[SOLVED] grep doesn't extract string!?

andrew.comly · 04-23-2016, 10:16 PM

grep doesn't work for extracting string from avprobe

Why does the syntax

Code:

lsb_release -a | grep 'Release'

just displays the desired line beginning with 'Release", but the command

Code:

avprobe ObamaFinalDebate.mp3 | grep 'Duration:'

displays a most undesired entire paragraph of stats?

Specific results:

Code:

a@a-NC210-NC110:~/Videos/Obama$ lsb_release -a | grep 'Release'
No LSB modules are available.
Release:	14.04

a@a-NC210-NC110:~/Videos/Obama$ avprobe ObamaFinalDebate.mp3 | grep 'Duration:'
avprobe version 9.18-6:9.18-0ubuntu0.14.04.1+fdkaac, Copyright (c) 2007-2014 the Libav developers
  built on Apr 10 2015 23:22:24 with gcc 4.8 (Ubuntu 4.8.2-19ubuntu1)
[mp3 @ 0xa081fe0] max_analyze_duration reached
Input #0, mp3, from 'ObamaFinalDebate.mp3':
  Metadata:
    major_brand     : dash
    minor_version   : 0
    compatible_brands: iso6mp41
    creation_time   : 2015-02-22 17:24:22
    encoder         : Lavf54.20.4
  Duration: 00:05:11.69, start: 0.000000, bitrate: 77 kb/s
    Stream #0.0: Audio: mp3, 44100 Hz, 2 channels, s16p, 77 kb/s

Can just the line beginning with "Duration" be extracted?

{Once I do this I think I could use awk to extract a certain word in that line.}

allend · 04-23-2016, 10:39 PM

Why does the output of your first command also include 'No LSB modules are available.' when only the line containing 'Release' should be shown?

Code:

a@a-NC210-NC110:~/Videos/Obama$ lsb_release -a | grep 'Release'
No LSB modules are available.
Release:	14.04

Do you have 'grep' aliased to something?
Perhaps the output from 'alias' will help.

syg00 · 04-23-2016, 11:28 PM

What happens if you pipe the output to a file then search it ?. Else dump it out in hex/octal and see what is being produced.

cnamejj · 04-24-2016, 01:32 AM

Looks like "avprobe" writes output to STDERR, which is why your grep isn't doing anything. And "lsb_release" also writes part of it's output, specifically the "No LSB modules are available." line to STDERR.

So you can do something like,

Code:

avprobe 2>&1 | grep 'Duration:'

to get the behavior you want.

andrew.comly · 04-25-2016, 07:49 PM

Quote:

Originally Posted by allend

Why does the output of your first command also include 'No LSB modules are available.' when only the line containing 'Release' should be shown?
Do you have 'grep' aliased to something?
Perhaps the output from 'alias' will help.

My alias output:

Code:

...
alias ftp='ncftp -d15'
alias grep='grep --color=auto'
alias h='history 10'
...

andrew.comly · 04-25-2016, 08:03 PM

Quote:

Originally Posted by syg00

What happens if you pipe the output to a file then search it ?

Attempt01:

Code:

$ avprobe ObamaFinalDebate.mp3 | grep 'Duration'>/home/a/Desktop/b.txt

b.txt output:
(blank)

Attempt02:

Code:

$avprobe ObamaFinalDebate.mp3>/home/a/Desktop/a.txt

Terminal output

Code:

avprobe version 9.18-6:9.18-0ubuntu0.14.04.1+fdkaac, Copyright (c) 2007-2014 the Libav developers
  built on Apr 10 2015 23:22:24 with gcc 4.8 (Ubuntu 4.8.2-19ubuntu1)
[mp3 @ 0x9b37fe0] max_analyze_duration reached
Input #0, mp3, from 'ObamaFinalDebate.mp3':
  Metadata:
    major_brand     : dash
    minor_version   : 0
    compatible_brands: iso6mp41
    creation_time   : 2015-02-22 17:24:22
    encoder         : Lavf54.20.4
  Duration: 00:05:11.69, start: 0.000000, bitrate: 77 kb/s
    Stream #0.0: Audio: mp3, 44100 Hz, 2 channels, s16p, 77 kb/s

a.txt:

Code:

# avprobe output

andrew.comly · 04-25-2016, 08:13 PM

Quote:

Originally Posted by cnamejj

Looks like "avprobe" writes output to STDERR,
So you can do something like,

Code:

avprobe 2>&1 | grep 'Duration:'

I tried this, results are below.

