new post

Author: wh0

_posts/2022-08-24-docker-quoting.md

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+---
+date: Wed Aug 24 2022 22:38:38 -0700 (Pacific Daylight Time)
+qualitative_time: 
+title: One time when it really was a Docker command quoting issue
+previous_teaser: The average number of slices of pizza a fork touches in its lifetime
+previous_first: false
+has_highlighting: true
+---
+A few days ago I was staring at a Docker command like this:
+
+```sh
+set -e
+image=debian # set externally
+tag=latest # derived from some command output
+cd /tmp
+docker pull "$image:$tag"
+docker run --rm \
+    --entrypoint echo \ 
+    -v $PWD/a:/mnt/a \
+    $image:$tag \
+    "hello world"
+```
+
+But it would give:
+
+```
+latest: Pulling from library/debian
+Digest: sha256:d52921d97310d0bd48dab928548ef539d5c88c743165754c57cfad003031386c
+Status: Image is up to date for debian:latest
+docker.io/library/debian:latest
+docker: invalid reference format.
+See 'docker run --help'.
+```
+
+And there were plenty of unquoted variables, which by now is well known to mess up Docker commands.
+But we were able to confirm that they didn't contain any spaces.
+(I think we later added quotes around stuff in order to be safer.)
+So I was thinking, this couldn't be a quoting issue.
+
+One interesting thing is that if any of `image` or `tag` were empty, it would give the exact error we're seeing:
+
+```
+$ docker run :latest
+docker: invalid reference format.
+See 'docker run --help'.
+$ docker run debian:
+docker: invalid reference format.
+See 'docker run --help'.
+$ docker run :
+docker: invalid reference format.
+See 'docker run --help'.
+```
+
+But the `docker pull` above it was working fine, so it couldn't have been those variables being empty.
+
+At that point, I tried typing in the whole command in my terminal and found that I couldn't reproduce the issue:
+
+```
+...
+$ docker run --rm \
+> --entrypoint echo \
+> -v $PWD/a:/mnt/a \
+> $image:$tag \
+> "hello world"
+hello world
+```
+
+Could it have been some sneaky non-ASCII characters hiding in the original?
+I checked with an online tool, and there was not.
+No specialized dashes instead of hyphens, no smart quotes instead of the straight ones.
+
+---
+
+It turned out that there was an extra space after the backslash in the `--entrypoint` line.
+(Apologies if anyone was reading a printed copy of this; you stood no chance.)
+
+The result was that the backslash-space would give `docker run` a space character as the first positional parameter, i.e. the image name.
+Then there would be a plain old newline marking the end of the command.
+
+We never saw any junk about `-v` not being found as a program because the thing we were using to run it bailed on the first failed command (I've tried to capture that with the `set -e` above).
+
+And I was looking up the terminology for this, and supposedly, escaping a character such as a space with a backslash in shell script is described under the "Quoting" section, so it was technically a quoting issue after all.