How to debug a single shell command call, viewing arguments accurately?

Question

When I'm debugging a script, I'll often take a line I'm not quite sure about e.g.:

hdiutil create -srcfolder "$3" -format UDRO -fs APFS -volname "$1" -ov "$2"

and simply change it to:

echo hdiutil create -srcfolder "$3" -format UDRO -fs APFS -volname "$1" -ov "$2"

Then when I run my script the first time, it won't run the (potentially incorrect) command but I can see it and check that it looks correct… and maybe even copy/paste it to run it manually.

But this isn't reliable/correct if any of the arguments have spaces in them. The echo shows spaces within an argument just the same as the separator between arguments.

I found the set -x command which helpfully logs every command with arguments properly escaped! That's what I'm using for now, but it's pretty broad-grained (noisy across a large script where I'm focused on just one line) and doesn't prevent the actual un-verified command from running.

Is there a quick/clean way to do this for just one line. Or some other trick using commonly available commands — maybe some combination of env and printf or whatnot? or a dedicated command that got included for basically this purpose?

Ideally it'd be something I could just pre-pend to the line in question which would both "comment it out" and show me exactly what it would have done if enabled.

You can do set -x right before the line you're interested in, and set +x after. — Benjamin W.
– Benjamin W., Commented Jun 15, 2018 at 17:21
If you want a more comprehensive debugging solution than set -x then you could use bashdb which can be installed via MacPorts or Homebrew. — l'L'l
– l'L'l, Commented Jun 15, 2018 at 17:37

that other guy · Accepted Answer · 2018-06-15 18:22:50Z

1

You can use printf %q:

#!/bin/bash
run() {
  printf "%q " "$@"  # show each argument shell escaped
  printf '\n'
  (( dryrun )) || "$@" # optionally run the command
}

dryrun=1
run hdiutil create -srcfolder "$3" -format UDRO -fs APFS -volname "$1" -ov "$2"

answered Jun 15, 2018 at 18:22

that other guy

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