Code:

a@a-NC210-NC110:~/Videos/Obama$ avprobe 2>&1 | grep 'Duration:'
a@a-NC210-NC110:~/Videos/Obama$ avprobe ObamaFinalDebate.mp3>&1 | grep 'Duration:'
avprobe version 9.18-6:9.18-0ubuntu0.14.04.1+fdkaac, Copyright (c) 2007-2014 the Libav developers
  built on Apr 10 2015 23:22:24 with gcc 4.8 (Ubuntu 4.8.2-19ubuntu1)
[mp3 @ 0x9ab8fe0] max_analyze_duration reached
Input #0, mp3, from 'ObamaFinalDebate.mp3':
  Metadata:
    major_brand     : dash
    minor_version   : 0
    compatible_brands: iso6mp41
    creation_time   : 2015-02-22 17:24:22
    encoder         : Lavf54.20.4
  Duration: 00:05:11.69, start: 0.000000, bitrate: 77 kb/s
    Stream #0.0: Audio: mp3, 44100 Hz, 2 channels, s16p, 77 kb/s

Looks like I'll use the topics you mentioned to try to find something related in the ebook: Advanced Bash Scripting by Mendell Cooper.

Thanks anyway.

cnamejj · 04-25-2016, 09:48 PM

Code:

avprobe ObamaFinalDebate.mp3 2>&1 | grep 'Duration:

andrew.comly · 04-26-2016, 09:36 PM

Quote:

Originally Posted by cnamejj

Code:

avprobe ObamaFinalDebate.mp3 2>&1 | grep 'Duration:

It works!

Code:

$ avprobe ObamaFinalDebate.mp3 2>&1 | grep 'Duration:'
  Duration: 00:05:11.69, start: 0.000000, bitrate: 77 kb/s

And with a simple awk pipe I can extract just the bitrate:

Code:

$ avprobe ObamaFinalDebate.mp3 2>&1 | grep 'bitrate:' | awk '{ print $6 }'
77

Thanks a lot cnamejj and everyone else!

pan64 · 04-27-2016, 01:03 AM

do not need both awk and grep, use:

Code:

avprobe ... 2>&1 | awk '/bitrate:/ { print $6 }'

andrew.comly · 04-27-2016, 02:45 AM

I extracted the following from my current Bash textbook, this will convenience future learners who encounter the same problem.

Chapter 20. I/O Redirection
p 371:
There are always three default files [104] open, stdin (the keyboard), stdout (the screen), and stderr
(error messages output to the screen). These, and any other open files, can be redirected.
...

Each open file gets assigned a file descriptor. [105] The file descriptors for stdin, stdout, and stderr
are 0, 1, and 2, respectively.
...

p372:
2>&1
# Redirects stderr to stdout.
# Error messages get sent to same place as standard output.
>>filename 2>&1
bad_command >>filename 2>&1
# Appends both stdout and stderr to the file "filename" ...
2>&1 | [command(s)]
bad_command 2>&1 | awk '{print $5}'
# found
# Sends stderr through a pipe.
# |& was added to Bash 4 as an abbreviation for 2>&1 |.
i>&j
# Redirects file descriptor i to j.
# All output of file pointed to by i gets sent to file pointed to by j.
>&j
# Redirects, by default, file descriptor 1 (stdout) to j.
# All stdout gets sent to file pointed to by j.

Footnotes:
[104] By convention in UNIX and Linux, data streams and peripherals (device files) are treated as files, in a
fashion analogous to ordinary files.
[105] A file descriptor is simply a number that the operating system assigns to an open file to keep track of it. Consider it a simplified type of file pointer. It is analogous to a file handle in C.