unix - split a huge .gz file by line

Question

I'm sure someone has had the below need, what is a quick way of splitting a huge .gz file by line? The underlying text file has 120million rows. I don't have enough disk space to gunzip the entire file at once so I was wondering if someone knows of a bash/perl script or tool that could split the file (either the .gz or inner .txt) into 3x 40mn line files. ie calling it like:

    bash splitter.sh hugefile.txt.gz 4000000 1
 would get lines 1 to 40 mn    
    bash splitter.sh hugefile.txt.gz 4000000 2
would get lines 40mn to 80 mn
    bash splitter.sh hugefile.txt.gz 4000000 3
would get lines 80mn to 120 mn

Is perhaps doing a series of these a solution or would the gunzip -c require enough space for the entire file to be unzipped(ie the original problem): gunzip -c hugefile.txt.gz | head 4000000

Note: I can't get extra disk.

Thanks!

Solution 1

How to do this best depends on what you want:

• Do you want to extract a single part of the large file?
• Or do you want to create all the parts in one go?

---

If you want a single part of the file, your idea to use gunzip and head is right. You can use:

gunzip -c hugefile.txt.gz | head -n 4000000

That would output the first 4000000 lines on standard out - you probably want to append another pipe to actually do something with the data.

To get the other parts, you'd use a combination of head and tail, like:

gunzip -c hugefile.txt.gz | head -n 8000000 |tail -n 4000000

to get the second block.

Is perhaps doing a series of these a solution or would the gunzip -c require enough space for the entire file to be unzipped

No, the gunzip -c does not require any disk space - it does everything in memory, then streams it out to stdout.

---

If you want to create all the parts in one go, it is more efficient to create them all with a single command, because then the input file is only read once. One good solution is to use split; see jim mcnamara's answer for details.

Solution 2

pipe to split use either gunzip -c or zcat to open the file

gunzip -c bigfile.gz | split -l 400000

Add output specifications to the split command.

Solution 3

As you are working on a (non-rewindable) stream, you will want to use the '+N' form of tail to get lines starting from line N onwards.

zcat hugefile.txt.gz | head -n 40000000
zcat hugefile.txt.gz | tail -n [phone] | head -n 40000000
zcat hugefile.txt.gz | tail -n [phone] | head -n 40000000

Solution 4

Directly split .gz file to .gz files:

zcat bigfile.gz | split -l 400000 --filter='gzip > $FILE.gz'

I think this is what OP wanted, because he don't have much space.

Solution 5

I'd consider using split.

split a file into pieces

View more solutions

Comments

[Admin]

Do you want the resulting files to be gziped again?

[Ingo]

You can use gunzip in a ipe. The rest can be done with head and tail

[Rajeshkumar]

@Tichodroma - no I don't need them gziped again. But I could not store all the split text files at once. So i would like to get the first split, do stuff with it, then delete the first split, and then get the 2nd split.etc finally removing the original gz

[sleske]

@toop: Thanks for the clarification. Note that it's generally better to edit your question if you want to clarify it, rather than put it into a comment; that way everyone will see it.

[b0fh]

The accepted answer is good if you only want a fraction of the chunks, and do not know them in advance. If you want to generate all the chunks at once, the solutions based on split will be a lot faster ,O(N) instead of O(N²).

[Alois Mahdal]

From performance view: does gzip actually unzip whole file? Or is it able to "magically" know that only 4mn lines are needed?

[sleske]

@AloisMahdal: Actually, that would be a good separate question :-). Short version: gzip does not know about the limit (which comes from a different process). If head is used, head will exit when it has received enough, and this will propagate to gzip (via SIGPIPE, see Wikipedia). For tail this is not possible, so yes, gzip will decompress everything.

[sleske]

But if you are interested, you should really ask this as a separate question.

[b0fh]

This is massively more efficient than the accepted answer, unless you only require a fraction of the split chunks. Please upvote.

[sleske]

@b0fh: Yes, your are right. Upvoted, and referenced in my answer :-).

[Stephen Blum]

Best answer for sure.

[Quetzalcoatl]

what are the output specs so that the outputs are .gz files themselves?