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Full Discussion: nawk use
Top Forums UNIX for Advanced & Expert Users nawk use Post 302157375 by Perderabo on Thursday 10th of January 2008 07:34:27 PM
Old 01-10-2008
'c-->0;$0~s{if(b)for(c=b+1;c>1;c--)print r[(NR-c+1)%b];print;c=a}b{r[NR%b]=$0}'
has 3 pattern-action statements:
1) c-->0;
2) $0~s{if(b)for(c=b+1;c>1;c--)print r[(NR-c+1)%b];print;c=a}
3) b{r[NR%b]=$0}

The first one has no explicit action so the action is to simply print the entire record. But in this case the semicolon is needed so that it doesn't run in to the second staement. The second statement has an explicit action which is in braces and the braces are enough to separate it from the third. Now consider these statements in reverse order...

3) b{r[NR%b]=$0}
The pattern is b, which is asking if b is equal to zero. If b is non-zero we need to save records in case we need them later. But if b is zero, we can skip this since we do not want any "before" records displayed. If b is, say, 5, we will always have the last 5 records in the r array.

2) $0~s{if(b)for(c=b+1;c>1;c--)print r[(NR-c+1)%b];print;c=a}
The pattern $0~s simply asks if the record matches the search criteria we stored in s. If so and if b is non-zero, we print those records that we saved in step 3 above. Then we print the current record. Then c=a sets up the next step to be explained...

1) c-->0;
c gets set to a (number of "after" records) when we find a match. The c-- part decrements c after we use it. And we use it to see if it is greater than zero. This is how the "after" records are printed.
Last edited by Perderabo; 01-17-2008 at 08:34 AM.. Reason: fix some typos
 

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LEARN ABOUT OSX

ucblinks

ucblinks(1B)					     SunOS/BSD Compatibility Package Commands					      ucblinks(1B)

NAME

       ucblinks - adds /dev entries to give SunOS 4.x compatible names to SunOS 5.x devices

SYNOPSIS

       /usr/ucb/ucblinks [-e rulebase] [-r rootdir]

DESCRIPTION

       ucblinks  creates symbolic links under the /dev directory for devices whose SunOS 5.x names differ from their SunOS 4.x names. Where possi-
       ble, these symbolic links point to the device's SunOS 5.x name rather than to the actual /devices entry.

       ucblinks does not remove unneeded compatibility links; these must be removed by hand.

       ucblinks should be called each time the system is reconfiguration-booted, after any new SunOS 5.x links that are needed have been  created,
       since the reconfiguration may have resulted in more compatibility names being needed.

       In  releases prior to SunOS 5.4, ucblinks used a  nawk rule-base to construct the SunOS 4.x compatible names. ucblinks no longer uses  nawk
       for the default operation, although  nawk rule-bases can still be specifed with the -e option.  The  nawk rule-base equivalent to the SunOS
       5.4 default operation can be found in /usr/ucblib/ucblinks.awk.

OPTIONS

       -e rulebase     Specify rulebase as the file containing nawk(1) pattern-action statements.

       -r rootdir      Specify rootdir as the directory under which dev and devices will be found, rather than the standard root directory /.

FILES

       /usr/ucblib/ucblinks.awk        sample rule-base for compatibility links

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Availability		     |SUNWscpu			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       devlinks(1M), disks(1M), ports(1M), tapes(1M), attributes(5)

SunOS 5.10							    13 Apr 1994 						      ucblinks(1B)
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