exit "Bad filename"
echo "Bad filename" >&2 exit 1
exit can only be used to signal success or failure (0 = success, 1-255 = failure). It can not be used to return string data, and it can not be used to print error messages.
String data should be written stdout, before an
exit 0 to exit with success.
Errors should instead be written to stderr, with an
exit 1 (or higher) to exit with failure:
if [ ! -f "$1" ] then echo "$1 is not a regular file" >&2 exit 1 fi
Note in particular that
exit -1 is equivalent to
exit 255, but that
exit 1 is the more canonical way of expressing the first possible error code.
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.