Simple bash argument parser

Question

In a bash script, I need to parse arguments that have the following form:

1. The main arguments can be thought of as being a single argument, but I do not want to force users to quote the entire thing, so when the argument contains spaces I must be able to handle multiple arguments.

2. One flag may be passed as -<flag> where <flag> can be an arbitrary word (without spaces)

3. Finally, an external command, including its own options and flags may be passed. If so, this should be separated by a double dash.

For example,

my_command test

should result in

"$inp" == "test"
"$flag" == ""
"$ext_command" == ""

and

my_command this is a test -new -- sed "s|a|b|"

should result in

"$inp" == "this is a test"
"$flag" == "new"
"$ext_command" == "sed \"s|a|b\""

I think the following script does what I want, but since it's my first bash script, I wanted to ask whether the script is idiomatic and whether I missed any border cases.

local inp=""
local flag=""
local ext_command=""
local count="1" 
local started=""
for i
do
 count=$((count+1))
 if [[ "$i" == '--' ]]
 then
 ext_command="${@:count}"
 break
 else
 if [[ "$i" == -* ]];
 then
 flag=${i#*-}
 else
 if [ ! "$started" ]
 then
 inp="$i"
 started=1
 else
 inp="$inp $i"
 fi
 fi
 fi
done

• Solomon Ucko: Is it possible to have multiple flags?
• Bananach: @SolomonUcko no, not at the moment

Answer 1

Don't go against the language

The main arguments can be thought of as being a single argument, but I do not want to force users to quote the entire thing, so when the argument contains spaces I must be able to handle multiple arguments.

If you want to a value containing spaces as a single argument, then you and your users should double-quote it, otherwise Bash will perform word splitting. This is a fundamental principle in Bash, and it's better to play along with it than to go against.

Trying to go against will get you into all kinds of trouble. For example, what would you expect for?

my_command this is a test -new -- sed "s|a|b|"

That is, multiple spaces between words. Those spaces will be lost, the script will behave the same way as if there was a single space in between.

Keep in mind that users will have to quote special characters anyway. You cannot shelter them from quoting. It's better to learn the basic rules of word splitting and quoting early, rather than trying to work around it with hacky solutions.

Assign arrays to arrays

This statement assigns an array to a non-array:

ext_command="${@:count}"

This way you lose the ability to expand the original value correctly quoted.

Take for example this input:

my_command test -new -- sed "s|a| |"

Notice the space in the sed pattern.

And let's say the script uses ext_command like this:

ls | "$ext_command"

This will not work as intended (replacing "a" with spaces), because the original arguments are not preserved correctly.

Using an array you could leave this option open, that is:

ext_command=("${@:count}")

And then later:

ls | "${ext_command[@]}"

Minor points

Instead of this:

local inp=""

You can write simply:

local inp

---

Instead of this:

count=$((count+1))

You can write simply:

((count++))

---

Instead of this:

if [[ "$i" == '--' ]]
then
 ext_command="${@:count}"
 break
else
 # a long block of code ...
fi

It's more readable like this:

if [[ "$i" == '--' ]]
then
 ext_command="${@:count}"
 break
fi
# a long block of code ... but less deeply nested