Here’s a little
find
trick that not many people seem to know:
Code:
# 13 seconds...
$ time find . -type f -exec stat {} \; > /dev/null
13.20s real 3.94s user 9.22s sys
# 1.5 seconds! That's almost 10 times faster!
$ time find . -type f -exec stat {} + > /dev/null
1.48s real 0.68s user 0.79s sys
# Run the first command again, to make sure we’re not being biased by fs
# cache or got some fluke
[~]% time find . -type f -exec stat {} \; > /dev/null
13.40s real 3.67s user 9.51s sys
# FYI...
[~]% find . -type f | wc -l
2641
That’s quite a large difference! All we did was swap the
;
for a
+
.
Let’s see what
POSIX has to say about it (emphases mine):
Quote:
If the primary expression is punctuated by a <semicolon> , the utility utility_name shall be invoked once for each pathname
[.. snip ..]
If the primary expression is punctuated by a <plus-sign> , the primary shall always evaluate as true, and the pathnames for which the primary is evaluated shall be aggregated into sets. The utility utility_name shall be invoked once for each set of aggregated pathnames.
|
Or in slightly more normal English: If you use
;
,
find
will execute the utility once for every path; if you use
+
, it will cram as many paths as it can in an invocation.
How many? Well, as many as
ARG_MAX
allows.
Quoting from POSIX Again:
Quote:
{ARG_MAX}
Maximum length of argument to the exec functions including environment data.
Minimum Acceptable Value: {_POSIX_ARG_MAX}
{_POSIX_ARG_MAX}
Maximum length of argument to the exec functions including environment data.
Value: 4096
|
Most contemporary systems have it set much higher though; Linux (3.16, x86_64) defines
ARG_MAX
as 131072 (128k), while FreeBSD (10, i386) gives it as 262144 (256k).
Let’s verify this with
truss
[^1]:
Code:
# Amount of files we have
$ find . -type f | wc -l
2641
$ truss find . -type f -exec stat {} \; >& truss-slow
$ truss find . -type f -exec stat {} + >& truss-fast
# Less than ARG_MAX, so we expect one fork()
$ find . -type f | xargs | wc -c
119528
# Yup!
$ grep fork truss-fast | wc -l
1
# And we fork() once for every file
$ grep fork truss-slow | wc -l
2641
Caveat
There is one small caveat, this won’t work:
Code:
# FreeBSD find
$ find . -type f -exec cp {} /tmp +
find: -exec: no terminating ";" or "+"
# GNU find is even more cryptic:
$ find: missing argument to `-exec'
Going
back to POSIX:
Quote:
Only a <plus-sign> that immediately follows an argument containing only the two characters shall punctuate the end of the primary expression. Other uses of the <plus-sign> shall not be treated as special.
|
In other words, the command [em]needs[/em] to end with
{} +
.
cp {} /tmp +
doesn’t, and thus gives an error.
We can work around this by spawning a
sh
one-liner:
Code:
$ find . -type f -exec sh -c 'cp "$@" /tmp' {} +