标签：Advanced Bash-Scripting Guide

9.2. Manipulating Strings

1、String Length

expr "$string" : '.*'

3、Index这个表达式跟java的用法好像不同

expr index $string $substring

    Numerical position in $string of first character in $substring that matches.

5 echo `expr index "$stringZ" 1c`              # 3    6 # 'c' (in #3 position) matches before '1'.

4?{string:position:length}负数参数的用法有点怪

    Extracts $length characters of substring from $string at $position.

16 echo ${stringZ:-4}                           # abcABC123ABCabc   17 # Defaults to full string, as in ${parameter:-default}.   18 # However . . .   19   20 echo ${stringZ:(-4)}                         # Cabc   21 echo ${stringZ: -4}                          # Cabc   22 # Now, it works.   23 # Parentheses or added space "escape" the position parameter.   24   25 # Thank you, Dan Jacobson, for pointing this out.

而且下面这段不懂：

If the $string parameter is "*" or "@", then this extracts a maximum of $length positional parameters, starting at $position.        1 echo ${*:2}          # Echoes second and following positional parameters.        2 echo ${@:2}          # Same as above.        3        4 echo ${*:2:3}        # Echoes three positional parameters, starting at second.

5、Example 9-12. Emulating getopt其中的字符串处理不甚明了

14       if [ ${1:0:1} = '/' ]   15       then   16           tmp=${1:1}               # Strip off leading '/' . . .   17           parameter=${tmp%%=*}     # Extract name.   18           value=${tmp##*=}         # Extract value.   19           echo "Parameter: '$parameter', value: '$value'"   20           eval $parameter=$value   21       fi

6、awk处理函数不懂

14 # The awk equivalent of ${string:pos:length} is substr(string,pos,length).   15 echo | awk '   16 { print substr("'"${String}"'",3,4)      # skid   17 }   18 '

7、9.2.2. Further Discussion进一步讨论的几个例子没看

英语疑问：

1、emulation, getopt

A simple emulation of getopt  using substring extraction constructs.

9.1. Internal Variables

1、不懂：

$DIRSTACK     the top value in the directory stack (affected by pushd and popd)     This builtin variable corresponds to the dirs command, however dirs shows     the entire contents of the directory stack.

2、

 

$EUID     "effective" user ID number     Identification number of whatever identity the current user has assumed,     perhaps by means of su.     Caution The $EUID is not necessarily the same as the $UID.

3、

$GLOBIGNORE     A list of filename patterns to be excluded from matching in globbing.

4、

root# echo ${GROUPS[1]}      1

5、

$IFS defaults to whitespace (space, tab, and newline), but may be changed,     for example, to parse a comma-separated data file. Note that $* uses the     first character held in $IFS. See Example 5-1.      bash$ echo $IFS | cat -vte      $      bash$ bash -c 'set w x y z; IFS=":-;"; echo "$*"'      w:x:y:z

6、IFS是空格时，字符间的多个空格视为一个分隔符，第一个字符前面的空格会忽略不计。见例子ifs.sh。

14 IFS=" "               15 var=" a  b c   "  ...               26 IFS=:               27 var=":a::b:c:::"               # Same as above, but substitute

运行结果：

[mdx@localhost abs-exercises]$ ./ex9-1.sh IFS=" " ------- the arg is: [a] the arg is: [b] the arg is: [c] IFS=: ----- the arg is: [] the arg is: [a] the arg is: [] the arg is: [b] the arg is: [c] the arg is: [] the arg is: [] [mdx@localhost abs-exercises]$

7、$LINENO 不懂

$LINENO     This variable is the line number of the shell script in which this variable     appears. It has significance only within the script in which it appears,     and is chiefly useful for debugging purposes.        1 # *** BEGIN DEBUG BLOCK ***        2 last_cmd_arg=$_  # Save it.        3        4 echo "At line number $LINENO, variable \"v1\" = $v1"        5 echo "Last command argument processed = $last_cmd_arg"        6 # *** END DEBUG BLOCK ***

8、$PIPESTATUS不懂，并且在本机上的执行结果与书中的不同

bash$ echo $PIPESTATUS      0      bash$ ls -al | bogus_command      bash: bogus_command: command not found      bash$ echo $PIPESTATUS      141      bash$ ls -al | bogus_command      bash: bogus_command: command not found      bash$ echo $?      127 ...           bash$ echo ${PIPESTATUS[@]}

9、不懂

$PWD的：

22 # rm -f .[^.]* ..?*   to remove filenames beginning with multiple dots.      23 # (shopt -s dotglob; rm -f *)   will also work.       24 # Thanks, S.C. for pointing this out.

10、不懂$SHELLOPTS的用法

[mdx@localhost abs-exercises]$ echo $SHELLOPTS braceexpand:emacs:hashall:histexpand:history:interactive-comments:monitor

11、$SHLVL的用法

$SHLVL     Shell level, how deeply Bash is nested. If, at the command line, $SHLVL is     1, then in a script it will increment to 2.

12、$TMOUT的用法不懂，例子example9-2~9-4不懂

13、$UID，下面这段话的意思不懂

This is the current user's real id, even if she has temporarily assumed

    another identity through su.

