问题
Quarrying out from another thread Move file that has aged x minutes, this question came up:
How does the find command found typically in Linux search for files in the current directory?
Consider a directory that contains a fairly large amount of files, then:
Firstly find MY_FILE.txt returns immediately and secondly find . -name MY_FILE.txt takes much longer.
I used strace -c to see what happens for both and I learned that the second command invokes a directory scan, which explains why it's slower.
So, the first command must be optimized. Can anybody point me to the appropriate resource or provide a quick explanation how this might be implemented?
回答1:
The syntax for find is find <paths> <expression>, where paths is a list of files and directories to start the search from. find starts from those locations and then recurses (if they're directories).
When you write find . -name MY_FILE.txt it performs a recursive search under the ./ directory. But if you write find MY_FILE.txt then you're telling it to start the search at ./MY_FILE.txt, and so it does:
$ strace -e file find MY_FILE.txt
...
newfstatat(AT_FDCWD, "MY_FILE.txt", 0x556688ecdc68, AT_SYMLINK_NOFOLLOW) = -1 ENOENT (No such file or directory)
...
(No such file or directory)
: No such file or directory
+++ exited with 1 +++
Since the path doesn't exist, it only takes a single system call to determine that there's no such file. It calls newfstat(), gets a No such file or directory error, and that's that.
In other words, find MY_FILE.txt isn't equivalent to find . -name MY_FILE.txt. Heck, I wouldn't even call it useful because you're not asking it to search. You're just asking it to tell you if MY_FILE.txt exists in the current directory or not. But you could find that out by simply calling ls MY_FILE.txt.
Here's the difference:
[~]$ cd /usr
[/usr]$ find . -name sha384sum
./bin/sha384sum
[/usr]$ find sha384sum
find: ‘sha384sum’: No such file or directory
The first one performs a recursive search and finds /usr/bin/sha384sum. The second one doesn't recurse and immediately fails bcause /usr/sha384sum doesn't exist. It doesn't look any deeper. It's done in a nanosecond.
来源：https://stackoverflow.com/questions/58833367/how-does-the-find-command-search-for-files