月份：2006年9月

Chapter 6. Exit and Exit Status

1、$? 是脚本中最后一个命令的退出状态。

2、!在起到否定的作用时，其后面需要一个空格，如! true

3、在!用在命令行中时，起到调动最近的历史命令的作用，例如，前面你执行过 man true,

当你执行!man时，就会调用man true.

1、Example 5-2(怎么样才能显示下面这个 $escape ?)

 

54 escape=$'\033'                    # 033 is octal for escape.       55 echo "\"escape\" echoes as $escape"       56 #                                   no visible output.

2、echo -e的用法：

[mdx@localhost abs-exercises]$ echo "\n" \n [mdx@localhost abs-exercises]$ echo -e "\n" [mdx@localhost abs-exercises]$ [mdx@localhost abs-exercises]$ echo "\\" \ [mdx@localhost abs-exercises]$ echo -e "\\" \ [mdx@localhost abs-exercises]$ echo -e '\\' \ [mdx@localhost abs-exercises]$ echo  '\\' \\

3、可是说是一些非常烦人的用法，不用去记它，平常也用不着吧。

Note The behavior of \ depends on whether it is itself escaped, quoted, or

     appearing within command substitution or a here document.

1                       #  Simple escaping and quoting         2 echo \z               #  z         3 echo \\z              # \z         4 echo '\z'             # \z         5 echo '\\z'            # \\z         6 echo "\z"             # \z         7 echo "\\z"            # \z         8         9                       #  Command substitution        10 echo `echo \z`        #  z        11 echo `echo \\z`       #  z        12 echo `echo \\\z`      # \z        13 echo `echo \\\\z`     # \z        14 echo `echo \\\\\\z`   # \z        15 echo `echo \\\\\\\z`  # \\z        16 echo `echo "\z"`      # \z        17 echo `echo "\\z"`     # \z        18        19                       # Here document        20 cat <<EOF        21 \z        22 EOF                   # \z        23        24 cat <<EOF        25 \\z        26 EOF                   # \z        27        28 # These examples supplied by St?phane Chazelas.

4、\可以起到续行符的作用，文中的叙述：

The escape also provides a means of writing a multi-line command. Normally,

each separate line constitutes a different command, but an escape at the end of

a line escapes the newline character, and the command sequence continues on to

the next line.

例如：下面的dir这个命令被分成3行来写，每行一个字符：）

[mdx@localhost abs-exercises]$ d\ > i\ > r ch4-2.txt  ctrl-h.sh  ex2-1.sh  ex2-3.sh  ex4-5.sh  ex4-7.sh  plan2.txt ch5-2.txt  ctrl-m.sh  ex2-2.sh  ex4-2.sh  ex4-6.sh  ex5-2.sh  plan.txt [mdx@localhost abs-exercises]$ dir ch4-2.txt  ctrl-h.sh  ex2-1.sh  ex2-3.sh  ex4-5.sh  ex4-7.sh  plan2.txt ch5-2.txt  ctrl-m.sh  ex2-2.sh  ex4-2.sh  ex4-6.sh  ex5-2.sh  plan.txt

Chapter 5. Quoting

1、那个IFS是什么意思？

Example 5-1. Echoing Weird Variables

1 #!/bin/bash    2 # weirdvars.sh: Echoing weird variables.    3    4 var="'(]\\{}\$\""    5 echo $var        # '(]\{}$"    6 echo "$var"      # '(]\{}$"     Doesn't make a difference.    7    8 echo    9   10 IFS='\'   11 echo $var        # '(] {}$"     \ converted to space. Why?   12 echo "$var"      # '(]\{}$"   13   14 # Examples above supplied by Stephane Chazelas.   15   16 exit 0

2、

[mdx@localhost mdx]$ echo '\'' > > > ...

(回车就显示>，永远不会出现'，除非'没有在两个'之中)

Since even the escape character (\) gets a literal interpretation within single quotes, trying to enclose a single quote within single quotes will not yield the expected result.

1 echo "Why can't I write 's between single quotes"    2    3 echo    4    5 # The roundabout method.    6 echo 'Why can'\''t I write '"'"'s between single quotes'    7 #    |-------|  |----------|   |-----------------------|    8 # Three single-quoted strings, with escaped and quoted single quotes between.    9   10 # This example courtesy of Stéphane Chazelas.

3、太令人难以把握了，没有规律吗？

Of more concern is the inconsistent behavior of "\" within double quotes.

bash$ echo hello\!  hello!      bash$ echo "hello\!"  hello\!          bash$ echo -e x\ty  xty      bash$ echo -e "x\ty"  x       y

4、生词：

Quoting can also suppress echo's  "appetite" for newlines.

Weird

discrete

1、

http://www.linuxsir.org/main/doc/abs/HTML/untyped.html

4.3. Bash Variables Are Untyped

Example 4-4. Integer or string?

