bash: ${...}: bad substitution

bad substitution means the shell could not parse a ${...} expansion. Two causes cover most cases: the expansion text is invalid (${name bad}), or a bash-only expansion such as ${name^^} runs in a POSIX shell like dash. Check how the script is started (sh or bash) and read the expansion against the bash manual.

Reproduce it

bash -c 'echo ${name bad}'

bash: line 1: ${name bad}: bad substitution

Arithmetic goes in $(( )), not in ${ }:

bash -c 'n=1; echo $((n + 1))'

2

It works in bash but not with sh

On Debian and Ubuntu sh is dash, so sh script.sh ignores the shebang and rejects bash-only syntax.

sh -c 'name=hello; echo ${name^^}'
# sh: 1: Bad substitution
bash -c 'name=hello; echo ${name^^}'
# HELLO

Check which shell sh is with ls -l /bin/sh. In CI and Dockerfiles RUN and run: steps may also use sh; select bash explicitly (shell: bash in GitHub Actions).