Docker and sed

Docker Desktop

I saw my teammate use the Terminal tab of Docker Desktop. I initially tried with running ubuntu:latestand alpine:latest. Eventually, I remembered that he used redis:latest as the container image. So why does the container using redis continue running, but the ubuntu does not?

Redis container terminal

The idea is that a docker container will exit running if there is no more processes left to run. It turns out that redis is “commonly used as an in-memory data store or cache, and it’s designed to run continuously to serve incoming requests. Therefore, the Redis container doesn’t exit immediately because the Redis process remains active.”

I am using sed to make changes to a configuration file when the docker image is built. The app will display the build time, and I am setting that in the Dockerfile.

First, let’s go over what sed is and its syntax. sed stands for "stream editor" and is a powerful command-line utility for text manipulation in Unix-like operating systems. It is commonly used for tasks such as searching, replacing, inserting, and deleting text in files or streams of data.

Personally, I use it for replacing a string of text in files.

The basic syntax of sed is as follows:

sed options 'command' file

Here’s a breakdown of each component:

• options: These are optional flags that modify the behavior of sed. Some commonly used options include:

-i: Modifies the file in-place (i.e., changes are made directly to the file).

-e: Allows specifying multiple commands.

• 'command': This represents the actual editing command(s) to be executed. Several commands can be combined, separated by semicolons (;) or using the -e option.

Commands can operate on a range of lines, specified using line numbers or patterns.

Common commands include:

• s/pattern/replacement/: Substitutes the first occurrence of pattern with replacement on each line.
• s/pattern/replacement/g: Substitutes all occurrences of pattern with replacement on each line.
• /pattern/command: Executes command if the line matches pattern.
• d: Deletes the current line.
• p: Prints the current line.

• file: Specifies the name of the file(s) to be processed. If omitted, sed reads from standard input.

So to replace a placeholder like <BUILD_TIME> with the date in the version.json at time of image build, I would type: RUN sed -i "s/<BUILD_TIME>/$(date)/g" version.json.

The -i makes the replacement inside the version.json instead of stdout. The $() takes the result of the command inside the parentheses and performs string interpolation, expanding it to:

"s/<BUILD_TIME>/Mon May 15 22:11:01 PDT 2023/g" version.json

date is a CLI utility. It can be customized, for example $(date +%d-%b-%Y) will display 15-May-2023.

For more information on the date utility:

The basic syntax of the date command is as follows:

date [options] [+FORMAT]

Here’s a breakdown of each component:

• options: These are optional flags that modify the behavior of the date command. Some commonly used options include:

-u or --utc: Display or set the date in Coordinated Universal Time (UTC).

-d or --date: Specify a specific date and time rather than the current system

• +FORMAT: This specifies the output format of the date and time. The + character is used to indicate that a format string follows. There are various format specifiers that can be used to represent different components of the date and time, such as %Y for the year, %m for the month, %d for the day, %H for the hour, %M for the minute, %S for the second, and more.

Ex: Display the current date and time in a specific format (e.g., “YYYY-MM-DD HH:MM:SS”):

date +"%Y-%m-%d %H:%M:%S"

PS: One lesson, when the Docker image is being built it may not have access to the K8s secrets in the production environment. So if I try to run RUN sed -i "s/<VERSION>/${APPLICATION_VERSION}/g" version.json, it would not necessarily have access to the APPLICATION_VERSION environment variable K8s secret.