14、example9-7太复杂了，留在这里备忘

Caution The $* and $@ parameters sometimes display inconsistent and             puzzling behavior, depending on the setting of $IFS.     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━     Example 9-7. Inconsistent $* and $@ behavior

15、set命令的用法

[mdx@localhost abs-exercises]$ set a b c [mdx@localhost abs-exercises]$ echo $1 $2 $3 a b c [mdx@localhost abs-exercises]$

16、$!的用法

书上的例子：

LOG=$0.log COMMAND1="sleep 100" echo "Logging PIDs background commands for script: $0" >> "$LOG" # So they can be monitored, and killed as necessary. echo >> "$LOG" # Logging commands. echo -n "PID of \"$COMMAND1\":  " >> "$LOG" ${COMMAND1} & echo $! >> "$LOG" # PID of "sleep 100":  1506 # Thank you, Jacques Lederer, for suggesting this. #下面这句不大懂：eval, kill -9 #possibly_hanging_job & { sleep ${TIMEOUT}; eval 'kill -9 $!' &> /dev/null; }       # Forces completion of an ill-behaved program. # Useful, for example, in init scripts. # Thank you, Sylvain Fourmanoit, for this creative use of the "!" varia

英语疑问:

1、subtleties, nuances

This requires learning their subtleties and nuances.

2、necessarily

$SHELL does not necessarily give the correct answer.

3、collation order, mishandled

$LC_COLLATE

    Often set in the .bashrc or /etc/profile files, this variable controls

    collation order in filename expansion and pattern matching. If mishandled,

    LC_COLLATE can cause unexpected results in filename globbing.

4、revert, customary

To revert to the customary behavior of bracket matching, set LC_COLLATE to C by an export LC_COLLATE=C in /etc/profile and/or ~/.bashrc.

5、expedient

In a script, it may be expedient to temporarily add a directory to the path

    in this way.

6、likewise

         $SHELL, the name of the user's login shell, may be set from /

         etc/passwd or in an "init" script, and it is likewise not a

         Bash builtin.

8.2. Numerical Constants

1、Example 8-4. Representation of numerical constants

同C语言一样，字符也是整数。但是@和_代表是多少不知道？而且把它们转化为十进制的方法忘记了。

  32 let "b64 = 64#@_"

  33 echo "base-64 number = $b64"             # 4031

  34 # This notation only works for a limited range (2 - 64) of ASCII characters  35 # 10 digits + 26 lowercase characters + 26 uppercase characters + @ + _

  36

  37

  38 echo

  39

  40 echo $((36#zz)) $((2#10101010)) $((16#AF16)) $((53#1aA))

  41                                          # 1295 170 44822 3375

英语疑难: evaluates

A number with an embedded # evaluates as BASE#NUMBER (with range and notational restrictions).

Chapter 8. Operations and Related Topics

8.1. Operators

1、Example 8-1. Greatest common divisor最大公约数的算法不懂，里面的练习：判断命令行参数是否是整数不懂，记在这里。

2、发现一个问题，bash的算术操作符只能进行整数的运算，要进行其它的复杂点的运算：如开方、小数等怎么办呢？

3、Example 8-2. Using Arithmetic Operations

这个例子有不少地方不明白，记在这里以后进一步解决：

我知道 let "z = 2 + 3" 相当于 (( z = 2 + 3 ))，根据：

Similar to the let command, the ((...)) construct permits arithmetic expansion and evaluation.

1）、

10 : $((n = $n + 1))       11 #  ":" necessary because otherwise Bash attempts       12 #+ to interpret "$((n = $n + 1))" as a command.

2)、      20 n=$(($n + 1))

3)、

      23 : $[ n = $n + 1 ]

      24 #  ":" necessary because otherwise Bash attempts

      25 #+ to interpret "$[ n = $n + 1 ]" as a command.

      26 #  Works even if "n" was initialized as a string.

4)、

      29 n=$[ $n + 1 ]

      30 #  Works even if "n" was initialized as a string.

      31 #* Avoid this type of construct, since it is obsolete and

nonportable.

      32 #  Thanks, Stephane Chazelas.

5)、      44 : $(( n++ ))       # : $(( ++n )) also works.

6）、      47 : $[ n++ ]         # : $[ ++n ]] also works

4、bash不能处理浮点数：

Bash does not understand floating point arithmetic. It treats numbers containing a decimal point as strings.

5、

if [ $condition1 -a $condition2 ] 相当于 if [[ $condition1 && $condition2 ]]

，但是&&不能用在[...]结构中，这些有什么规律吗？真不好掌握啊。

6、Example 8-3. Compound Condition Tests Using && and ||

6 if [ "$a" -eq 24 ] && [ "$b" -eq 47 ] 17 #  Note:  if [[ $a -eq 24 && $b -eq 24 ]]  works. 20 #    (The "&&" has a different meaning in line 17 than in line 6.)

我不懂&&在6行和17行有什么区别？

7、不懂 $((...))的用法

bash$ echo $(( 1 && 2 )) $((3 && 0)) $((4 || 0)) $((0 || 0))      1 0 1 0

英语疑问:

1、Modulo turns up surprisingly often in various numerical recipes.

2、erroneous

An operation that takes a variable outside these limits will give an erroneous result.

3、flip, relevant, fly

"Bit flipping" is more relevant to compiled languages, such

as C and C++, which run fast enough to permit its use on the fly.

4、miscellaneous operators

5、evaluated

All the operations are evaluated (with possible side effects), but only the last operation is returned.