What's the difference between grep -r and -R

Question

In the man page:

-r

Read all files under each directory, recursively, following symbolic links only if they are on the command line.

what exactly does "being on the command line" means?

Thanks.

Answer 1

"Being on the command line" refers to the arguments passed to grep. If you give a symbolic link as an argument to grep -r it follows it. However, if grep -r encounters a symbolic link while traversing a directory it does not follow it (in contrast to grep -R which does).

Imagine you have a directory with a bunch of files in it, including one containing a symbolic link to .. (the parent directory):

$ ls -la
total 0
drwxr-xr-x 2 aw aw 47 Mar 31 16:05 .
drwxr-xr-x 3 aw aw 27 Mar 31 16:04 ..
-rw-r--r-- 1 aw aw  0 Mar 31 16:05 bar
-rw-r--r-- 1 aw aw  0 Mar 31 16:05 baz
lrwxrwxrwx 1 aw aw  2 Mar 31 16:04 foo -> ..
-rw-r--r-- 1 aw aw  0 Mar 31 16:05 quux
$

Then,

• grep -r foobar . will only grep the files inside this directory,
• grep -r foobar foo will grep the files in the parent directory (..) (following the symlink given as an argument),
• grep -R foobar . will also grep the files in the parent directory (following the symlink not given as an argument but found while traversing the current directory).