Dtyjlui

I have a directory that contains, among other files, 3 named pipes: FIFO, FIFO1, and FIFO11. If I try something like

grep mypattern *

in this directory, grep hangs forever on the named pipes, so I need to exclude them. Unexpectedly,

grep --exclude='FIF*' mypattern *

does not solve the problem; grep still hangs forever. However,

grep -r --exclude='FIF*' mypattern .

does solve the hanging problem (albeit with the undesired side effect of searching all the subdirectories).

I did some testing that shows that grep --exclude ='FIF*' mypattern * works as expected if FIFO etc. are regular files, not named pipes.

Questions:

1. Why does grep skip --excludes in both cases if they're regular files, and skips --excluded named pipes in the recursive case, but doesn't skip named pipes in the non-recursive case?


2. Is there another way to format the exclusion that will skip these files in all cases?


3. is there a better way to accomplish what I'm after? (EDIT: I just discovered the
   --devices=skip flag in grep, so that's the answer to this part ... but I'm still curious about the first two parts of the question)

It seems grep still opens files even if the regex tells it to skip them:

$ ll

total 4.0K

p-w--w---- 1 user user 0 Feb  7 16:44 pip-fifo

--w--w---- 1 user user 4 Feb  7 16:44 pip-file

lrwxrwxrwx 1 user user 4 Feb  7 16:44 pip-link -> file

(Note: none of these have read permissions.)

$ strace -e openat grep foo --exclude='pip*' pip-file pip-link pip-fifo

openat(AT_FDCWD, "pip-file", O_RDONLY|O_NOCTTY) = -1 EACCES (Permission denied)

grep: pip-file: Permission denied

openat(AT_FDCWD, "pip-link", O_RDONLY|O_NOCTTY) = -1 ENOENT (No such file or directory)

grep: pip-link: No such file or directory

openat(AT_FDCWD, "pip-fifo", O_RDONLY|O_NOCTTY) = -1 EACCES (Permission denied)

grep: pip-fifo: Permission denied

+++ exited with 2 +++

Granting read permissions, it appears that it doesn't try to read them after opening if they are excluded:

$ strace -e openat grep foo --exclude='pip*' pip-file pip-link pip-fifo

openat(AT_FDCWD, "pip-file", O_RDONLY|O_NOCTTY) = 3

openat(AT_FDCWD, "pip-link", O_RDONLY|O_NOCTTY) = -1 ENOENT (No such file or directory)

grep: pip-link: No such file or directory

openat(AT_FDCWD, "pip-fifo", O_RDONLY|O_NOCTTY^Cstrace: Process 31058 detached

 <detached ...>



$ strace -e openat,read grep foo --exclude='pip*' pip-file

read(3, "177ELF2113>100025"..., 832) = 832

read(3, "177ELF2113>1240r"..., 832) = 832

read(3, "177ELF21133>1Pt2"..., 832) = 832

read(3, "177ELF2113>1260`"..., 832) = 832

openat(AT_FDCWD, "pip-file", O_RDONLY|O_NOCTTY) = 3

+++ exited with 1 +++



$ strace -e openat,read grep foo --exclude='pipe*' pip-file

read(3, "177ELF2113>100025"..., 832) = 832

read(3, "177ELF2113>1240r"..., 832) = 832

read(3, "177ELF21133>1Pt2"..., 832) = 832

read(3, "177ELF2113>1260`"..., 832) = 832

openat(AT_FDCWD, "pip-file", O_RDONLY|O_NOCTTY) = 3

read(3, "foon", 32768)                 = 4

foo

read(3, "", 32768)                      = 0

+++ exited with 0 +++

And since openat wasn't called with O_NONBLOCK, the opening itself hangs, and grep doesn't reach the part where it excludes it from reading.

Looking at the source code, I believe the flow is like this:

1. If not recursive, call grep_command_line_arg on each file.


2. That calls grepfile if not on stdin.


3. 
   grepfile calls grepdesc after opening the file.


4. 
   grepdesc checks for excluding the file.

When recursive:

1. 
   grepdirent checks for excluding the file before calling grepfile, so the failing openat never happens.

why don't you combine with "find"? Get a list of nothing but files and grep into them:

find /path/to/dir -type f -exec grep pattern {} ;