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File Generation

OSImager generates installer configuration files (kickstart, preseed, cloud-init, AutoYaST, Autounattend) at build time by assembling template fragments and applying template substitution.

How It Works

The files Array

Each spec can define a files array. Each entry specifies source fragments to concatenate and a destination filename:

{
  "files": [
    {
      "sources": [
        "rhel/kickstart_8.cfg",
        "rhel/ks-part.sh",
        "rhel/kickstart_end.cfg",
        "rhel/kickstart_post.cfg"
      ],
      "dest": "ks.cfg"
    }
  ]
}

Source paths are relative to the data package's files/ directory (osimager/data/files/, or a ~/.config/osimager/files/ override).

The gen_files() Method

gen_files() (core.py:1300-1334) processes the files array:

  1. Apply do_sub() to the files array itself (resolves >>major<< in source paths like rhel/kickstart_>>major<<.cfg).
  2. For each entry in the array:
  3. Read each source file from the resolved files/ directory (resolve_data_path() -- user override first, then the bundled osimager/data/ baseline).
  4. Concatenate all sources in order into a single string.
  5. Apply do_substr() on the concatenated content — all 12 substitution actions run against the full text.
  6. Write the result to {temp_dir}/{dest}.

Source filenames can contain template markers. For example, RHEL uses rhel/kickstart_>>major<<.cfg which resolves to rhel/kickstart_8.cfg or rhel/kickstart_9.cfg based on the version being built.

When gen_files() Runs

gen_files() is called from run_packer() (core.py:1350), after changing to the data directory and checking required files, but before writing the Packer JSON and executing packer.

Installer Types by Distribution

Kickstart (RHEL Family, ESXi)

Used by: RHEL, CentOS, AlmaLinux, Rocky Linux, Oracle Linux, ESXi

Kickstart files are assembled from versioned fragments in files/rhel/:

  • kickstart_3.cfg through kickstart_10.cfg — version-specific base configs
  • ks-part.sh — partition script
  • kickstart_end.cfg — final section
  • kickstart_post.cfg — post-install script
  • install_vmwtools.sh — VMware tools installation (for VMware/vSphere platforms)

The version_specific entries in the RHEL spec select which kickstart fragment to use based on the major version. For example, RHEL 8.x uses kickstart_8.cfg.

Template variables in kickstart files include:

  • >>name<< — hostname
  • >>domain<< — DNS domain
  • >>ip<<, >>gateway<<, >>netmask<< — network config
  • >>dns1<<, >>dns2<< — DNS servers
  • >>ntp1<< — NTP server
  • 6>images/linux:password<6 — SHA512 root password hash
  • |>images/linux:username<| — root username

ESXi uses its own kickstart files in files/esxi/.

Oracle Linux overrides the RHEL kickstart for version 6 with a custom kickstart_6.cfg in files/oel/.

Preseed (Debian, Ubuntu 18.04)

Used by: Debian 8-9, Ubuntu 18.04

Preseed files are debconf answer files in files/debian/:

  • debian.seed — main preseed template
  • debian.fix — post-install fix script

Template variables include network configuration, DNS, timezone, and password hashes.

Cloud-Init (Debian 10+, Ubuntu 20.04+)

Used by: Debian 10+, Ubuntu 20.04+

Cloud-init configuration in files/ubuntu/ (also used by modern Debian):

  • user-data — cloud-init user-data YAML
  • meta-data — cloud-init meta-data

These are served via a virtual CD with label cidata (set via cd_label def). The boot command references the autoinstall endpoint.

AutoYaST (SLES)

Used by: SLES 12.x, 15.x, 16.x

AutoYaST XML configuration files in files/sles/:

  • autoinst_12.xml — SLES 12 configuration
  • autoinst_15.xml — SLES 15 configuration
  • autoinst_16.xml — SLES 16 configuration

The version_specific entries select the appropriate XML file based on the major version.

Autounattend (Windows)

Used by: Windows, Windows Server

XML answer file in files/windows/:

  • Autounattend.xml — unattended Windows installation configuration

Contains product keys, disk partitioning, locale settings, and administrator credentials via template substitution.

How Files Flow Into the Build

  1. Generated files are written to temp_dir (a temporary directory created per build).
  2. The platform config references these files via cd_files or cd_content in the Packer builder: 
    "cd_files": "%>cd_files<%"
    The cd_files def is typically set to `>>temp_dir<