Passing a variable to bc in bash

rodhull · 01-14-2009, 10:26 AM

I cannot for the life of me get this little (simple) script I wrote to work.

Here is the entire script:

Code:

#!/bin/bash

ASPECT=`mediainfo $1 |grep "Display aspect ratio" |cut -d : -f 2`
HEIGHT=`echo "320 / $ASPECT" |bc`
SIZE=`echo 320x$HEIGHT`

ffmpeg -i $1 -s $SIZE -f mp4 -vcodec libxvid -maxrate 2500 -qmin 3 -qmax 5 -bufsize 4096 -g 300 -acodec libfaac -ab 128k $2

So, here's what should happen:
An input filename ($1) is fed into mediainfo, which by the use of grep and cut spits out a single number which is the aspect ratio. This is then divided by bc into 320, which gives the desired height dimension for the file that I want ffmpeg to create for me. Finally, ffmpeg runs using the calculated dimensions...

Basically, it's the passing of the $ASPECT variable to bc that seems to fail.

It looks like bc won't read the output from the mediainfo line...

It always crashes out with:

Code:

(standard_in) 1: illegal character: ^M

I've tried doing something even simpler like this to debug by just trying it to display the calculation on the screen:

Code:

#!/bin/bash

ASPECT=`mediainfo $1 |grep "Display aspect ratio" |cut -d : -f 2`
HEIGHT=`echo "320 / $ASPECT" |bc`
echo $HEIGHT

...and it does the same, so it's definitely bc that won't accept the output from mediainfo...

Can anyone help?

wje_lq · 01-14-2009, 10:45 AM

I'm sure that others will come along with solutions which are more easily typed than the following, but this one is sure to work in pretty much any distribution of Linux or UNIX:

Code:

ASPECT=`mediainfo $1 |sed -e 's/[^ -~]//g' |grep "Display aspect ratio" |cut -d : -f 2`
                                  ^ ^
                                  | |
                              space tilde

mediainfo probably outputs a carriage return before the line feed at the end of the line, and this filters out the carriage return.

Hope this helps.

Disillusionist · 01-14-2009, 03:06 PM

Not sure it's easier to type but:

Code:

ASPECT=`mediainfo $1 |sed 's/^M//g' |grep "Display aspect ratio" |cut -d : -f 2`

^M in this is a single character created by typing [Ctrl]+[v] followed by [Ctrl]+[m]

colucix · 01-14-2009, 04:17 PM

It is the output of media info which is in DOS format, that is each line terminates with a line feed (carriage return + newline). You can filter the output using the dos2unix command:

Code:

mediainfo $1 | grep "Display aspect ratio" | dos2unix | cut -d : -f 2

You can see the difference piping the output to od -c. For example:

Code:

$ mediainfo file.avi | grep "Display aspect ratio" | od -c
0000000   D   i   s   p   l   a   y       a   s   p   e   c   t       r
0000020   a   t   i   o
0000040       :       4   /   3  \r  \n
0000050
$ mediainfo file.avi | grep "Display aspect ratio" | dos2unix | od -c
0000000   D   i   s   p   l   a   y       a   s   p   e   c   t       r
0000020   a   t   i   o
0000040       :       4   /   3  \n
0000047

rodhull · 01-14-2009, 06:59 PM

Thanks, guys.

I decided to go with the dos2unix variant in the end. Strange how mediainfo produces a DOS-like output - nevermind!

Here's the full line I used for the HEIGHT var. in the end after a little more experimentation:

Code:

HEIGHT=`echo "scale=3;320/($ASPECT)"|bc|cut -d . -f 1`

Some aspect ratios given by mediainfo are decimal numbers, but others are formats like 4/3 or 16/9 etc. so I had to enclose the $ASPECT var. in brackets. The "scale" option is needed to give a more accurate figure from the calculation (3 dec. places), but then the second "cut" is required to remove the decimal places and provide an integer in pixels for the height for ffmpeg - it obviously won't allow a fraction of a pixel!

The script now converts virtually anything into an iPod-ready video at the original aspect ratio.

Many thanks again!