I have a Windows text file which contains a line (with ending CRLF)
aline
The following is several commands' output:
[root@panel ~]# grep aline file.txt
aline
[root@panel ~]# grep aline$'\r' file.txt
[root@panel ~]# grep aline$'\r'$'\n' file.txt
[root@panel ~]# grep aline$'\n' file.txt
aline
The first command's output is normal. I'm curious about the second and the third output. Why is it an empty line? And the last output, I think it can not find the string but it actually finds it, why? The commands are run on CentOS/bash.
In this case grep really matches the string "aline\r" but you just don't see it because it was overwritten by the ANSI sequence that prints color. Pass the output to od -c and you'll see
$ grep aline file.txt aline $ grep aline$'\r' file.txt $ grep aline$'\r' --color=never file.txt aline $ grep aline$'\r' --color=never file.txt | od -c 0000000 a l i n e \r \n 0000007 $ grep aline$'\r' --color=always file.txt | od -c 0000000 033 [ 0 1 ; 3 1 m 033 [ K a l i n e 0000020 \r 033 [ m 033 [ K \n 0000030
With --color=never you can see the output string because grep doesn't print out the color. \r simply resets the cursor to the start of the line and then a new line is printed out, nothing is overwritten. But by default grep will check whether it's running on the terminal or its output is being piped and prints out the matched string in color if supported, and it seems resetting the color then print \n clears the rest of the line
To match \n you can use the -z option to make null bytes the line separator
$ grep -z aline$'\r'$'\n' --color=never file.txt aline $ grep -z aline$'\r'$'\n' --color=never file.txt | od -c 0000000 a l i n e \r \n \0 0000010 $ grep -z aline$'\r'$'\n' --color=always file.txt | od -c 0000000 033 [ 0 1 ; 3 1 m 033 [ K a l i n e 0000020 \r 033 [ m 033 [ K \n \0 0000031
Your last command grep aline$'\n' file.txt works because \n is simply a word separator in bash, so the command is just the same as grep aline file.txt. Exactly the same thing happened in the 3rd line: grep aline$'\r'$'\n' file.txt To pass a newline you must quote the argument to prevent word splitting
$ echo "aline" | grep -z "aline$(echo $'\n')" aline
To demonstrate the effect of the quote with the 3rd line I added another line to the file
$ cat file.txt aline another line $ grep -z "aline$(echo $'\n')" file.txt | od -c 0000000 a l i n e \r \n a n o t h e r l 0000020 i n e \n \0 0000025 $ grep -z "aline$(echo $'\n')" file.txt aline another line $