标签：高级Bash脚本编程指南

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.

read echo >$2 不懂

(http://www.linuxsir.org/main/doc/abs/HTML/special-chars.html)

Ctl-M

Carriage return.

   1 #!/bin/bash

   2 # Thank you, Lee Maschmeyer, for this example.

   3

   4 read -n 1 -s -p $'Control-M leaves cursor at beginning of this line. Press Enter. \x0d'

   5                                   # Of course, '0d' is the hex equivalent of Control-M.

   6 echo >&2   #  The '-s' makes anything typed silent,

   7            #+ so it is necessary to go to new line explicitly.

   8

   9 read -n 1 -s -p $'Control-J leaves cursor on next line. \x0a'

  10            #  '0a' is the hex equivalent of Control-J, linefeed.

  11 echo >&2

  12

  13 ###

  14

  15 read -n 1 -s -p $'And Control-K\x0bgoes straight down.'

  16 echo >&2   #  Control-K is vertical tab.

  17

  18 # A better example of the effect of a vertical tab is:

  19

  20 var=$'\x0aThis is the bottom line\x0bThis is the top line\x0a'

  21 echo "$var"

  22 #  This works the same way as the above example. However:

  23 echo "$var" | col

  24 #  This causes the right end of the line to be higher than the left end.

  25 #  It also explains why we started and ended with a line feed --

  26 #+ to avoid a garbled screen.

  27

  28 # As Lee Maschmeyer explains:

  29 # --------------------------

  30 #  In the [first vertical tab example] . . . the vertical tab

  31 #+ makes the printing go straight down without a carriage return.

  32 #  This is true only on devices, such as the Linux console,

  33 #+ that can't go "backward."

  34 #  The real purpose of VT is to go straight UP, not down.

  35 #  It can be used to print superscripts on a printer.

  36 #  The col utility can be used to emulate the proper behavior of VT.

  37

  38 exit 0

 

习题的问题：

Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting Prev Chapter 2. Starting Off With a Sha-Bang Next 2.2. Preliminary Exercises    1.       System administrators often write scripts to automate common tasks. Give several instances where such scripts would be useful.    2.       Write a script that upon invocation shows the time and date, lists all logged-in users, and gives the system uptime. The script then saves this information to a logfile. Prev Home Next Starting Off With a Sha-Bang Up Basics

我的答案：

第一题：

backup database; restart web service; backup files; etc..

第二题：

#!/bin/bash

#exercise 2.2

#show date and time

echo "The date and time is: "

date

echo "lists all logged-in users: "

who

echo "the uptime is: "

uptime

echo "write the information above into file ./ex2.2.out"

#if the file ./ex2.2.out exists, delete it at first.

#to do.

echo "The date and time is: " >>  ./ex2.2.out

date >> ./ex2.2.out

echo "lists all logged-in users: " >>  ./ex2.2.out

who  >> ./ex2.2.out

echo "the uptime is: " >>  ./ex2.2.out

uptime >> ./ex2.2.out

echo "finished the work,exit."

exit 0

shell脚本在unix，linux世界是一种重要和基础的技术，你看看linux中大量的系统配置文件都是shell脚本就知道了。它对这类系统的灵活和高效起着非常大的作用。

用好linux就必须有shell脚本编程的能力。

为此，我特地把学习这本电子书作为一个独立的目标，这个目标是学习linux总目标的一个分目标。

学习教程：《Advanced Bash-Scripting Guide》

学习时间：有空就看，不限时间。

准备学习两遍，第一遍粗看，看英文版，对其中的全部例子过一遍;第二遍看中文版，细看重点，并纠正自己的英文理解错误，争取学英语和学shell编程一举两得。

随时把学习心得和问题记成日记，并把英文的疑问也记下来。

欢迎有兴趣、有需要的朋友加入到这个学习目标的进行中来，大家互相交流和进步。

这两天在看那本讲bash编程的电子书。

在这本电子书中有几个有趣的shell脚本。

1、(abs-guide-3.7/HTML/sha-bang.html)

在一个文本文件的前面加上：#!/bin/more 就可以让文本文件自己显示自己; 在一个文件前面加上：#!/bin/rm 就可以自己删除自己;

2、找出存储设备的某类文件并把它们打包：

(abs-guide-3.7/HTML/special-chars.html)

  find . -mtime -1 -type f -print0 | xargs -0 tar rvf "bak.tar"

3、一个执行结果不像书上说的那样的shell:

(abs-guide-3.7/HTML/special-chars.html)

Ctl-H "Rubout" (destructive backspace). Erases characters the cursor backs over while backspacing.    1 #!/bin/bash    2 # Embedding Ctl-H in a string.    3    4 a="^H^H"                  # Two Ctl-H's (backspaces).    5 echo "abcdef"             # abcdef    6 echo -n "abcdef$a "       # abcd f    7 #  Space at end  ^              ^ Backspaces twice.    8 echo -n "abcdef$a"        # abcdef    9 #  No space at end                Doesn't backspace (why?).   10                           # Results may not be quite as expected.   11 echo; echo