How to use double or single brackets, parentheses, curly braces
我对bash中括号、圆括号、花括号的用法以及它们的双形式或单形式之间的区别感到困惑。有明确的解释吗？
在bash中，
双支架，使额外的功能。例如，你可以使用
此外，delimiting的牙套，该变量的名称是用于参数展开的东西：你可以像操作系统
-
截断变量的内容
$ var="abcde"; echo ${var%d*} abc -
sed 结构类似于make$ var="abcde"; echo ${var/de/12} abc12 -
使用默认值
$ default="hello"; unset var; echo ${var:-$default} hello -
和多更多
另外，支撑expansions创建列表，这是在typically strings中迭代循环：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 | $ echo f{oo,ee,a}d food feed fad $ mv error.log{,.OLD} (error.log is renamed to error.log.OLD because the brace expression expands to"mv error.log error.log.OLD") $ for num in {000..2}; do echo"$num"; done 000 001 002 $ echo {00..8..2} 00 02 04 06 08 $ echo {D..T..4} D H L P T |
请注意，在领先的零和增量功能没有不可用在bash 4。
我感谢我们expansions reminding gboffi是支撑。
双parentheses是用于算术操作：
1 2 3 4 5 6 7 | ((a++)) ((meaning = 42)) for ((i=0; i<10; i++)) echo $((a + b + (14 * c))) |
他们使你omit美元符号的整数变量和数组，包括在线和readability空间是在运营商。
单阵列也被用于例如指数：
1 2 3 | array[4]="hello" element=${array[index]} |
卷曲的支撑是必要的(几乎全是(？)数组引用的对侧手。
我reminded ephemient的评论也被用于parentheses壳层。这就是他们用来创建数组。
1 2 3 | array=(1 2 3) echo ${array[1]} 2 |
单支架(
1 2 3 | $ VARIABLE=abcdef $ if [ $VARIABLE == abcdef ] ; then echo yes ; else echo no ; fi yes |
在双括号(
1 2 3 | $ VARIABLE=abcdef $ if [[ $VARIABLE == 123456 ]] ; then echo yes ; else echo no ; fi no |
parentheses(
1 2 3 4 5 6 | $ pwd /home/user $ (cd /tmp; pwd) /tmp $ pwd /home/user |
你可以看到的，允许你执行操作的计算没有影响当前shell环境。
(一)支撑(
1 2 3 4 5 6 7 | $ VARIABLE=abcdef $ echo Variable: $VARIABLE Variable: abcdef $ echo Variable: $VARIABLE123456 Variable: $ echo Variable: ${VARIABLE}123456 Variable: abcdef123456 |
(二)支撑也被用来运行在当前的shell命令序列上下文。
1 2 3 4 5 6 | $ { 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 |
差分有一个微妙的句法与
例如
1 2 3 4 5 | 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 /脚本/过时
卷曲支架
1 2 3 4 5 6 | ${variable} Parameter substitution ${!variable} Indirect variable reference { command1; command2; . . . commandN; } Block of code {string1,string2,string3,...} Brace expansion {a..z} Extended brace expansion {} Text replacement, after find and xargs |
parentheses
1 2 3 4 5 | ( command1; command2 ) Command group executed within a subshell Array=(element1 element2 element3) Array initialization result=$(COMMAND) Command substitution, new style >(COMMAND) Process substitution <(COMMAND) Process substitution |
双parentheses
1 2 3 4 5 | (( var = 78 )) Integer arithmetic var=$(( 20 + 5 )) Integer arithmetic, with variable assignment (( var++ )) C-style variable increment (( var-- )) C-style variable decrement (( var0 = var1<98?9:21 )) C-style ternary operation |
我只是想添加这些来自tldp：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 | ~:$ 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 |
测试之间的差异的解释是，[与[在bashfaq在伟大的细节。
To cut a long story short: test implements the old, portable syntax of
the command. In almost all shells (the oldest Bourne shells are the
exception), [ is a synonym for test (but requires a final argument of
]). Although all modern shells have built-in implementations of [,
there usually still is an external executable of that name, e.g.
/bin/[.[[ is a new improved version of it, which is a keyword, not a program.
This has beneficial effects on the ease of use, as shown below. [[ is
understood by KornShell and BASH (e.g. 2.03), but not by the older
POSIX or BourneShell.
的结论：
When should the new test command [[ be used, and when the old one [?
If portability to the BourneShell is a concern, the old syntax should
be used. If on the other hand the script requires BASH or KornShell,
the new syntax is much more flexible.
在函数定义parentheses
parentheses
1 | function_name () { command1 ; command2 ; } |
这是你逃避的原因，甚至在parentheses命令参数：
1 2 3 4 5 6 7 8 9 | $ echo ( bash: syntax error near unexpected token `newline' $ echo \( ( $ echo () { command echo The command echo was redefined. ; } $ echo anything The command echo was redefined. |
1 2 3 4 5 6 7 8 9 10 11 12 13 14 | Truncate the contents of a variable $ var="abcde"; echo ${var%d*} abc Make substitutions similar to sed $ var="abcde"; echo ${var/de/12} abc12 Use a default value $ default="hello"; unset var; echo ${var:-$default} hello |