How do I write a BASH script to download and unzip file on a Mac?

If you do not want change directory context, use the following script:

#!/bin/bash

unzip-from-link() {
 local download_link=$1; shift || return 1
 local temporary_dir

 temporary_dir=$(mktemp -d) \
 && curl -LO "${download_link:-}" \
 && unzip -d "$temporary_dir" \*.zip \
 && rm -rf \*.zip \
 && mv "$temporary_dir"/* ${1:-"$HOME/Downloads"} \
 && rm -rf $temporary_dir
}

Usage:

# Either launch a new terminal and copy `git-remote-url` into the current shell process, 
# or create a shell script and add it to the PATH to enable command invocation with bash.

# Place zip contents into '~/Downloads' folder (default)
unzip-from-link "http://example.com/file.zip"

# Specify target directory
unzip-from-link "http://example.com/file.zip" "/your/path/here"

Output:

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 17.8M  100 17.8M    0     0  22.6M      0 --:--:-- --:--:-- --:--:-- 22.6M
Archive:  file.zip
  inflating: /tmp/tmp.R5KFNvgYxr/binary

OSX uses the same GNU sh/bash as Linux

#!/bin/sh

mkdir /tmp/some_tmp_dir                         && \
cd /tmp/some_tmp_dir                            && \
curl -sS http://foo.bar/filename.zip > file.zip && \
unzip file.zip                                  && \
rm file.zip

the first line #!/bin/sh is so called "shebang" line and is mandatory

BSD Tar can open a zip file and decompress through a stream.The -L or --location flag is to follow redirects. So the following will work:

curl --show-error --location http://example.org/file.zip | tar -xf - -C path/to/save