In Bash, note there are lots of kinds of variable expansion.
Checking a variable exists: ${name:?error message}. For example, if a Bash script requires a single argument, just write input_file=${1:?usage: $0 input_file}.
Using a default value if a variable is empty: ${name:-default}.
If you want to have an additional (optional) parameter added to the previous example, you can use something like output_file=${2:-logfile}. If $2 is omitted and thus empty, output_file will be set to logfile.
Arithmetic expansion: i=$(( (i + 1) % 5 )).
Sequences: {1..10}.
Trimming of strings: ${var%suffix} and ${var#prefix}. For example if var=foo.pdf, then echo ${var%.pdf}.txt prints foo.txt.