grep behavior with patterns of '-' character

Question

Actual answer to my question might be very simple(probably I just have to read the reg-ex tutorials again). But this is more on the behavior which I saw with grep and '-' character.

I've a patch file(diff.txt) which contains entries like.

--- a/path/to/xyz

+++ b/path/to/modified/xyz

Now, for some reason I wanted to grep all lines in the file which exactly contains "---" pattern.

After simple grep commands failed to give me what I wanted, I tried below three commands:

$ grep "-" ./diff.txt

The above one yields the results showing all the existence of '-' symbol(single occurence, multiple occurence ).

$ grep  "--" ./diff.txt

This one yields nothing. No output. Grep got stuck in some loop?(trying to find out something which is not there?). Had to Ctrl-C to stop the command.

$ grep "---"  ./diff.txt

After this one, Grep complains of Unrecognized pattern and prints the help message.

In case, someone is wondering, I did try the same command with a "+++" pattern and a simple grep command worked like a charm:

$ grep "+++" ./diff.txt   --> output all the lines correctly.

I'll appreciate if someone could explain this strange behavior with '-' character.

Answer 1

Since your pattern starts with a dash it's being interpretted as an option. Either explicitly indicate the end of options with the -- option, or explicitly indicate that your pattern is a pattern with the -e option.

$ echo -e "---abc\ndef\n---ghi\njkl"
---abc
def
---ghi
jkl

$ echo -e "---abc\ndef\n---ghi\njkl" | grep -- ---
---abc
---ghi

$ echo -e "---abc\ndef\n---ghi\njkl" | grep -e ---
---abc
---ghi

(The -e option to the echo command allows c-style escapes like \n.)

Answer 2

A pattern starting with a - is interpreted as an option.

-- means end of options.

Use -- to tell grep that any following arguments are not options.

grep -- pattern path...

Answer 3

Grep may be trying to interpret your pattern as an option.

Use -e to make it explicit:

grep -e --- ./diff.txt