Shell scripting: extracting absolute path from default value of path variable

Question

I am writing a shell script that takes argument $1 as a destination directory and $2 as a source directory. Script:

#!/bin/zsh
# readlink is used to get absolute path when relative path is provided
output_path=`readlink -f "${1:-.}"`
src_dir=`readlink -f -- "${2:-~/Downloads}"`
echo "destination dir: $output_path"
echo "source dir: $src_dir"
return

Output:

destination dir: /home/ashwin
source dir: 

I get blank in source directory variable. Changing the default value of src_dir to the absolute path of folder solves the problem. Script:

#!/bin/zsh
# readlink is used to get absolute path when relative path is provided
output_path=`readlink -f "${1:-.}"`
src_dir=`readlink -f -- "${2:-/home/ashwin/Downloads}"`
echo "destination dir: $output_path"
echo "source dir: $src_dir"
return

Output:

destination dir: /home/ashwin
source dir: /home/ashwin/Downloads

Also providing ~/Downloads as an argument to script works fine. I want to understand the reason why ~/Downloads as the default value of src_dir does not work.

Answer 1

I want to understand the reason why ~/Downloads as the default value of src_dir does not work.

From bash manual tilde expansion:

If a word begins with an unquoted tilde character (‘~’), ...

The "${2:-~/Downloads}" doesn't begin with an unquoted tilde character. And the result ~/Downloads is quoted within double ". The ~ is not expanded. So readlink -f -- "~/Downloads" is run. Because the path named literally /home/ashwin/~/Downloads doesn't exists on your system, readlink generates no output and exits with a nonzero exit status (1 on my system).

Please do not use backticks `. Use $( ) instead.