20 c=BB34   21 echo "c = $c"            # c = BB34   22 d=${c/BB/23}             # Substitute "23" for "BB".   23                          # This makes $d an integer.   24 echo "d = $d"            # d = 2334

2、

http://www.linuxsir.org/main/doc/abs/HTML/othertypesv.html

The space allotted to the environment is limited. Creating too many environmental variables or ones that use up excessive space may cause problems.

bash$ eval "`seq 10000 | sed -e 's/.*/export var&=ZZZZZZZZZZZZZZ/'`"    bash$ du  bash: /usr/bin/du: Argument list too long

3、

命令:basename(# Strips out path name info (see 'basename'))

[mdx@localhost abs-exercises]$ basename /home/mdx/abs-guide-3.7/ex15.sh

ex15.sh

4、比较：

16 if [ -n "$1" ]              # Tested variable is quoted.   17 then   18  echo "Parameter #1 is $1"  # Need quotes to escape #   19 fi

5、得到最后一个参数：

1 args=$#           # Number of args passed.    2 lastarg=${!args}    3 # Or:       lastarg=${!#}    4 #           (Thanks, Chris Monson.)    5 # Note that lastarg=${!$#} doesn't work.

6、待查

If a script expects a command line parameter but is invoked without one, this may cause a null variable assignment, generally an undesirable result. One way to prevent this is to append an extra character to both sides of the assignment statement using the expected positional parameter.

1 variable1_=$1_  # Rather than variable1=$1    2 # This will prevent an error, even if positional parameter is absent.    3    4 critical_argument01=$variable1_    5    6 # The extra character can be stripped off later, like so.    7 variable1=${variable1_/_/}    8 # Side effects only if $variable1_ begins with an underscore.    9 # This uses one of the parameter substitution templates discussed later.   10 # (Leaving out the replacement pattern results in a deletion.)   11   12 #  A more straightforward way of dealing with this is   13 #+ to simply test whether expected positional parameters have been passed.   14 if [ -z $1 ]   15 then   16   exit $E_MISSING_POS_PARAM   17 fi   18   19   20 #  However, as Fabian Kreutz points out,   21 #+ the above method may have unexpected side-effects.   22 #  A better method is parameter substitution:   23 #         ${1:-$DefaultVal}   24 #  See the "Parameter Substition" section   25 #+ in the "Variables Revisited" chapter.

7、在Example 4-6. wh, whois domain name lookup中，域名查询的服务器是无效的，可以换成："wh-inn" ) whois $1@whois.internic.net;;

8、生词：

positional

notch

1、echo -n的作用简单的说就是不换行，像java中的System.out.print("something");

19 # In a 'for' loop (really, a type of disguised assignment):     20 echo -n "Values of \"a\" in the loop are: "     21 for a in 7 8 9 11     22 do     23   echo -n "$a "     24 done

结果：Values of "a" in the loop are: 7 8 9 11

2、

下面这段的“!”不懂：

  

10 a=`echo Hello!`   # Assigns result of 'echo' command to 'a'     11 echo $a     12 #  Note that including an exclamation mark (!) within a     13 #+ command substitution construct #+ will not work from the command line,    14 #+ since this triggers the Bash "history mechanism."     15 #  Inside a script, however, the history functions are disabled.

3、``相当于$()

2 R=$(cat /etc/redhat-release)

4、英语生词：

disguised

naked

trailing

1、双引号内的变量可以保留变量值当中的超过一个空格的空格：

(abs-guide-3.7/HTML/variables.html#VARSUBN)

Example 4-1. Variable assignment and substitution

33 hello="A B  C   D"   34 echo $hello   # A B C D   35 echo "$hello" # A B  C   D   36 # As you see, echo $hello   and   echo "$hello"   give different results.   37 #                                      ^      ^   38 # Quoting a variable preserves whitespace.

2、可以在一行中对多个变量赋值，但是在有可能引起版本兼容性问题：

(abs-guide-3.7/HTML/variables.html#VARSUBN)

Example 4-1. Variable assignment and substitution

57 #  It is permissible to set multiple variables on the same line,   58 #+ if separated by white space.   59 #  Caution, this may reduce legibility, and may not be portable.   60   61 var1=21  var2=22  var3=$V3   62 echo   63 echo "var1=$var1   var2=$var2   var3=$var3"   64   65 # May cause problems with older versions of "sh".

3、在没有初始化的变量上进行算术运算不是非法的，当作是"0"，但是在有可能引起版本兼容性问题：

(abs-guide-3.7/HTML/variables.html#VARSUBN)

An uninitialized variable has a "null" value - no assigned value at all (not zero!). Using a variable before assigning a value to it will usually cause problems. It is nevertheless possible to perform arithmetic operations on an uninitialized variable.    1 echo "$uninitialized"                                # (blank line)    2 let "uninitialized += 5"                             # Add 5 to it.    3 echo "$uninitialized"                                # 5    4    5 #  Conclusion:    6 #  An uninitialized variable has no value, however    7 #+ it acts as if it were 0 in an arithmetic operation.    8 #  This is undocumented (and probably non-portable) behavior.