On the Ubuntu-based machines I use at work, SIZE defaults to the ridiculously small 1MB. It's somewhat hidden in the manpage: the documentation about the -S option doesn't mention it, but later it says (bold added):
SIZE may be followed by the following multiplicative suffixes: % 1% of memory, b 1, K 1024 (default), and so on for M, G, T, P, E, Z, Y.
You can verify this by running seq 1 1000000000000 | sort and, while that's happening, ls -lh /tmp/sort*.
I actually generally don't want it to write its data to disk, because I'm usually using machines where the data fits very comfortably in RAM (data size is maybe up to 1GB) and writing to disk (even SSD) just adds slowness. Though splitting it unnecessarily also adds slowness. For my use case, specifying a much bigger buffer is appropriate.
Wow! That's way too low for modern machines, especially since sort can query the environment to get a sense of how much memory it might be worth using.
The unix
sort command is clever: to sort very large
files it does a series of in-memory sorts, saving sorted chunks to
temporary files, and then does a merge sort on those chunks. Except
this often doesn't work anymore.
Here's what I see if I run man sort and look at the
documentation for --buffer-size:
use SIZE for main memory buffer
That's pretty terse! What does my Mac say?
Use size for the maximum size of the memory buffer. Size modifiers %,b,K,M,G,T,P,E,Z,Y can be used. If a memory limit is not explicitly specified, sort takes up to about 90% of available memory. If the file size is too big to fit into the memory buffer, the temporary disk files are used to perform the sorting.
Makes sense! But then the docs for --temporary-directory
say:
use DIR for temporaries, not$TMPDIRor/tmp; multiple options specify multiple directories
And these days /tmp is often memory-backed, via tmpfs. This changed in
Fedora
18 (2013) and Ubuntu
24.10 (2024), and is changing in Debian 13
(in a month or two).
It seems to me that these days it would be better for
--temporary-directory to default to
/var/tmp, which is preserved
across reboots and so will generally be backed by disk even on
systems that use tmpfs for /tmp. In the meantime,
sort --temporary-directory /var/tmp will do the trick.