Docker exec - Write text to file in container

Question

I want to write a line of text to a textfile INSIDE a running docker container. Here's what I've tried so far:

docker exec -d app_$i eval echo "server.url=$server_url" >> /home/app/.app/app.config

Response:

/home/user/.app/app.config: No such file or directory

Second try:

cfg_add="echo 'server.url=$server_url' >> /home/user/.app/app.config"
docker exec -i app_$i eval $cfg_add

Response:

exec: "eval": executable file not found in $PATH

Any ideas?

Answer 1

eval is a shell builtin , whereas docker exec requires an external utility to be called, so using eval is not an option.

Instead, invoke a shell executable in the container ( bash ) explicitly , and pass it the command to execute as a string , via its -c option:

docker exec "app_$i" bash -c "echo 'server.url=$server_url' >> /home/app/.app/app.config"

By using a double-quoted string to pass to bash -c , you ensure that the current shell performs string interpolation first, whereas the container's bash instance then sees the expanded result as a literal , as part of the embedded single-quoted string.

---

As for your symptoms :

• /home/user/.app/app.config: No such file or directory was reported, because the redirection you intended to happen in the container actually happened in your host's shell - and because dir. /home/user/.app apparently doesn't exist in your host's filesystem, the command failed fundamentally , before your host's shell even attempted to execute the command ( bash will abort command execution if an output redirection cannot be performed).

  • Thus, even though your first command also contained eval , its use didn't surface as a problem until your second command, which actually did get executed.

• exec: "eval": executable file not found in $PATH happened, because, as stated, eval is not an external utility , but a shell builtin , and docker exec can only execute external utilities.

Answer 2

Additionally:

If you need to write text from outside the container, this also works:

(docker exec -i container sh -c "cat > c.sql") < c.sql

This will pipe you input into the container. Of course, this would also work for plain text (no file). It is important to leave off the -t parameter.

See https://github.com/docker/docker/pull/9537

UPDATE (in case you just need to copy files, not parts of files):

Docker v17.03 has docker cp which copies between the local fs and the container: https://docs.docker.com/engine/reference/commandline/cp/#usage

Answer 3

try to use heredoc:

(docker exec -i container sh -c "cat > /test/iplist") << EOF
10.99.154.146
10.99.189.247
10.99.189.250
EOF