关注bash新特性

最近无意中再次查阅abs文档时，发现了bash 4 4.1 4.2新特性，清除了困扰多年不愿使用bash的阻碍。
在这里我只列出了自己关注的特性。

2009年2月发布的bash4：
1. 关联数组/Hash表

abs中的样例：
#创建

declare -A address
address[Charles]="414 W. 10th Ave., Baltimore, MD 21236"
address[John]="202 E. 3rd St., New York, NY 10009"
address[Wilma]="1854 Vermont Ave, Los Angeles, CA 90023"

#通过键提取值
echo "Charles's address is ${address[Charles]}."
# Charles

2. ;;& ;&  用于case语句中的连续匹配
;;& 继续匹配下一个模式条件，并根据结果执行相关语句。
;&  直接执行下一条语句，(我的理解是不进行模式匹配)。

例子:

test_char ()
{
case "$1" in
[[:print:]] ) echo "$1 is a printable character.";;& # |
# The ;;& terminator continues to the next pattern test. |
[[:alnum:]] ) echo "$1 is an alpha/numeric character.";;& # v
[[:alpha:]] ) echo "$1 is an alphabetic character.";;& # v
[[:lower:]] ) echo "$1 is a lowercase alphabetic character.";;&
[[:digit:]] ) echo "$1 is an numeric character.";& # |
# The ;& terminator executes the next statement ... # |
%%%@@@@@ ) echo "********************************";; # v
# ^^^^^^^^ ... even with a dummy pattern.
esac
}

3. bash新的协程语句 coproc
coproc， 协程可以命名，可以让主程获取协程的输入输出。由于协程是异步执行的，所以有可能会比主程和它交互之前结束运行。

coproc { cat mx_data.txt; sleep 2; }
# ^^^^^^^
# Try running this without "sleep 2" and see what happens.
while read -u ${COPROC[0]} line # ${COPROC[0]} is the
do #+ file descriptor of the coprocess.
echo "$line" | sed -e 's/line/NOT-ORIGINAL-TEXT/'
done
kill $COPROC_PID

4. 新的内置mapfile功能，使文件调入数组避免使用循环和替换语句。

mapfile Arr1 < $0
# Same result as Arr1=( $(cat $0) )
echo "${Arr1[@]}" # Copies this entire script out to stdout.

5. 参数变量描述符，使用大小写描述符可以指定变量内容的大小写情况。

#!/bin/bash4
var=veryMixedUpVariable
echo ${var} # veryMixedUpVariable
echo ${var^} # VeryMixedUpVariable
# * First char --> uppercase.
echo ${var^^} # VERYMIXEDUPVARIABLE
# ** All chars --> uppercase.
echo ${var,} # veryMixedUpVariable
# * First char --> lowercase.
echo ${var,,} # verymixedupvariable
# ** All chars --> lowercase.

6. declare 语句支持 -l 全部小写,  -c  第一字符大写其他小写。

#!/bin/bash4
declare -l var1 # Will change to lowercase
var1=MixedCaseVARIABLE
echo "$var1" # mixedcasevariable
# Same effect as echo $var1 | tr A-Z a-z
declare -c var2 # Changes only initial char to uppercase.
var2=originally_lowercase
echo "$var2" # Originally_lowercase
# NOT the same effect as echo $var2 | tr a-z A-Z

7. 大括号扩展指令

#!/bin/bash4
echo {40..60..2}
# 40 42 44 46 48 50 52 54 56 58 60
# All the even numbers, between 40 and 60.
echo {60..40..2}
# 60 58 56 54 52 50 48 46 44 42 40
# All the even numbers, between 40 and 60, counting backwards.
# In effect, a decrement.
echo {60..40..-2}
# The same output. The minus sign is not necessary.
# But, what about letters and symbols?
echo {X..d}
# X Y Z [ ] ^ _ ` a b c d
# Does not echo the \ which escapes a space.

补零指令，在大括号中第一个位置，表示补零状态的初始值，

8. 位置参数的字符串提取，纠正了bash3的索引号未和位置参数索引对齐问题。

#!/bin/bash
# show-params.bash
# Requires version 4+ of Bash.
# Invoke this scripts with at least one positional parameter.

