Assign variable the grep value of command output

So... To answer your question about the error, you need a space after the echo command.

But wait, there's more.

You probably don't really want the word "echo" as part of your duration variable. You'd use echo to display output of some kind, not to gather data and store it in a variable. Variable assignments are in the form of variable=value or read variable which can take a value from a variety of sources.

Note that without the -v quiet, the ffprobe command will dump a plethora of data to stderr, so it's important to include it to reduce the noise of your script.

The output of your (quieted) ffprobe command would include something like this:

duration=171.805000

You can do a number of things with this format of data (my goodness, it looks like variable=value...), but the easiest might be to use cut.

file="media/myvideo.mp4"
str_duration=$(ffprobe -i "$file" -show_format -v quiet | grep ^duration | cut -d= -f2)

Note that this approach of using the equals as a field separator might fail for other fields which might include their own equals sign. But it should be safe for duration.

Alternately, you could use tools like awk or sed to reduce the number of pipes:

str_duration=$(ffprobe -i "$file" -show_format -v quiet | sed -ne '/^duration=/s///p')

or

str_duration=$(ffprobe -i "$file" -show_format -v quiet | awk -F= '$1=="duration"{print $2}')

I am purposely not going to tell you how to use the eval command to take ffprobe's output and assign it directly as a variable within your script, because the eval command is very dangerous if you don't check and limit its input. As a beginning shell programmer, you should read about eval and then not use it until you feel that you're somewhere between "intermediate" and "advanced". :)

Alternately, you can use Input Field Separators to parse this data in bash:

declare -A details
while IFS== read field value; do
  details[$field]="$value"
done < <(ffprobe -i "$file" -show_format -v quiet | grep '.=.')

echo "${details[duration]}"

This takes the output of the ffprobe command and commits it to the elements of an array, which you can read as per the echo line. It is safe for content that includes an equals sign, because of the limited number of variables mentioned in the read command.

Note that except in the case of this last example, ALL the commands in this answer are compatible with traditional /bin/sh, not just bash.