Difference of two big files

Sounds like a job for comm:

$ comm -3 <(sort test1.csv) <(sort test2.csv)
100,300,500,700
    100,4,2,1,7
    21,22,23,24,25
    50,25,700,5

As explained in man comm:

   -1     suppress column 1 (lines unique to FILE1)

   -2     suppress column 2 (lines unique to FILE2)

   -3     suppress column 3 (lines that appear in both files)

So, the -3 means that only lines that are unique to one of the files will be printed. However, those are indented according to which file they were found in. To remove the tab, use:

$ comm -3 <(sort test1.csv) <(sort test2.csv) | tr -d '\t'
100,300,500,700
100,4,2,1,7
21,22,23,24,25
50,25,700,5

In this case, you don't really even need to sort the files and you can simplify the above to:

comm -3 test1.csv test2.csv | tr -d '\t' > difference.csv

Using grep with bash process substitution:

$ cat <(grep -vFf test2.csv test1.csv) <(grep -vFf test1.csv test2.csv)
100,300,500,700
100,4,2,1,7
21,22,23,24,25
50,25,700,5

To save the output as results.csv:

cat <(grep -vFf test2.csv test1.csv) <(grep -vFf test1.csv test2.csv) >results.csv

• <() is the bash process substitution pattern

• grep -vFf test2.csv test1.csv will find the lines unique to only test1.csv

• grep -vFf test1.csv test2.csv will find the lines unique to only test2.csv

• Finally we are summing up the results by cat

Or as Oli suggested, you can use command grouping also:

$ { grep -vFf test2.csv test1.csv; grep -vFf test1.csv test2.csv; }
100,300,500,700
100,4,2,1,7
21,22,23,24,25
50,25,700,5

Or just run one after another, as they are both writing to STDOUT they will ultimately get added:

$ grep -vFf test2.csv test1.csv; grep -vFf test1.csv test2.csv
100,300,500,700
100,4,2,1,7
21,22,23,24,25
50,25,700,5

If the order of rows is not relevant, use awk or perl:

awk '{seen[$0]++} END {for (i in seen) {if (seen[i] == 1) {print i}}}' 1.csv 2.csv

Use grep to get the common lines and filter those out:

grep -hxvFf <(grep -Fxf 1.csv 2.csv) 1.csv 2.csv

The internal grep gets the common lines, then the external grep finds lines which don't match these common lines.

Use the --*-line-format=... options of diff

You can tell diff exactly what you need - explained below:

diff --old-line-format='%L' --new-line-format='%L' --unchanged-line-format='' f1.txt f2.txt

It is possible to specify the output of diff in a very detailed way, similar to a printf number format.

The lines from the first file, test1.csv are called "old" lines, and the lines from the second, test2.csv, are "new" lines. That makes sense when diff is used to see what changed in a file.

The options we need are the ones to set the format for "old" lines, "new" lines, and "unchanged" lines.
The formats we need are very simple:
For the changed lines, new and old, we want to output only the text of the lines. %L is the format symbol for the line text.
For the unchanged lines, we want to show nothing.

With this, we can write options like --old-line-format='%L', and put it all together, using your example data:

$ diff --old-line-format='%L' --new-line-format='%L' --unchanged-line-format='' test1.csv test2.csv
100,4,2,1,7
100,300,500,700
21,22,23,24,25
50,25,700,5

Notes on performance

Because the files have different size, try to exchange the input files if it does not matter, it could be that the inner workings of diff can handle one way better than the other. Better is either needing less memory, or less computation.

There is an optimisation option for using diff with large files: --speed-large-files. It uses assumptions about the file structure, so it's not clear whether it helps in your case, but worth trying it.

The format options are described in the man diff under --LTYPE-line-format=LFMT.

Since the order doesn't need to be preserved, simply:

sort test1.csv test2.csv | uniq -u

• sort test1.csv test2.csv: merges and sorts test1.csv and test2.csv
• uniq -u: prints only the lines which have no duplicate