How to grep for special character NUL (^@^@^@)

Question

File:

O000000667520994000000074720121112000000N^@^@^@ 

I used the below command but it doesn't work.

grep "^@^@^@" *

Answer 1

^@ is not a carat ^ and at-sign @, it's one character. It's how some programs display the NUL character—ASCII value 0, also known as \0 in C.

Here I've created a file with a NUL byte in it.^† Notice that I use cat -v to show non-printing characters.

$ cat -v blah
hello
null^@
hi
$ hexdump -C blah
00000000  68 65 6c 6c 6f 0a 6e 75  6c 6c 00 0a 68 69 0a     |hello.null..hi.|
0000000f

Grep has trouble finding NULs since they're used to terminate strings in C. Sed, however, can do the job:

$ sed -n '/\x0/p' blah
null
$ sed -n '/\x0/p' blah | cat -v
null^@

---

^† In vi, in insert mode press Ctrl-V, Ctrl-Shift-@ to insert a null byte.

Answer 2

Character ^@ is the NUL char, so I'm afraid that it cannot be grepped directly.

Your best option would be probably to write a simple program that searches for this sequence of bytes.

Alternatively you may try to convert it into some form of hexadecimal dump (od, xxd or so) and grep into the output of it. But frankly speaking, it would be tricky to get it right.

Answer 3

^@ is the NUL character. What do you want to do with the lines of your files containing it ?

You could have a look at Identifying and removing null characters in UNIX which deals with a similar issue.

Answer 4

In bash you can add special characters when prefixed with C-q or C-v. So you can, for example

grep 'Ctrl-vCtrl-a' file.txt

The search string should be read as control key + character v, followed by control key + character a, which searches for ASCII value SOH (01). Unfortunately this doesn't work for the NUL character.