2008/08/25

重温 shell 编程的 变量操作

Shell编程的时候,总会觉得字符串的处理很麻烦! 其实人家也觉得麻烦,ksh/bash中自有一套办法来帮助你,只是很多时候我们都不知道或者没注意而已!
特别是有正则表达式

The character $ is used to introduce substitutable parameters.

${parameter}

The shell reads all the characters from ${ to the matching } as part of the same word even if it contains braces or metacharacters. The value, if any, of the parameter is substituted. The braces are required when parameter is followed by a letter, digit, or underscore that is not to be interpreted as part of its name, when the variable name contains a .. The braces are also required when a variable is subscripted unless it is part of an Arithmetic Expression or a Conditional Expression. If parameter is one or more digits then it is a positional parameter. A positional parameter of more than one digit must be enclosed in braces. If parameter is * or @, then all the positional parameters, starting with $1, are substituted (separated by a field separator character). If an array vname with subscript * or @ is used, then the value for each of the elements is substituted, separated by the first character of the value of IFS.


${#parameter}

If parameter is * or @, the number of positional parameters is substituted. Otherwise, the length of the value of the parameter is substituted.


${#vname[*]}
${#vname[@]}

The number of elements in the array vname is substituted.


${!vname}

Expands to the name of the variable referred to by vname. This will be vname except when vname is a name reference.


${!vname[subscript]}

Expands to name of the subscript unless subscript is * or @. When subscript is *, the list of array subscripts for vname is generated. For a variable that is not an array, the value is 0 if the variable is set. Otherwise it is null. When subscript is @, same as above, except that when used in double quotes, each array subscript yields a separate argument.


${!prefix*}

Expands to the names of the variables whose names begin with prefix.


${parameter:-word}

If parameter is set and is non-null then substitute its value; otherwise substitute word.


${parameter:=word}

If parameter is not set or is null then set it to word; the value of the parameter is then substituted. Positional parameters may not be assigned to in this way.


${parameter:?word}

If parameter is set and is non-null then substitute its value; otherwise, print word and exit from the shell (if not interactive). If word is omitted then a standard message is printed.


${parameter:+word}

If parameter is set and is non-null then substitute word; otherwise substitute nothing.

In the above, word is not evaluated unless it is to be used as the substituted string, so that, in the following example, pwd is executed only if d is not set or is null:

print ${d:-$(pwd)}

If the colon ( : ) is omitted from the above expressions, then the shell only checks whether parameter is set or not.


${parameter:offset:length}
${parameter:offset}

Expands to the portion of the value of parameter starting at the character (counting from 0) determined by expanding offset as an arithmetic expression and consisting of the number of characters determined by the arithmetic expression defined by length. In the second form, the remainder of the value is used. If A negative offset counts backwards from the end of parameter.
Note that one or more blanks is required in front of a minus sign to prevent the shell from interpreting the operator as :-. If parameter is * or @, or is an array name indexed by * or @, then offset and length refer to the array index and number of elements respectively. A negative offset is taken relative to one greater than the highest subscript for indexed arrays. The order for associate arrays is unspecified.



${parameter#pattern}
${parameter##pattern}

If the shell pattern matches the beginning of the value of parameter, then the value of this expansion is the value of the parameter with the matched portion deleted; otherwise the value of this parameter is substituted. In the first form the smallest matching pattern is deleted and in the second form the largest matching pattern is deleted. When parameter is @, *, or an array variable with subscript @ or *, the substring operation is applied to each element in turn.



${parameter%pattern}
${parameter%%pattern}

If the shell pattern matches the end of the value of parameter, then the value of this expansion is the value of the parameter with the matched part deleted; otherwise substitute the value of parameter. In the first form the smallest matching pattern is deleted and in the second form the largest matching pattern is deleted. When parameter is @, *, or an array variable with subscript @ or *, the substring operation is applied to each element in turn.



${parameter/pattern/string}
${parameter//pattern/string}
${parameter/#pattern/string}
${parameter/%pattern/string}

Expands parameter and replaces the longest match of pattern with the given string. Each occurrence of \n in string is replaced by the portion of parameter that matches the n-th sub-pattern. In the first form, only the first occurrence of pattern is replaced. In the second form, each match for pattern is replaced by the given string. The third form restricts the pat-
tern match to the beginning of the string while the fourth form restricts the pattern match to the end of the string. When string is null, the pattern will be deleted and the / in front
of string may be omitted. When parameter is @, *, or an array variable with subscript @ or *, the substitution operation is applied to each element in turn. In this case, the string portion of word will be re-evaluated for each element.

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