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Top Forums Shell Programming and Scripting awk removing data before or after a pattern Post 302443406 by agama on Sunday 8th of August 2010 07:05:09 PM
Old 08-08-2010
Quote:
Originally Posted by gvj
Could you pls explain why it is working??
His script is taking advantage of the fact that if the action part of a pattern-action pair is missing, the action {print;} is assumed. Further, !p is the same as saying 'p == 0' in the pattern portion of the statement. The value of an undefined variable is either 0 or "" depending on how it is used. To write the script in 'long form' might make more sense to you:

Code:
awk ' 
BEGIN { p = 0;}
p == 0 { print; }       # pattern not matched; print  (!p;)
/^03:00/ { p = 1; }  # pattern is matched, stop printing (exit(0) would be more efficient)
' file

Similarly, printing everything after the pattern line:
Code:
BEGIN { p = 0;}
/^03:00/ { p = 1; }  # pattern is matched, allow printing
p != 0 { print; }       # p;  in the original script
' file

This User Gave Thanks to agama For This Post:
 

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match(1)							Mail Avenger 0.8.3							  match(1)

NAME
match - Match strings against glob paterns SYNOPSIS
match [-gilrqs] [-n <n>] [-c cmd] [-x code] {[-p] pattern | -f <file>} str1 [str2 ...] DESCRIPTION
match checks strings against pattern, which should be a shell-like glob pattern. pattern may contain the following special characters: ? A "?" character in pattern matches any single character in the string, except that the "/" character is only matched if match was given the -s option. * A "*" character in pattern matches zero or more characters in the string. The exception is that it will only match "/" characters if match was given the -s option. [...] A set of characters between square brackets matches any character in the set. In addition, the "-" character can be used to specify a range. For example "[+e0-3]" would match any of the characters "+", "e", 0, 1, 2, or 3 in the input string. To include a hyphen ("-") in the set of characters matched, either include the hyphen first or last, or escape it with a "". [!...] A character class preceded by a "!" matches any character but those specified in the class. The exception is that the negated character class will match a "/" only if match was given the -s option. c The backslash character escapes the next character c. Thus, to match a literal "*", you would use the pattern "*". match prints each string that matches pattern, one per line, and exits 0 if one or more strings matched. If no string matches, match exits with status 67 (or whatever alternate status was specified by the -x flag). If the -n n flag was specified, match prints only the text that matched the nth occurrence of "*" in the patten. OPTIONS -f file Specifies that the pattern should be read from file. match will read each line of the file and consider it as pattern to match against the argument strings. For each argument string, match stops when it hits the first matching line of the file. If file does not exist, match exits 67, or whatever code was specified by -x. -g Normally, the -n option selects text matching particular "*" characters in the patern. -g changes this behavior to use parentheses for grouping. Thus, for instance, the text "foo.c" would match pattern "*(.[ch])", and the output with option -n 1 would be ".c". To include a literal "(" or ")" in the pattern with the -g option, you must precede the character with a "". -i Makes the match case insensitive. str will be considered to match if any variation on its capitalization would match. For example, string "G" would match pattern "[f-h]". -l When a pattern matches the string in more than one way, the -l flag says to assign as much text as possible to the leftmost "*"s in the pattern. For example, pattern "*+*" would match text "a+b+c", and the first "*" would match "a+b". This behavior is the default, thus -l's effect is only to undo a previous -r flag. -n n With this flag, match prints the text that matched the nth "*" in the pattern, as opposed to printing the whole string. The leftmost "*" corresponds to -n 1. Specifying -n 0 causes match to print the whole matching string. Specifying -n -1 or using a value greater than the number of "*"s in the pattern causes match not to print anything, in which case you can still use the exit status to see if there is a match. The default value for n is 0, unless -g has also been specified, in which case the default is 1. -c command When -c is specified, match runs command with the system shell (/bin/sh), giving it as argument $0 the full string that matched, and as arguments $1, $2, etc., the parts of the string that matched any "*"s in pattern. If the command does not exit with status 0, match will exit immediately, before processing further matches, with whatever status command returned. The -c and -n flags are mutually exclusive. -p pattern Specifies the pattern to match against. The -p flag is optional; you can specify pattern as the first argument following the options. However, if you want to try matching the same input string against multiple patterns, then you must specify each pattern with a -p flag. -q This option is synonymous with -n -1; it suppresses output when there is a match. You can still determine whether a match occurred by the exit status. -r When a pattern matches the string in more than one way, the -r flag says to assign as much text as possible to the rightmost "*"s in the pattern. For example, with -r, pattern "*+*" would match text "a+b+c" with the "*" matching "a", and the second matching "b+c". -s Ordinarily, "*", "?", and negated character classes ("[!...]") do not match "/" characters. -s changes this behavior to match slashes. -x code By default, when there is no match, match exits with status 67. With this option, match exits with status code, instead. EXAMPLES
Suppose you have a directory with a bunch of files ending .c and .o. If, for each file named foo.c you want to attempt to delete the file foo.o, you can run the following command: match -p '*.c' -c 'rm -f $1.o' *.c Servers running the mailman list manager often send mail from bounce addresses of the form listname-bounces@host.com. If you subscribe to multiple lists on the same server, the mailman interface makes it easier if you subscribe under the same address. To split the mail into multiple folders based on the bounce address in the environment variable SENDER, you might chose a mailbox with the following shell code: name=`match -n1 "*-bounces@host.com" "$SENDER"` && echo "$HOME/Mail/incoming/host-$name.spool" SEE ALSO
avenger(1), avenger.local(8) The Mail Avenger home page: <http://www.mailavenger.org/>. AUTHOR
David Mazieres Mail Avenger 0.8.3 2012-04-05 match(1)
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