Pass Bash Variables to FFmpeg

This article provides a quick overview and practical guide on how to safely and effectively pass Linux Bash variables into an FFmpeg command. You will learn the correct syntax for standard variable substitution, how to handle complex arguments containing spaces, and how to safely environment-export variables for advanced filtering scripts.

Standard Variable Substitution

The most straightforward way to pass a Bash variable into an FFmpeg command is by using double quotes (""). In Bash, variables inside double quotes are expanded by the shell before the command is executed, whereas single quotes ('') treat the variable name as literal text.

Here is the basic syntax for passing an input file, an output file, and a video bitrate variable:

#!/bin/bash

INPUT_FILE="input.mp4"
OUTPUT_FILE="output.mkv"
BITRATE="5M"

ffmpeg -i "$INPUT_FILE" -b:v "$BITRATE" "$OUTPUT_FILE"

Handling Spaces and Special Characters

Always wrap your Bash variables in double quotes when passing them to FFmpeg. If your file names or paths contain spaces, omitting the quotes will cause Bash to split the variable into multiple arguments, leading to an FFmpeg “No such file or directory” error.

• Correct: ffmpeg -i "$INPUT_FILE" "$OUTPUT_FILE"
• Incorrect: ffmpeg -i $INPUT_FILE $OUTPUT_FILE

Passing Variables Inside Complex Filters

When using FFmpeg’s complex filtergraphs (-filter_complex or -vf), you often need to use single quotes to wrap the filter chain. Because Bash does not expand variables inside single quotes, you must step out of the single quotes to inject your variable.

Here is the syntax to pass a variable into a drawtext filter:

#!/bin/bash

WATERMARK_TEXT="Property of ACME"

ffmpeg -i input.mp4 -vf "drawtext=text=''"$WATERMARK_TEXT"'':x=10:y=10" output.mp4

Alternatively, you can use double quotes for the entire filter string, but you must escape the internal quotes required by FFmpeg:

ffmpeg -i input.mp4 -vf "drawtext=text=\"$WATERMARK_TEXT\":x=10:y=10" output.mp4