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Ansible File Editing with lineinfile and blockinfile

Sometimes you cannot template a file because you do not own all of it - a package-managed config, or one another team also edits. Three modules edit in place, and each has a distinct job and a distinct failure. The important one is `lineinfile` without a `regexp`, which appends a duplicate instead of changing anything.

Templates, Files and Vault Guide 19 of 45 Intermediate

Written against the versions above. `lineinfile` acts on the **last** line matching `regexp`. If several lines match, the others are left alone - which is why `replace` exists, and why a config with a repeated directive needs care.

One managed node, with a small `/etc/demo.conf` created for the guide and removed at the end.
Server NameIP AddressOSRolesCPURAMHDD
ANS-CTL01192.168.0.36Ubuntu 26.04 LTSAnsible Control Node2 Core3 GB50 GB
ANS-A01192.168.0.37Ubuntu 26.04 LTSManaged Node (group: web)2 Core3 GB50 GB

Before you start

  1. The configuration this guide assumes

  2. A file to edit

  3. lineinfile with a regexp

  4. The duplicate trap

  5. replace changes every match

  6. blockinfile for more than one line

Official sources

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