Pau*_*ce. 583
在Bash中,test并且[是内置的.
的双托架使附加功能.例如,您可以使用&&and ||而不是-a和,-o并且有一个正则表达式匹配运算符=~.
除了分隔变量名之外,大括号还用于参数扩展,因此您可以执行以下操作:
截断变量的内容
sed
test
使替换类似于 [
&&
||
使用默认值
-a
-o
还有几个
此外,大括号扩展创建字符串列表,这些字符串通常在循环中迭代:
$ time for ((i=0; i<10000000; i++)); do [[ "$i" = 1000 ]]; done
real 0m24.548s
user 0m24.337s
sys 0m0.036s
$ time for ((i=0; i<10000000; i++)); do [ "$i" = 1000 ]; done
real 0m33.478s
user 0m33.478s
sys 0m0.000s
请注意,在Bash 4之前,前导零和增量功能不可用.
感谢gboffi提醒我关于支撑扩张的信息.
双括号用于算术运算:
$ var="abcde"; echo ${var%d*}
abc
它们使您可以省略整数和数组变量上的美元符号,并在运算符周围包含空格以提高可读性.
单括号也用于数组索引:
$ var="abcde"; echo ${var/de/12}
abc12
右侧(大多数/全部？)阵列参考需要使用卷曲支撑.
ephemient的评论提醒我,圆括号也用于子壳.并且它们用于创建数组.
$ default="hello"; unset var; echo ${var:-$default}
hello
Car*_*rum 326
单个括号([)通常实际上调用一个名为的程序[; man test或man [了解更多信息.例:
$ VARIABLE=abcdef
$ if [ $VARIABLE == abcdef ] ; then echo yes ; else echo no ; fi
yes
双括号([[)与单个括号完全相同(基本上),但是内置的是bash.
$ VARIABLE=abcdef
$ if [[ $VARIABLE == 123456 ]] ; then echo yes ; else echo no ; fi
no
圆括号(())用于创建子shell.例如:
$ pwd
/home/user
$ (cd /tmp; pwd)
/tmp
$ pwd
/home/user
如您所见,子shell允许您在不影响当前shell环境的情况下执行操作.
4A.大括号({})用于明确识别变量.例:
$ VARIABLE=abcdef
$ echo Variable: $VARIABLE
Variable: abcdef
$ echo Variable: $VARIABLE123456
Variable:
$ echo Variable: ${VARIABLE}123456
Variable: abcdef123456
4B.大括号还用于在当前 shell上下文中执行一系列命令,例如
$ { date; top -b -n1 | head ; } >logfile
# 'date' and 'top' output are concatenated,
# could be useful sometimes to hunt for a top loader )
$ { date; make 2>&1; date; } | tee logfile
# now we can calculate the duration of a build from the logfile
但是,有一个微妙的句法差异( )(参见bash参考); 本质上,一个分号;括号内的最后一个命令后是必须的,而括号{,} 必须用空格包围.
Yol*_*ola 287
括号
if [ CONDITION ] Test construct
if [[ CONDITION ]] Extended test construct
Array[1]=element1 Array initialization
[a-z] Range of characters within a Regular Expression
$[ expression ] A non-standard & obsolete version of $(( expression )) [1]
[1] http://wiki.bash-hackers.org/scripting/obsolete
大括号
${variable} Parameter substitution
${!variable} Indirect variable reference
{ command1; command2; . . . commandN; } Block of code
{string1,string2,string3,...} Brace expansion
{a..z} Extended brace expansion
{} Text replacement, after find and xargs
括弧
( command1; command2 ) Command group executed within a subshell
Array=(element1 element2 element3) Array initialization
result=$(COMMAND) Command substitution, new style
>(COMMAND) Process substitution
<(COMMAND) Process substitution
双括号
(( var = 78 )) Integer arithmetic
var=$(( 20 + 5 )) Integer arithmetic, with variable assignment
(( var++ )) C-style variable increment
(( var-- )) C-style variable decrement
(( var0 = var1<98?9:21 )) C-style ternary operation
kzh*_*kzh 23
我只是想从TLDP添加这些:
~:$ echo $SHELL
/bin/bash
~:$ echo ${#SHELL}
9
~:$ ARRAY=(one two three)
~:$ echo ${#ARRAY}
3
~:$ echo ${TEST:-test}
test
~:$ echo $TEST
~:$ export TEST=a_string
~:$ echo ${TEST:-test}
a_string
~:$ echo ${TEST2:-$TEST}
a_string
~:$ echo $TEST2
~:$ echo ${TEST2:=$TEST}
a_string
~:$ echo $TEST2
a_string
~:$ export STRING="thisisaverylongname"
~:$ echo ${STRING:4}
isaverylongname
~:$ echo ${STRING:6:5}
avery
~:$ echo ${ARRAY[*]}
one two one three one four
~:$ echo ${ARRAY[*]#one}
two three four
~:$ echo ${ARRAY[*]#t}
one wo one hree one four
~:$ echo ${ARRAY[*]#t*}
one wo one hree one four
~:$ echo ${ARRAY[*]##t*}
one one one four
~:$ echo $STRING
thisisaverylongname
~:$ echo ${STRING%name}
thisisaverylong
~:$ echo ${STRING/name/string}
thisisaverylongstring
小智 18
测试之间的区别,[和[[在BashFAQ中详细解释.
简而言之:测试实现了命令的旧的可移植语法.在几乎所有的shell中(最老的Bourne shell都是例外),[是test的同义词(但需要最终参数为)).尽管所有现代shell都具有[的内置实现,但通常仍然存在该名称的外部可执行文件,例如/ bin/[.
[[是一个新的改进版本,它是一个关键字,而不是一个程序.这对易用性有益,如下所示.[[由KornShell和BASH(例如2.03)理解,但不是旧的POSIX或BourneShell.
结论是:
什么时候应该新的测试命令[[使用,当旧的[？] 如果关注BourneShell的可移植性,则应使用旧语法.另一方面,如果脚本需要BASH或KornShell,则新语法更灵活.
pab*_*ouk 18
括号()用于函数定义:
function_name () { command1 ; command2 ; }
这就是你必须在命令参数中转义括号的原因:
$ echo (
bash: syntax error near unexpected token `newline'
$ echo \(
(
$ echo () { command echo The command echo was redefined. ; }
$ echo anything
The command echo was redefined.