E_BADPARAMS=99
if [ -z "$1" ]
then
echo "Usage $0 param1 ..."
exit $E_BADPARAMS
fi
echo ${@:0}
# bash3 show-params.bash4 one two three
# one two three
# bash4 show-params.bash4 one two three
# show-params.bash4 one two three
# $0 $1 $2 $3

9. 新的通配符操作，可以对文件名或目录名，进行递归匹配。

#!/bin/bash4
# filelist.bash4
shopt -s globstar # Must enable globstar, otherwise ** doesn

10. 新的$BASHPID全局内部变量

11. 新的出错处理插入函数

#!/bin/bash4
command_not_found_handle ()
{ # Accepts implicit parameters.
echo "The following command is not valid: \""$1\"""
echo "With the following argument(s): \""$2\"" \""$3\""" # $4, $5 ...
} # $1, $2, etc. are not explicitly passed to the function.
bad_command arg1 arg2
# The following command is not valid: "bad_command"
# With the following argument(s): "arg1" "arg2"

Bash 4.1  2010 5月
1. printf 函数开始支持,-v 参数，支持设置数据的索引值
2. 在双方括号中，<>号开始支持地方字符集的比较,由于地方字符集的不同，可以引起字符序号的大小变化。
3. 内置 read 函数，开始支持 -N选项，指定读入多少字符后结束。
4. 内嵌文档标识功能，可以用简单的)符号进行终结。
例如：

#!/bin/bash
# here-commsub.sh
# Requires Bash version -ge 4.1 ...
multi_line_var=$( cat <<ENDxxx
------------------------------
This is line 1 of the variable
This is line 2 of the variable
This is line 3 of the variable
------------------------------
ENDxxx)
# Rather than what Bash 4.0 requires:
#+ that the terminating limit string and
#+ the terminating close-parenthesis be on separate lines.
# ENDxxx
# )
echo "$multi_line_var"
# Bash still emits a warning, though.
# warning: here-document at line 10 delimited
#+ by end-of-file (wanted `ENDxxx

Bash 4.2
2011年2月发布
1. 开始支持 unicode 字符表达
例如：

echo -e '\u2630' # Horizontal triple bar character.
# Equivalent to the more roundabout:
echo -e "\xE2\x98\xB0"
# Recognized by earlier Bash versions.
echo -e '\u220F' # PI (Greek letter and mathematical symbol)
echo -e '\u0416' # Capital "ZHE" (Cyrillic letter)
echo -e '\u2708' # Airplane (Dingbat font) symbol
echo -e '\u2622' # Radioactivity trefoil
echo -e "The amplifier circuit requires a 100 \u2126 pull-up resistor."

2. lastpipe 功能。有了lastpipe, 终于bash可以像 korn shell 一样进行工作了。korn shell之所以可以优雅的工作，这个功能占的比例很大。当然，还有其他一些细节目前bash还是不具备。但有这个功能，bash已经可以干很多了。

#!/bin/bash
# lastpipe-option.sh
line='' # Null value.
echo "\$line = "$line"" # $line =
echo
shopt -s lastpipe # Error on Bash version -lt 4.2.
echo "Exit status of attempting to set \"lastpipe\" option is $?"
# 1 if Bash version -lt 4.2, 0 otherwise.
echo

head -1 $0 | read line # Pipe the first line of the script to read.
# ^^^^^^^^^ Not in a
echo "\$line = "$line""
# Older Bash releases $line =
# Bash version 4.2 $line = #!/bin/bash

3. 数组下标负值，允许从数组末尾开始进行标注
4. 子串提取，可以长度采用负值，表示从目标字串的末尾开始算起。
点击(此处)折叠或打开

# Important: It changes the interpretation of this
stringZ=abcABC123ABCabc
echo ${stringZ} # abcABC123ABCabc
# Position within string: 0123456789.....
echo ${stringZ:2:3} # cAB
# Count 2 chars forward from string beginning, and extract 3 chars.
# ${string:position:length}
# So far, nothing new, but now ...
# abcABC123ABCabc
# Position within string: 0123....6543210
echo ${stringZ:3:-6} # ABC123
# ^
# Index 3 chars forward from beginning and 6 chars backward from end,
#+ and extract everything in between.
# ${string:offset-from-front:offset-from-end}
# When the "length" parameter is negative,
#+ it serves as an offset-from-end parameter.
# See also neg-array.sh.
echo ${stringZ: -4}   #Cabc
echo ${stringZ:(-4)}  #Cabc