How can I list unique characters used in a text file using linux command line tools?

Question

I would like to list a set of characters used in a text file using linux command line tools. How can I achieve this ? `uniq` utility works only on lines.

Accepted Answer

I'd use `od`

```
od -cvAnone -w1
```

This lists characters, showing `\escapes` for non-displayables. Other formats are available

Examples:

So, to list the uniques:

```
od -cvAnone -w1 | sort -bu
```

Or to produce a top-20 histogram:

```
od -cvAnone -w1 | sort -b | uniq -c | sort -rn | head -n 20
```

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How can I list unique characters used in a text file using linux command line tools?

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