What's the best way to join files again after splitting them?

If I have a large file and need to split it into 100 megabyte chunks I will do

split -b 100m myImage.iso

That usually give me something like


And to get them back together I have been using

cat x* > myImage.iso

Seems like there should be a more efficient way than reading through each line of code in a group of files with cat and redirecting the output to a new file. Like a way of just opening two files, removing the EOF marker from the first one, and connecting them – without having to go through all the contents.

Windows/DOS has a copy command for binary files. The help mentions that this command was designed to able able to combine multiple files. It works with this syntax: (/b is for binary mode)

copy /b file1 + file2 + file3 outputfile

Is there something similar or a better way to join large files on Linux than cat?


It seems that cat is in fact the right way and best way to join files. Glad to know i was using the right command all along 🙂 Thanks everyone for your feedback.

Asked By: cwd


That’s just what cat was made for. Since it is one of the oldest GNU tools, I think it’s very unlikely that any other tool does that faster/better. And it’s not piping – it’s only redirecting output.

Answered By: rozcietrzewiacz

Seems like there should be a more efficient way than piping all of the contents through the system’s stdin / stdout

Except that’s not really what’s happening. The shell is connecting the stdout of cat directly to the open file, which means that “going through stdout” is the same as writing to disk.

I once had exactly this problem: I wanted to join some files, but had not enough disk space to hold them doubly.

So I wrote a bunch of programs:

  • one to “suck up” a file by reading it, sending it to stdout and, if finished, removing it
  • and one to buffer data “on the fly”.

This enabled me to do something like

partto sourcefile | mybuffer 128M >>cumufile

and thus removing the source file while 128M was still unwritten. A little bit dangerous, but if the data are not that precious, or they exist somewhere else as well, it is feasible.

If needed, I can provide the source.

Answered By: glglgl

Under the hood

There is no more efficient way than copying the first file, then copying the second file after it, and so on. Both DOS copy and cat do that.

Each file is stored independently of other files on the disk. Almost every filesystem designed to store data on a disk-like device operates by blocks. Here’s a highly simplified presentation of what happens: the disk is divided into blocks of, say 1kB, and for each file the operating system stores the list of blocks that make it up. Most files aren’t an integer number of blocks long, so the last block is only partially occupied. In practice, filesystems have many optimizations, such as sharing the last partial block between several files or storing “blocks 46798 to 47913” rather than “block 46798, block 46799, …”. When the operating system needs to create a new file, it looks for free blocks. The blocks don’t have to be consecutive: if only blocks 4, 5, 98 and 178 are free, you can still store a 4kB file. Using blocks rather than going down to the byte level helps make finding free blocks for a new or growing file considerably faster, and reduces the problems due to fragmentation when you create or grow and delete or shrink a lot of files (leaving an increasing number of holes).

You could support partial blocks in mid-file, but that would add considerable complexity, particularly when accessing files non-sequentially: to jump to the 10340th byte, you could no longer jump to the 100th byte of the 11th block, you’d have to check the length of every intervening block.

Given the use of blocks, you can’t just join two files, because in general the first file ends in mid-block. Sure, you could have a special case, but only if you want to delete both files when concatenating. That would be a highly specific handling for a rare operation. Such special handling doesn’t live on its own, because on a typical filesystem, many file are being accessed at the same time. So if you want to add an optimization, you need to think carefully: what happens if some other process is reading one of the files involved? What happens if someone tries to concatenate A and B while someone is concatenating A and C? And so on. All in all, this rare optimization would be a huge burden.

All in all, you can’t make joining files more efficient without making major sacrifices elsewhere. It’s not worth it.

On splitting and joining

split and cat are simple ways of splitting and joining files. split takes care of producing files named in alphabetical order, so that cat * works for joining.

A downside of cat for joining is that it is not robust against common failure modes. If one of the files is truncated or missing, cat will not complain, you’ll just get damaged output.

There are compression utilities that produce multipart archives, such as zipsplit and rar -v. They aren’t very unixy, because they compress and pack (assemble multiple files into one) in addition to splitting (and conversely unpack and uncompress in addition to joining). But they are useful in that they verify that you have all the parts, and that the parts are complete.

Technically speaking, this is a way of accessing the entire file without having to read and write the entire contents, and could be useful for huge files or if there is little space left:

$ mkfifo myImage.iso
$ cat xa{a..g} > myImage.iso &

And then use myImage.iso, for example

$ md5sum myImage.iso

Though of course myImage.iso is a special file (named pipe) and not a regular file, so this may be of use or not depending on what you’re trying to do.

Answered By: golimar

Files Spliting

Split By Size

If you want to split big file into small files and choose name and size of small output files this is the way.

split -b 500M videosBigVideoFile.avi SmallFile.

In this way you choose to split one big file to smaller parts of 500 MB. Also you want that names of part files is SmallFile. Note that you need dot after file name.
The result should be generation of new files like this:

SmallFile.ab SmallFile.ad SmallFile.af SmallFile.ah SmallFile.aj
SmallFile.aa SmallFile.ac SmallFile.ae SmallFile.ag SmallFile.ai SmallFile.ak

Split By Number Of Lines

This way you’ll split textual file into smaller files limited to 50 lines.

split -l 50 text_to_split.txt

The result should be something like this:

xaa xab xac ...

Split By Bytes

Split to small files with custom size of small files in bytes:

split -b 2048 BigFile.mp4

The result should be similar to result from Spliting By Number Of Lines.

Files Joining

You can join files in two ways. The first one is:

cat SmallFile.* > OutputBigVideoFile.avi

or with:

cat SmallFile.?? > OutputBigVideoFile.avi

Note: When you are joining files small files should not be damaged. Also all small (part) files should be in the same directory.

Answered By: Nole
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