Finding a string in docker logs of container

Question

this can happen if the container is logging to stderr, piping works only for stdout, so try:

docker logs nginx 2>&1 | grep "127."

"2>&1" will tells the shell to redirect stderr to stdout and give the output to grep using pipe.

As vim fan I prefer to use less and search with /

docker logs nginx 2>&1 | less

docker logs <container_name> 2>&1 | grep <string>

You could also use an anonymous pipe:

docker logs nginx 2> >(grep '127.')

To follow up on the comments and clarify this for anyone else hitting this issue. Here is the simplest way I can see to search an nginx container log.

docker logs nginx > stdout.log 2>stderr.log
cat stdout.log | grep "127."

IMO its kinda messy because you need to create and delete these potentially very large files. Hopefully we'll get some tooling to make it a bit more convenient.

Additionally, I found it usefull to highlight some terms that I'm searching for in the logs. Especially on productive installations where a lot of log output is generated. In my case I want to highlight COUNT(*) statements. But with a simple grep I can't see the entire SQL statement since it's a multi line statement. This is possible with -E switch and some regex:

For example, the following snippet search for all queries that contain COUNT(*) as well as count(*):

docker logs <containerName> -f | grep --line-buffered -i -E --color "select count\(\*\)|$"

Some explanation:

• docker logs -f tell docker to follow the logs, so the filter applys also to new entrys like when viewing it using tail -f
• greps --line-buffered switch flushes the output on every line, which is required to grep in real time when docker logs -f is used
• -E is an extended regex pattern, required to apply our pattern that allow us returning also the non matching results
• --color highlights the matched parts (seems the default behaviour on my Ubuntu 16.04 LTS, but maybe not on other distributions, so I included it here to be safe)
• * is escaped to disable its special glob functionality, where (, and ) are masked to avoid their regex meaning as group, which is enabled by -E switch

If the container logs to stderr, you can pipe them as Edoardo already wrote for a simple grep:

docker logs <containerName> -f 2>&1 | grep --line-buffered -i -E --color "select count\(\*\)|$"

The -f switch could be omitted if no live grep is wanted. In both cases, you see the entire log buth with highlighted search term like this: