How to write command line argument input redirection within a bash script?

Question

I'm putting together a bash script to run movie rendering jobs. ffmpeg expects a multi-line text file as one of the arguments for concatenation filter to join several files into one. Like this:

ffmpeg -f concat -i mylist.txt -c copy output

There's also an option to write it all in a single line like so:

ffmpeg -f concat -i <(printf "file '%s'\n" A.mp4 B.mp4) -c copy Output.mp4

How do I write the latter for into a bash script, substituting the file names from other variables? Tryed splitting the variables, but its not the one that works. $A and $B variables contains paths to input files.

#!/bin/bash
...
TMP_LIST=$(printf "file '%s'\n" ${A} ${B})
APPEND="${FFMPEG} -f concat -i <<< {${TMP_LIST}} -c copy ${OUTPUT}"
# run the concatenation:
$APPEND

dogbane · Accepted Answer · 2013-09-03 11:09:41Z

3

Try the following:

ffmpeg -f concat -i <(printf "file '%s'\n" "$A" "$B") -c copy Output.mp4

answered Sep 3, 2013 at 11:09

dogbane

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2 Comments

Serge Over a year ago

It works! Thank you. Is there a way to put the whole command into a variable to execute then by just calling that variable? — asking for the sake of knowledge.

Gordon Davisson Over a year ago

@SergikS: Use a function, variables are parsed in an way that makes this sort of thing difficult. See BashFAQ #50:I'm trying to put a command in a variable, but the complex cases always fail!

konsolebox · Accepted Answer · 2013-09-03 11:38:14Z

This is one way to have a list and store the command in a variable (array) before it gets executed. Unfortunately process substitutions can only be kept alive by passing them as arguments to a function, so we have to use a function here:

#!/bin/bash

LIST_IN_ARRAYS=("$A" "$B")  ## You can add more values.

function append {
    local APPEND=("$FFMPEG" -f concat -i "$1" -c copy "$2")  ## Store the command in an array.
    "${APPEND[@]}"  ## This will run the command.
}

append <(printf "file '%s'\n" "${LIST_IN_ARRAYS[@]}") "$OUTPUT"

Although I would have done it this way to make things simpler:

function append {
    "$FFMPEG" -f concat -i <(printf "file '%s'\n" "${@:2}") -c copy "$1"
}

append "$OUTPUT" "${LIST_IN_ARRAYS[@]}"  ## Or
append "$OUTPUT" "$A" "$B"

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