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How to update a file using the sed command?

i found this commande in internet, to update image tag inside a file, can someone please explain this command please ?

               sh """
                    sed -i 's|image: .*|image: ${ECR_URL}/${ECR_REPO_NAME}:${env.BUILD_NUMBER}|' ${env}/values.yaml
                """
Solution

  • Let's start decomposing this:

    sh """
...
"""

    That's for Groovy scripts to invoke multiline shells. If you're not writing a Groovy script, remove that.

    The rest exhibits the beauty of sed:

    sed -i ... ${env}/values.yaml means the operation between single quotes will be case insensitive and will apply to the file ${env}/values.yaml ; -i is short for --in-place, which means it applies the changes to the file. ${env} is defined somewhere else in the script and is likely the path where the file values.yaml resides.

    Now, for what it does on this file: the s command is for "substitute" and the next character is the separator. It's most often a '/', sometimes a '%', rarely a '|' or a '@'. \7f is a valid separator only used to explain how separators work in sed (but the sed formula must then be in double quotes).

    The syntax of s is s|searched_string|replaced_string|options (replace searched_string by replaced_string in the entire file, once per line at most - using the pipe as the separator). You have no option in your snippet.

    In image: .*, .* means any number of occurrences of any character. Basically, you're searching for a line containing "image: " followed by anything, including nothing: replace everything starting by image: up to the end of the line. Note that the occurrence of image: can be anywhere in a line, including after something else, it will match splash-image: splash.png

    image: ${ECR_URL}/${ECR_REPO_NAME}:${env.BUILD_NUMBER}|' is the replaced_string, as per above. ${ECR_URL}, ${ECR_REPO_NAME} and ${env.BUILDNUMBER} are environment variables probably defined elsewhere in the Groovy script or by the Groovy engine. No witchcraft here, it doesn't even retain the original value of image: (but that you could do with regular expressions and/or sed-scripts).

    For more information, just type man sed.