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Building a script with functions that encodes and shrinks videos
The previous tutorials were on the short side. Heres one to make up for my brevity :^) If something is unclear check those other tutorials to see if it is explained there. For more insight into the use of any of the commands below check out the man page.
I often save movies and documentaries for viewing later and sometimes I want to burn them onto a DVD to share with family and friends. In order to burn the show I crop anything extra before and after plus remove any commercials in the middle. I use gopdit http://gopdit.ath.cx/ for my mpg-ps editing because its fast and simple. I have it set to save without encoding so I'm left with the basic mpg-ps (program stream) format that I saved the original show with.
I can use a a short one command script to save me the trouble of remembering (and typing) a very long mencoder command that encodes the video to a good DVD quality mpg-ps but the problem is that often I end up with a video that is too big to fit on a DVD and then I have to shrink the video before I can burn it. Below is an example of a movie before and after encoding
bash-4.1$ ls -l total 8849532 -rwx------ 2 rick root 5068703744 Jun 17 06:18 encoded-hidden-blade.mpg -rwx------ 1 rick root 3937532284 Jan 12 2011 hidden-blade.mpg
In order to save the time and bother of encoding and shrinking separately I can do them both in the same script. Here's what it looks like
!/bin/bash
####################################################################
# con2vob
# Converts and shrinks an mpg-ps video.
# Requires mencoder, vamps, bc and assumes system (not file system)
# block size = 1024 bytes (1kB).
####################################################################
# Variables
input_file=$1
dvd_size=4400000
# Test input.
test -n "$input_file"
if [ $? -eq 1 ]; then
echo -e "\nUsage: con2vob [input file]"
exit
fi
# Encode video.
mencoder $input_file -oac lavc -ovc lavc -of mpeg -mpegopts format=dvd:tsaf \
-vf scale=720:576,harddup -srate 48000 -af lavcresample=48000 \
-lavcopts vcodec=mpeg2video:vrc_buf_size=1835:vrc_maxrate=9800:vbitrate=5000:\
keyint=15:vstrict=0:acodec=ac3:abitrate=192:aspect=16/9 -ofps 25 \
-o encoded-$input_file
# Shrink video (if necessary).
file_size=`ls -s encoded-$input_file | awk -F " " '{ print $1 }'`
if [ $file_size -gt $dvd_size ]; then
requant_factor=`echo "$file_size/$dvd_size" | bc -l`
vamps -e $requant_factor -a 1 < encoded-$input_file > shrunk-encoded-$input_file
fi
# End of script.
In this script the video is assigned to the variable input_file and then input_file is encoded with mencoder. If no video file was entered the user is given the correct usage of the script and the script exits.
The output of mencoder is named encoded-input_file and if its size is greater than my variable dvd_size the script will shrink the video using vamps.
The linux operating systems use a block size of 1024 bytes, i.e.1kb. I want the script to shrink the video if the video exceeds 4.2gb and 4,400,000kb equals 1.19gb, close enough for me.
I will need to compare the size of encoded-input_file to my dvd_size of 4400000 so first I need to get the size of encoded-input_file in kilobytes. I use ls -s to give me the file size in blocks which you now know is a good as asking for the size in kilobytes and use awk to print just the file size.
bash-4.1$ ls -l encoded-hidden-blade.mpg
-rwx------ 1 rick root 5068703744 Jun 3 11:51 encoded-hidden-blade.mpg
bash-4.1$ ls -s encoded-hidden-blade.mpg
4949908 encoded-hidden-blade.mpg
bash-4.1$ ls -s encoded-hidden-blade.mpg | awk -F " " '{ print $1 }'
4949908
Bash will do simple math but for floating point math I have to use bc. The requant factor (read shrink factor) is found by dividing the file size by my set limit of 4,400,000kb which will result in a decimal. In this usage the equation is piped to bc using an echo command.
bash-4.1$ file_size=`ls -s encoded-hidden-blade.mpg | awk -F " " '{ print $1 }'` \
> echo "$file_size/4400000" | bc -l
1.12497909090909090909
The vamps command syntax might seem a bit different. It uses redirection for input and output files.
vamps -e $requant_factor -a 1 < encoded-$input_file > shrunk-encoded-$input_file
The encoded-input_file is redirected to stdin with “<” and shrunk-encoded-input_file is redirected to stdout with “>”.
Here are the results when con2vob is run and the movie hidden-blade has been shrunk.
bash-4.1$ ls -lSr total 13274848 -rwx------ 1 rick root 3937532284 Jan 12 2011 hidden-blade.mpg -rwx------ 1 rick root 4531521536 Jun 17 11:04 shrunk-encoded-hidden-blade.mpg -rwx------ 1 rick root 5068703744 Jun 17 10:52 encoded-hidden-blade.mpg bash-4.1$ ls -1 -sSr total 13274848 3845248 hidden-blade.mpg 4425316 shrunk-encoded-hidden-blade.mpg 4949908 encoded-hidden-blade.mpg
The video file shrunk-encoded-hidden-blade.mpg has been reduced to 4425316kb. One gigabyte is 1048576kb. Consequently shrunk-encoded-hidden-blade.mpg is 4425316kb / 1048576kb = 4.22031gb.
the same script with functions
In programming a function is a subroutine that contains a command or more usually a group of commands that is called by the script when required. It is intended to do just one thing. For me it is much easier to write a long script using functions. Each subroutine can be written and tested as a mini script and then be incorporated into the final script.
The syntax for a function in a bash script is:
function_name()
{
commands.........
)
or alternatively:
function function_name
{
commands........
}
Functions are called by invoking their names in a script and they must be listed in the script before they are called.
My con2vob script listed above is too short to require functions but it will serve as a good example of how to use functions as well as serve as the core of a more complex script that also burns DVDs which I will introduce in Part 5 of this series.
Con2vob comprises some variables and then three sections each beginning with a comment explaining what is happening. These comments will now become my function names and the sections of code will become the functions.
#!/bin/bash
####################################################################
# new-con2vob
# Converts and shrinks an mpg-ps video.
# Requires mencoder, vamps, bc and assumes system (not file system)
# block size = 1024 bytes (1kB).
####################################################################
# Variables
input_file=$1
dvd_size=4400000
test_input()
{
test -n "$input_file"
if [ $? -eq 1 ]; then
echo -e "\nUsage: con2vob [input file]"
exit
fi
}
encode_video()
{
mencoder $input_file -oac lavc -ovc lavc -of mpeg -mpegopts format=dvd:tsaf \
-vf scale=720:576,harddup -srate 48000 -af lavcresample=48000 \
-lavcopts vcodec=mpeg2video:vrc_buf_size=1835:vrc_maxrate=9800:vbitrate=5000:\
keyint=15:vstrict=0:acodec=ac3:abitrate=192:aspect=16/9 -ofps 25 \
-o encoded-$input_file
}
shrink_video() # (if necessary).
{
file_size=`ls -s encoded-$input_file | awk -F " " '{ print $1 }'`
if [ $file_size -gt $dvd_size ]; then
requant_factor=`echo "$file_size/$dvd_size" | bc -l`
vamps -e $requant_factor -a 1 < encoded-$input_file > shrunk-encoded-$input_file
fi
}
####################################################################
# Now call the functions
test_input
encode_video
shrink_video
# End of script.
in progess