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It is pretty clear that with shell scripting this sort of thing can be accomplished in a huge number of ways (more than most programming languages) because of all the different variable expansion methods and programs like test and [ and [[, etc.
Right now I'm just looking for
DIR=$1 or . Meaning, my DIR variable should contain either what is specified in the first arg or the current directory.
What is the difference between this and DIR=${1-.}?
I find the hyphen syntax confusing, and seek more readable syntax.
Why can't I do this?
DIR="$1" || '.' I'm guessing this means "if $1 is empty, the assignment still works (DIR becomes empty), so the invalid command '.' never gets executed."
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To provide a default value for a variable, include a DEFAULT clause. The value can be specified as an expression; it need not be a constant. If the DEFAULT clause is missing, the initial value is NULL. When a variable is first declared, its value is set to NULL.
bash [filename] runs the commands saved in a file. $@ refers to all of a shell script's command-line arguments. $1 , $2 , etc., refer to the first command-line argument, the second command-line argument, etc. Place variables in quotes if the values might have spaces in them.
We can run dest=$source to assign one variable to another. The dest denotes the destination variable, and $source denotes the source variable.
I see several questions here.
“Can I write something that actually reflects this logic”
Yes. There are a few ways you can do it. Here's one:
if [[ "$1" != "" ]]; then DIR="$1" else DIR=. fi “What is the difference between this and DIR=${1-.}?”
The syntax ${1-.} expands to . if $1 is unset, but expands like $1 if $1 is set—even if $1 is set to the empty string.
The syntax ${1:-.} expands to . if $1 is unset or is set to the empty string. It expands like $1 only if $1 is set to something other than the empty string.
“Why can't I do this? DIR="$1" || '.'”
Because this is bash, not perl or ruby or some other language. (Pardon my snideness.)
In bash, || separates entire commands (technically it separates pipelines). It doesn't separate expressions.
So DIR="$1" || '.' means “execute DIR="$1", and if that exits with a non-zero exit code, execute '.'”.
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