ECS Exec is not a shell

I needed to run a Rake task with a dry-run flag inside a staging container on ECS Fargate. A small wrapper script calls aws ecs execute-command --interactive --command "<string>". Plain bundle exec rake ... commands worked through it. This one didn’t:

--command "DRY_RUN=true bundle exec rake my:task[3,42]"

Unable to start command: Failed to start pty: exec: "DRY_RUN=true": executable file not found in $PATH

VAR=value command is shell syntax, and ECS Exec doesn’t run the string through a shell. It splits it into words and runs the first word as the program, so it looked for a program named DRY_RUN=true. Pipes, &&, and globs don’t work there either.

There are two general fixes. env sets the variable and runs the program, and sh -c gives you a real shell:

--command "env DRY_RUN=true bundle exec rake my:task[3,42]"
--command "sh -c 'DRY_RUN=true bundle exec rake my:task[3,42]'"

The image is Alpine, so sh is there but bash may not be.

For Rake there’s a third way. The task’s usage line showed it, and Rake reads KEY=value arguments after the task name into ENV:

--command "bundle exec rake my:task[3,42] DRY_RUN=true"

This is the second ECS Exec surprise I’ve written down, after finding that ECS Exec always runs as root.