Bash auto completion not working after certain command

Question

I am using graphViz under bash / Cygwin, but somehow when calling it, bash tab autocomplete does not do anything.

Example files:

• myGraph.graphviz is the input file
• myGraph.graphviz.png should be the output file.

The command to make the graphic is

dot -T png -o myGraph.graphviz.png myGraph.graphviz

Of course I want to input

dot -T png -o myTab.png myTab

to autocomplete these file names, but nothing happens! I can enter cat myTab and it gives cat myGraph.graphviz, but why doesn't it work after dot? Does bash need an extra autocomplete module for every command I could enter?

Answer 1

I created the example files in my Debian. Indeed, dot -T png -o myTab doesn't work. Then (after completing manually) dot -T png -o myGraph.graphviz.png myTab also doesn't work. There are two separate issues.

---

The general manual for Graphviz programs specifies:

-ooutfile
Write output to file outfile. […]

Bash completion in my Debian obeys this literally: it supports -ooutfile, not -o outfile. Even if the latter works with dot (and I don't know this), the completion doesn't support it. You should proceed like this:

dot -T png -omyTab

without any space after -o. In my tests the above works.

---

To make the second Tab work I inspected /usr/share/bash-completion/completions/dot. The relevant line near the end is:

_filedir dot

This will offer you directories to descend into and files to specify, but only files with names ending with .dot or .DOT. Probably you can manually provide a file not matching this pattern and nothing bad will happen, yet if you rely on the completion mechanism then the file name should end properly.

In fact I don't know dot at all, I know Bash completions a little. Maybe the requirement doesn't make sense and the completion for dot would be more helpful if it allowed any file. In such case I copy the original file to /etc/bash_completion.d/ and modify the copy, so the line

_filedir dot

at the end of the function becomes

_filedir

This is a system-wide change. The change will not affect existing shells, so start a new bash to test it. If you cannot (or don't want to) modify /etc/bash_completion.d/ then see this answer. It provides a way to have your private (user-specific) bash_completion.d/.

Note this change will allow you to type dot -T png -o myTab and the completion will work. However in this particular case it has nothing to do with -o internally, it's the general completion for operands and it would work even after some unexpected or invalid option (e.g. dot -T png -: myTab).

Answer 2

This can happen if there is another dot command on the system for which a real autocomplete definition is installed, and that command doesn't expect a file at that point. You get file auto-completion on all parameters by default for unknown commands, but specific commands can set their own specific auto-completion. Check /etc/bash_completion and /etc/bash_completion.d/.

PS: Another explanation is that the dot auto-completion script isn't coherent with the doc, or that you made a mistake in your arguments.