I am trying to check whether a docker container exists using grep.
The following script signals an error exit during running when IMAGE is empty and is fine when IMAGE is set. I can't find out the cause.
#!/usr/bin/env bash
set -e
IMAGE=$(docker ps | grep my-dev-image)
if [[ -z "$IMAGE" ]]; then
echo "It is not there"
else
echo "It is there"
fi
When you use set -e in a script the shell will exit whenever a command fails. It interacts badly with your grep call because grep exits with an error code if it doesn't find a match. If grep fails then the entire IMAGE=$(...) assignment fails and the script exits instead of setting IMAGE to an empty string.
You can fix this by ensuring the assignment always succeeds. A common idiom for this is to append || :. Adding || cmd will run cmd whenever the command on the left fails. And : is a command that always succeeds. (Yes, it's a command name consisting of a single colon. Strange, but it's a legal identifier.)
IMAGE=$(docker ps | grep my-dev-image) || :
Alternatively, you could check grep's exit code directly by using it in the if statement. This is what I would do if I didn't care about grep's output:
if docker ps | grep -q my-dev-image; then
echo "It is there"
else
echo "It is not there"
fi
set -e(orset -o errexit, ortrap ERR) do what I expected? -- if in a hurry, be sure to read the exercises section.set -e.