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Top Forums Shell Programming and Scripting Need shell script to read two file at same time and print out in single file Post 302295455 by mjd_tech on Sunday 8th of March 2009 11:37:03 AM
Old 03-08-2009
ripat,

I knew there would be a better way, most likely awk.
Here, I try to explain how it works...
Code:
awk '
NR==FNR {_[NR]=$0}  
                    # NR is total number of lines read in both files.
                    # FNR is number of lines read, reset for each file.
                    # _[NR] is an array named _ with NR as index.
                    # This loads array with contents of file1,
                    # and ignores file2.
                    
NR!=FNR {print _[FNR] "\n" $0 "\n---"}
                    # This part ignores file1.
                    # For each line in file 2,
                    # Print the corresponding line from the array,
                    # using FNR as index.
                    # then a newline, then the line from file2,
                    # another newline, then the dashes, then another newline.
' file1 file2

Hopefully, this is a correct explanation.
I was initially confused by the _[NL] thing, thinking this was some kind of special awk reserved variable or something, but then I realized it's just an array named _
I guess that if file1 were super huge, you could run out of memory because it gets loaded into an array. There's probably a way to do this in perl that can handle huge files without slurping into an array...

Regards,
MD
 

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DIFF(1) 						      General Commands Manual							   DIFF(1)

NAME
diff - print differences between two files SYNOPSIS
diff [-c | -e | -C n] [-br]file1 file2 OPTIONS
-C n Produce output that contains n lines of context -b Ignore white space when comparing -c Produce output that contains three lines of context -e Produce an ed-script to convert file1 into file2 -r Apply diff recursively to files and directories of EXAMPLES
diff file1 file2 # Print differences between 2 files diff -C 0 file1 file2 # Same as above diff -C 3 file1 file2 # Output three lines of context with every diff -c file1 file2 # Same diff /etc /dev # Compares recursively the directories /etc and /dev diff passwd /etc # Compares ./passwd to /etc/passwd DESCRIPTION
the same name, when file1 and file2 are both directories" difference encountered" Diff compares two files and generates a list of lines telling how the two files differ. Lines may not be longer than 128 characters. If the two arguments on the command line are both directories, diff recursively steps through all subdirectories comparing files of the same name. If a file name is found only in one directory, a diagnostic message is written to stdout. A file that is of either block special, character special or FIFO special type, cannot be compared to any other file. On the other hand, if there is one directory and one file given on the command line, diff tries to compare the file with the same name as file in the directory directory. SEE ALSO
cdiff(1), cmp(1), comm(1), patch(1). DIFF(1)
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