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I've got 10k+ files totaling over 20GB that I need to concatenate into one file.

Is there a faster way than

cat input_file* >> out

?

The preferred way would be a bash command, Python is acceptable too if not considerably slower.

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  • Updated my answer, find does not sort files the same as a shell glob. Commented Mar 5, 2014 at 13:18
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    Any and all (sane) solutions will have an equivalent speed here since the time will be 99% system I/O. Commented Mar 5, 2014 at 15:04
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    See also A virtual file containing the concatenation of other files Commented Mar 5, 2014 at 15:09
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    Considering writing the concatenated file in a different disk than the one(s) you're reading from. Commented Mar 5, 2014 at 15:29
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    It will be faster if out is located on another disk. Commented Mar 5, 2014 at 22:41

2 Answers 2

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Nope, cat is surely the best way to do this. Why use python when there is a program already written in C for this purpose? However, you might want to consider using xargs in case the command line length exceeds ARG_MAX and you need more than one cat. Using GNU tools, this is equivalent to what you already have:

find . -maxdepth 1 -type f -name 'input_file*' -print0 | sort -z | xargs -0 cat -- >>out
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    Can you insure in this case that your files will be read in the order ? Commented Mar 5, 2014 at 13:34
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    Yes, because the output of find is piped through sort. Without this, the files would be listed in an arbitrary order (defined by the file system, which could be file creation order). Commented Mar 5, 2014 at 13:43
  • @scai I missread sorry, with sort it's pretty obvious Commented Mar 5, 2014 at 13:46
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    @Kiwy, the only case I can see is if the locale isn't properly set in the environment, then sort might behave differently from a bash glob. Otherwise I don't see any cases where xargs or cat would not behave as expected. Commented Mar 5, 2014 at 14:05
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    @MarcvanLeeuwen, xargs will call as may cat as is necessary to avoid an E2BIG error of execve(2). Commented Mar 6, 2014 at 9:41
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Allocating the space for the output file first may improve the overall speed as the system won't have to update the allocation for every write.

For instance, if on Linux:

size=$({ find . -maxdepth 1 -type f -name 'input_file*' -printf '%s+'; echo 0;} | bc) fallocate -l "$size" out && find . -maxdepth 1 -type f -name 'input_file*' -print0 | sort -z | xargs -r0 cat 1<> out

Another benefit is that if there's not enough free space, the copy will not be attempted.

If on btrfs, you could copy --reflink=always the first file (which implies no data copy and would therefore be almost instantaneous), and append the rest. If there are 10000 files, that probably won't make much difference though unless the first file is very big.

There's an API to generalise that to ref-copy all the files (the BTRFS_IOC_CLONE_RANGE ioctl), but I could not find any utility exposing that API, so you'd have to do it in C (or python or other languages provided they can call arbitrary ioctls).

If the source files are sparse or have large sequences of NUL characters, you could make a sparse output file (saving time and disk space) with (on GNU systems):

find . -maxdepth 1 -type f -name 'input_file*' -print0 | sort -z | xargs -r0 cat | cp --sparse=always /dev/stdin out
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    @XTian, no, it should be neither > nor >>, but 1<> as I said to write into the file. Commented Mar 5, 2014 at 16:41
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    @grebneke, <> is the standard Bourne/POSIX read+write redirection operator. See your shell manual or the POSIX spec for details. The default fd is 0 for the <> operator (<> is short for 0<>, like < is short for 0< and > short for 1>), so you need the 1 to explicitly redirect stdout. Here, it's not so much that we need read+write (O_RDWR), but that we don't want O_TRUNC (as in >) which would deallocate what we've just allocated. Commented Mar 5, 2014 at 20:30
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    @grebneke, "">unix.stackexchange.com/search?q=user%3A22565+%22%3C%3E%22 will give you a few. ksh93 has seek operators BTW, and you can seek forward with dd or via reading. Commented Mar 5, 2014 at 21:15
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    @StephaneChazelas - thanks a lot, your help and knowledge is deeply appreciated! Commented Mar 5, 2014 at 21:19
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    I'm not convinced that there will be many cases where fallocate will negate the overhead of the extra find, even though it will be faster the second time round. btrfs certainly opens up some interesting possibilities though. Commented Mar 6, 2014 at 1:28

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