unexpected result: grep from a changing line - shell

Question

I wrote a bash command to test grep from a changing line:
for i in $(seq 0 9); do echo -e -n "\r"$i; sleep 0.1; done | grep 5
The result shows:
9
Update
The real problem is as follows:
mplayer shows and refreshes a single-line playing progress when playing a media file. A sample result is:
A: 17.2 (17.2) of 213.0 (03:33.0) 0.5%
And I'm trying to grep this playing progress and ingore other lines. I used this command:
mplayer xxx.mp3 | grep ^A:
The result does not contain the line expected.
Update 2
mplayer xxx.mp3 | od -xda
shows:
0002140 4a5b 410d 203a 2020 2e31 2033 3028 2e31
[ J \r A : 1 . 3 ( 0 1 .
133 112 015 101 072 040 040 040 061 056 063 040 050 060 061 056
0002160 2932 6f20 2066 3132 2e33 2030 3028 3a33
2 ) o f 2 1 3 . 0 ( 0 3 :
062 051 040 157 146 040 062 061 063 056 060 040 050 060 063 072
0002200 3333 302e 2029 3020 342e 2025 5b1b 0d4a
3 3 . 0 ) 0 . 4 % 033 [ J \r
063 063 056 060 051 040 040 060 056 064 045 040 033 133 112 015
0002220 3a41 2020 3120 352e 2820 3130 342e 2029
A : 1 . 5 ( 0 1 . 4 )
101 072 040 040 040 061 056 065 040 050 060 061 056 064 051 040
0002240 666f 3220 3331 302e 2820 3330 333a 2e33
o f 2 1 3 . 0 ( 0 3 : 3 3 .
157 146 040 062 061 063 056 060 040 050 060 063 072 063 063 056
And
mplayer xxx.mp3 | tr '\r' '\n'
shows
A: 0.2 (00.1) of 213.0 (03:33.0) 0.3%
A: 0.3 (00.3) of 213.0 (03:33.0) 0.3%
A: 0.5 (00.5) of 213.0 (03:33.0) 0.4%
A: 0.6 (00.6) of 213.0 (03:33.0) 0.4%
A: 0.8 (00.8) of 213.0 (03:33.0) 0.4%
A: 1.0 (01.0) of 213.0 (03:33.0) 0.4%
While,
mplayer xxx.mp3 | tr '\r' '\n' | grep ^A
shows empty result.
Any tip will be appreciated.

Answer 1

It's your definition of "line" that's causing the problem here. The -n means that all the numbers are output on a single line, according the the definition used by grep (a series of characters, terminated by the \n character):
\r1\r2\r3\r4\r5\r6\r7\r8\r9
If you pipe the output through something like a hex dump, you can see what's happening:
$ for i in $(seq 0 9); do echo -e -n "\r"$i; sleep 0.1; done | grep 5 | od -xcb
0000000 300d 310d 320d 330d 340d 350d 360d 370d
\r 0 \r 1 \r 2 \r 3 \r 4 \r 5 \r 6 \r 7
015 060 015 061 015 062 015 063 015 064 015 065 015 066 015 067
0000020 380d 390d 000a
\r 8 \r 9 \n
015 070 015 071 012
0000025
That single line containing all the carriage returns (and not newlines) will, when output, appear to be a single line with just the 9 on it. Removing the -n will result instead in:
$ for i in $(seq 0 9); do echo -e "\r"$i; sleep 0.1; done | grep 5 | od -xcb
0000000 350d 000a
\r 5 \n
015 065 012
0000003
which would look like just the 5 was being output.
If you have a process that outputs "lines" separated by carriage returns rather than newlines, there's nothing to stop you changing them on the fly so as to be able to handle them as real lines:
$ echo -e "junk\rA: good 1\rjunk\rA: good 2\rjunk" | tr '\r' '\n' | grep '^A'
A: good 1
A: good 2
Applying that back to your original question, it would be (with the sleep removed since it's irrelevant):
$ for i in $(seq 0 9); do echo -e -n "\r"$i; done | tr '\r' '\n' | grep 5
5
$ for i in $(seq 0 9); do echo -e -n "\r"$i; done | tr '\r' '\n' | grep 5 | od -xcb
0000000 0a35
5 \n
065 012
0000002