�要細說的話�Q�要扯上 shell �?interpret 一�?command line 時的 priority �?/span>
基本上，光��序如下：(x��)
1�Q�將 line 拆成 words �Q�IFS很重要）(j��)
2�Q�括�?alias
3�Q�擴�?�?�?/span>
4�Q�擴�?~
5�Q�擴�?$variable, $(command), `command`
6�Q�重�i�再拆成 words
7�Q�括�?wildcards
8�Q�處�?I/O redirection
9�Q�載(j��)入命令運�?/span>
如果大家有O'Reilly英文版的 Learning the Bash(2nd)的話�Q�請多端詳p178的圖�Q�細��略異�Q?/span>

回到LZ的問��，看上�?5 �?6 的順序然後才�?9 �?/span>
也就是在 6 重組命��o(h��)�?$A 已經完成替換�Q�當時的 environment 是沒賦��|��
因此重組後就�?A=B echo
然後在第 9 的步驟運行命令時�Q?A=B 是�� echo 命��o(h��)�?local environment�Q?/span>
不管是否 built-in command�Q�都不媄(ji��ng)響當前的 shell �Q�不同的 shell 在實作上或有差異�Q?/span>
所以第二行�?echo $A 也是得到沒��c�?/span>

我通过eval说明赋值是成功的，而不�?5楼所说的赋��g��成功�?br style="word-wrap: break-word; line-height: normal; ">
�W�一步��?metacharacter�Q�与IFS没有关系

The following is a brief description of the shell's operation when it
reads and executes a command.  Basically, the shell does the following:

  1. Reads its input from a file (*note Shell Scripts::), from a string
   supplied as an argument to the `-c' invocation option (*note
   Invoking Bash::), or from the user's terminal.

  2. Breaks the input into words and operators, obeying the quoting
   rules described in *Note Quoting::.  These tokens are separated by
   `metacharacters'.  Alias expansion is performed by this step
   (*note Aliases::).
QUOTE:
`IFS'
   A list of characters that separate fields; used when the shell
   splits words as part of expansion.
`metacharacter'
   A character that, when unquoted, separates words.  A metacharacter
   is a `blank' or one of the following characters: `|', `&', `;',
   `(', `)', `<', or `>'.
8.05 命��o(h��)行的评�h(hu��n)�Q�evaluation�Q?br style="word-wrap: break-word; line-height: normal; ">下面是C shell 解释命��o(h��)行的��序�Q?br style="word-wrap: break-word; line-height: normal; ">1. 历史替换
2. 分裂词（包括�Ҏ(gu��)��字符�Q?br style="word-wrap: break-word; line-height: normal; ">3. 更新历史�?br style="word-wrap: break-word; line-height: normal; ">4. 解释单引��P��'�Q?�?双引��P��"�Q?br style="word-wrap: break-word; line-height: normal; ">5. 别名替换
6. 输入和输出的重定向（�?>  < �?|�Q?br style="word-wrap: break-word; line-height: normal; ">7. 变量替换
8. 命��o(h��)替换
9. 文�g名扩�?br style="word-wrap: break-word; line-height: normal; ">�Q�Bourne shell 的解释顺序本质上是一��L(f��ng)��Q�除�?ji��n)它不执行历史替换和别名替换之外�Q?br style="word-wrap: break-word; line-height: normal; ">
所�?br style="word-wrap: break-word; line-height: normal; ">A=B  echo $A

的执行过�E�应该是�q�样的：(x��)
1. 没有历史操作�W�，因此不进行历史替换（Bourne shell 不执行这一步）(j��)
2. 分裂词，每碰到未加引��L(f��ng)��I�白字符��׃��(x��)产生一个新“�?#8221;。这些词�?A=B、echo�?A�?br style="word-wrap: break-word; line-height: normal; ">3. shell ��命令行攑ֈ�历史列表中。（Bourne shell 不执行这一步）(j��)
4. 没有引号需要解�?br style="word-wrap: break-word; line-height: normal; ">5. 没有别名需要替�?br style="word-wrap: break-word; line-height: normal; ">6. 没有输入或输出重定向需要处�?br style="word-wrap: break-word; line-height: normal; ">7. shell注意到变�?A�Q��ƈ把它替换成空
8. shell��L��左单引号�Q�执行左单引号中的�Q何命令，�q�且��命令的输出插入到命令行中。在本例中，没有�q�方面的事需要做。（如果左单引号内有通配�W�或者变量，那么在shell�q�行左单引号中的命��o(h��)之前它们是不�?x��)被解释的�?j��)
9. shell��L��通配�W�。本例中没有�Q�不需要处�?br style="word-wrap: break-word; line-height: normal; ">10. shell 执行 A=B�Q?执行 echo �?/span>
8.05 命��o(h��)行的评�h(hu��n)�Q�evaluation�Q?br style="word-wrap: break-word; line-height: normal; ">下面是C shell 解释命��o(h��)行的��序�Q?br style="word-wrap: break-word; line-height: normal; ">1. 历史替换
2. 分裂词（包括�Ҏ(gu��)��字符�Q?br style="word-wrap: break-word; line-height: normal; ">3. 更新历史�?br style="word-wrap: break-word; line-height: normal; ">4. 解释单引��P��'�Q?�?双引��P��"�Q?br style="word-wrap: break-word; line-height: normal; ">5. 别名替换
6. 输入和输出的重定向（�?>  < �?|�Q?br style="word-wrap: break-word; line-height: normal; ">7. 变量替换
8. 命��o(h��)替换
9. 文�g名扩�?br style="word-wrap: break-word; line-height: normal; ">�Q�Bourne shell 的解释顺序本质上是一��L(f��ng)��Q�除�?ji��n)它不执行历史替换和别名替换之外�Q?br style="word-wrap: break-word; line-height: normal; ">
所�?br style="word-wrap: break-word; line-height: normal; ">A=B  echo $A

