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Top Forums UNIX for Dummies Questions & Answers Faster than nested while read loops? Post 302545657 by chstr_14 on Tuesday 9th of August 2011 05:06:46 AM
Old 08-09-2011
Faster than nested while read loops?

Hi experts,
I just want to know if there is a better solution to my nested while read loops below:

Code:
 
while read line; do
    while read line2; do
        while read line3; do
            echo "$line $line2 $line3"
        done < file3.txt
    done < file2.txt
done < file1.txt > output.txt

file1.txt:
1a
1b

file2.txt:
2a
2b

file3.txt:
3a
3b

output.txt:
1a 2a 3a
1a 2a 3b
1a 2b 3a
1a 2b 3b
1b 2a 3a
1b 2a 3b
1b 2b 3a
1b 2b 3b

I am thinking that a perl or an awk script could do this, which are much faster than while loops.

Thanks!
 

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merge(1)						      General Commands Manual							  merge(1)

NAME
merge - three-way file merge SYNOPSIS
file1 file2 file3 DESCRIPTION
combines two files that are revisions of a single original file. The original file is file2, and the revised files are file1 and file3. identifies all changes that lead from file2 to file3 and from file2 to file1, then deposits the merged text into file1. If the option is used, the result goes to standard output instead of file1. An overlap occurs if both file1 and file3 have changes in the same place. prints how many overlaps occurred, and includes both alterna- tives in the result. The alternatives are delimited as follows: lines in file1 lines in file3 If there are overlaps, edit the result in file1 and delete one of the alternatives. This command is particularly useful for revision control, especially if file1 and file3 are the ends of two branches that have file2 as a common ancestor. EXAMPLES
A typical use for is as follows: 1. To merge an RCS branch into the trunk, first check out the three different versions from RCS (see co(1)) and rename them for their revision numbers: 5.2, 5.11, and 5.2.3.3. File 5.2.3.3 is the end of an RCS branch that split off the trunk at file 5.2. 2. For this example, assume file 5.11 is the latest version on the trunk, and is also a revision of the "original" file, 5.2. Merge the branch into the trunk with the command: 3. File 5.11 now contains all changes made on the branch and the trunk, and has markings in the file to show all overlapping changes. 4. Edit file 5.11 to correct the overlaps, then use the command to check the file back in (see ci(1)). WARNINGS
uses the ed(1) system editor. Therefore, the file size limits of ed(1) apply to AUTHOR
was developed by Walter F. Tichy. SEE ALSO
diff3(1), diff(1), rcsmerge(1), co(1). merge(1)
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