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Q&A In a bash shell script, how to filter the command line argument list to unique entries only, for processing each?

The solution is to use an associative array to store what you've already seen: #!/bin/bash unset seen declare -A seen for arg in "$@" do if [[ -z "${seen[$arg]}" ]] then ...

posted 2y ago by celtschk‭

Answer
#1: Initial revision by user avatar celtschk‭ · 2021-09-28T17:58:47Z (over 2 years ago)
The solution is to use an associative array to store what you've already seen:

```bash
#!/bin/bash

unset seen
declare -A seen

for arg in "$@"
do
    if [[ -z "${seen[$arg]}" ]]
    then
        echo "Doing something omplicated with $arg"
        seen["$arg"]=1
    fi
done
```
The `unset seen` is just in case the caller had an exported variable named `seen`. The `declare -A seen` tells bash to treat `seen` as an associative array, that is, it takes arbitrary strings as index.

The loop then tests for each argument whether it has not yet been seen (in which case `"${seen[arg]}"` is empty). If so, it processes it and then marks it as seen by storing something in `seen["$arg"]` (what it stores doesn't really matter as long as it is not empty; if you want, you can store additional information about the argument here, e.g. the result of processing this argument).