的执行过�E�应该是�q�样的：(x��)
1. 没有历史操作�W�，因此不进行历史替换（Bourne shell 不执行这一步）(j��)
2. 分裂词，每碰到未加引��L(f��ng)��I�白字符��׃��(x��)产生一个新“�?#8221;。这些词�?A=B、echo�?A�?br style="word-wrap: break-word; line-height: normal; ">3. shell ��命令行攑ֈ�历史列表中。（Bourne shell 不执行这一步）(j��)
4. 没有引号需要解�?br style="word-wrap: break-word; line-height: normal; ">5. 没有别名需要替�?br style="word-wrap: break-word; line-height: normal; ">6. 没有输入或输出重定向需要处�?br style="word-wrap: break-word; line-height: normal; ">7. shell注意到变�?A�Q��ƈ把它替换成空
8. shell��L��左单引号�Q�执行左单引号中的�Q何命令，�q�且��命令的输出插入到命令行中。在本例中，没有�q�方面的事需要做。（如果左单引号内有通配�W�或者变量，那么在shell�q�行左单引号中的命��o(h��)之前它们是不�?x��)被解释的�?j��)
9. shell��L��通配�W�。本例中没有�Q�不需要处�?br style="word-wrap: break-word; line-height: normal; ">10. shell 执行 A=B�Q?执行 echo �?/span>

c++ 学习(f��n) 2010-06-25 16:29 发表评论

c++ 学习(f��n) — Tue, 22 Jun 2010 10:44:00 GMT

原文地址�Q?a >http://bbs.chinaunix.net/thread-201843-1-1.html
得到长度
%x="abcd"
#�Ҏ(gu��)��一
%expr length $x
4
# �Ҏ(gu��)��?br style="word-wrap: break-word; line-height: normal; ">
%echo ${#x}
4
# �Ҏ(gu��)��?br style="word-wrap: break-word; line-height: normal; ">
%expr "$x" : ".*"
4
# expr 的帮�?br style="word-wrap: break-word; line-height: normal; ">
# STRING : REGEXP anchored pattern match of REGEXP in STRING
复制代码

查找子串
%expr index  $x "b"
2
%expr index  $x "a"
1
%expr index  $x "b"
2
%expr index  $x "c"
3
%expr index  $x "d"
4
复制代码

得到子字�W�串
# �Ҏ(gu��)��一
# expr startpos length
%expr substr "$x" 1 3
abc
%expr substr "$x" 1 5
abcd
%expr substr "$x" 2 5
bcd
# �Ҏ(gu��)��?br style="word-wrap: break-word; line-height: normal; ">
# ${x:pos:lenght}
%echo ${x:1}
bcd
%echo ${x:2}
cd
%echo ${x:0}
abcd
%echo ${x:0:2}
ab
%pos=1
%len=2
%echo ${x:$pos:$len}
bc
复制代码

匚w��正则表达�?/span>
# 打印匚w��长度
%expr match $x "."
1
%expr match $x "abc"
3
%expr match $x "bc"
0
复制代码

字符串的掐头��d��
%x=aabbaarealwwvvww
%echo "${x%w*w}"
aabbaarealwwvv
%echo "${x%%w*w}"
aabbaareal
%echo "${x##a*a}"
lwwvvww
%echo "${x#a*a}"
bbaarealwwvvww
复制代码

其中 , # 表示掐头�Q?因�ؓ(f��)键盘?sh��)?# �?$ 的左面�?/span>
其中 , % 表示%�Q?nbsp; 因�ؓ(f��)键盘?sh��)?% �?$ 的右面�?/span>
单个的表�C�最��匹配，双个表示最大匹配�?/span>
也就是说�Q�当匚w��的有多种�Ҏ(gu��)��的时候，选择匚w��的最大长度还是最��长度�?/span>

字符串的替换
%x=abcdabcd
%echo ${x/a/b} # 只替换一�?br style="word-wrap: break-word; line-height: normal; ">
bbcdabcd
%echo ${x//a/b} # 替换所�?br style="word-wrap: break-word; line-height: normal; ">
bbcdbbcd
复制代码

不可以��?regexp �Q?只能�?* ? �?/span>文�g扩展方式�?/span>

c++ 学习(f��n) 2010-06-22 18:44 发表评论