Why does my bash code fail when I run it with sh?

I have a line of code that works fine in my terminal:

for i in *.mp4; do echo ffmpeg -i "$i" "$i/.mp4/.mp3"; done


Then I put the exact same line of code in a script myscript.sh:

myscript.sh

#!/bin/sh
for i in *.mp4; do echo ffmpeg -i "$i" "$i/.mp4/.mp3"; done


However, now I get an error when running it:

$ sh myscript.sh
myscript.sh: 2: myscript.sh: Bad substitution


Based on other questions I tried changing the shebang to #!/bin/bash, but I get the exact same error. Why can't I run this script?

#!/bin/bash

bash

#!/bin/sh

sh

#!/bin/bash

2 Answers
2

TL;DR: Since you are using bash specific features, your script has to run with bash and not with sh:

bash

bash

sh

$ sh myscript.sh
myscript.sh: 2: myscript.sh: Bad substitution

$ bash myscript.sh
ffmpeg -i bar.mp4 bar.mp3
ffmpeg -i foo.mp4 foo.mp3


bash is basically to sh what C++ is to C. See Difference between sh and bash.

bash

sh

The best practices are to both:

#!/bin/sh

#!/bin/bash

./myscript.sh

/path/to/myscript.sh

sh

bash

Here's an example:

$ cat myscript.sh
#!/bin/bash
for i in *.mp4
do
echo ffmpeg -i "$i" "$i/.mp4/.mp3"
done

$ chmod +x myscript.sh # Ensure script is executable

$ ./myscript.sh
ffmpeg -i bar.mp4 bar.mp3
ffmpeg -i foo.mp4 foo.mp3


(Related: Why ./ in front of scripts?)

#!/bin/sh

The shebang suggests which shell the system should use to run a script. This allows you to specify #!/usr/bin/python or #!/bin/bash so that you don't have to remember which script is written in what language.

#!/usr/bin/python

#!/bin/bash

People use #!/bin/sh when they only use a limited set of features (defined by the POSIX standard) for maximum portability. #!/bin/bash is perfectly fine for user scripts that take advantage of useful bash extensions.

#!/bin/sh

#!/bin/bash

/bin/sh is usually symlinked to either a minimal POSIX compliant shell or to a standard shell (e.g. bash). Even in the latter case, #!/bin/sh may fail because bash wil run in compatibility mode as explained in the manpage:

/bin/sh

#!/bin/sh

bash

If bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of sh as closely as possible, while conforming to the POSIX standard as well.

sh myscript.sh

The shebang is only used when you run ./myscript.sh, /path/to/myscript.sh, or when you drop the extension, put the script in a directory in your $PATH, and just run myscript.

./myscript.sh

/path/to/myscript.sh

$PATH

myscript

If you explicitly specify an interpreter, that interpreter will be used. sh myscript.sh will force it to run with sh, no matter what the shebang says. This is why changing the shebang is not enough by itself.

sh myscript.sh

sh

You should always run the script with its preferred interpreter, so prefer ./myscript.sh or similar whenever you execute any script.

./myscript.sh

"$i"

$i

"$i%.mp4.mp3"

"$i/.mp4/.mp3"

$parameter%word

foo.mp4.backup

#!/bin/bash

sh

#!/bin/sh

bash

#!/usr/bin/env bash

sh yourscript

#!/bin/bash

./myscript.sh

The $var/x/y/ construct is not POSIX. In Your case, where you just remove a string at the end of a variable and tack on another string, the portable POSIX solution is to use

$var/x/y/

#!/bin/sh
for i in *.mp4; do
ffmpeg -i "$i" "$i%.mp4.mp3"
done


or even shorter, ffmpeg -i "$i" "$i%43".

ffmpeg -i "$i" "$i%43"

The definitive dope for these constructs is the chapter on Parameter Expansion for the POSIX shell.

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