bash : Bad Substitution

The default shell (/bin/sh) under Ubuntu points to dash, not bash.

me@pc:~$ readlink -f $(which sh)
/bin/dash

So if you chmod +x your_script_file.sh and then run it with ./your_script_file.sh, or if you run it with bash your_script_file.sh, it should work fine.

Running it with sh your_script_file.sh will not work because the hashbang line will be ignored and the script will be interpreted by dash, which does not support that string substitution syntax.

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