如何将awk与多值定界符一起使用

Question

How I can use is awk delimiter which contains multivalue: "#@$"

I have file like this: Test1#@$Test2#@$Test3#@$Test4 I need to extract 'Test2'. After I execute this command: awk -F "#@$" '{print $2}' , nothing is displayed>

And after that awk -F "#@$" '{print $1}' i get full line

Any ideas?

Answer 1

The issue you are having is that the field separator FS is considered to be a regular expression. The <dollar>-character ( $ ) has a special meaning in regular expressions as it represents an anchor for the end-of-the-line. The solution is to escape it twice as the <backslash>-escapes are interpreted twice; once in lexical processing of the string and once in processing the regular expression:

awk -F '#@\\$' '{print $1}'

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An extended regular expression can be used to separate fields by assigning a string containing the expression to the built-in variable FS , either directly or as a consequence of using the -F sepstring option. The default value of the FS variable shall be a single <space>. The following describes FS behaviour:

1. If FS is a null string, the behaviour is unspecified.

2. If FS is a single character:

   • If FS is <space>, skip leading and trailing <blank> and <newline> characters; fields shall be delimited by sets of one or more <blank> or <newline> characters.
   • Otherwise, if FS is any other character c , fields shall be delimited by every single occurrence of c .

3. Otherwise, the string value of FS shall be considered to be an extended regular expression . Each occurrence of a sequence matching the extended regular expression shall delimit fields.

_{source: POSIX awk standard}

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A <dollar-sign> ( $ ) outside a bracket expression shall anchor the expression or subexpression it ends to the end of a string; such an expression or subexpression can match only a sequence ending at the last character of a string. For example, the EREs ef$ and (ef$) match ef in the string abcdef , but fail to match in the string cdefab , and the ERE e$f is valid, but can never match because the f prevents the expression e$ from matching ending at the last character.

_{source: POSIX Extended Regular Expressions}

Answer 2

Just wrap $ in brackets [] to remove its special significance

> cat t1
Test1#@$Test2#@$Test3#@$Test4
> awk -F '#@[$]' '{print $2}' t1
Test2
>