guestfs-erlang(3) Virtualization Support guestfs-erlang(3)

guestfs-erlang - How to use libguestfs from Erlang

{ok, G} = guestfs:create(),
ok = guestfs:add_drive_opts(G, Disk,
                            [{format, "raw"}, {readonly, true}]),
ok = guestfs:launch(G),
[Device] = guestfs:list_devices(G),
ok = guestfs:close(G).

This manual page documents how to call libguestfs from the Erlang programming language. This page just documents the differences from the C API and gives some examples. If you are not familiar with using libguestfs, you also need to read guestfs(3).

The Erlang bindings are implemented using an external program called "erl-guestfs". This program must be on the current PATH, or else you should specify the full path to the program:

{ok, G} = guestfs:create().
{ok, G} = guestfs:create("/path/to/erl-guestfs").

"G" is the libguestfs handle which you should pass to other functions.

To close the handle:

ok = guestfs:close(G).

For functions that take optional arguments, the first arguments are the non-optional ones. The last argument is a list of tuples supplying the remaining optional arguments.

ok = guestfs:add_drive_opts(G, Disk,
                            [{format, "raw"}, {readonly, true}]).

If the last argument would be an empty list, you can also omit it:

ok = guestfs:add_drive_opts(G, Disk).

On success, most functions return a "Result" term (which could be a list, string, tuple etc.). If there is nothing for the function to return, then the atom "ok" is returned.

On error, you would see one of the following tuples:

"{error, Msg, Errno}"
This indicates an ordinary error from the function.

"Msg" is the error message (string) and "Errno" is the Unix error (integer).

"Errno" can be zero. See "guestfs_last_errno" in guestfs(3).

"{unknown, Function}"
This indicates that the function you called is not known. Generally this means you are mixing "erl-guestfs" from another version of libguestfs, which you should not do.

"Function" is the name of the unknown function.

"{unknownarg, Arg}"
This indicates that you called a function with optional arguments, with an unknown argument name.

"Arg" is the name of the unknown argument.

#!/usr/bin/env escript
%%! -smp enable -sname create_disk debug verbose
% Example showing how to create a disk image.

main(_) ->
    Output = "disk.img",

    {ok, G} = guestfs:create(),

    % Create a raw-format sparse disk image, 512 MB in size.
    {ok, File} = file:open(Output, [raw, write, binary]),
    {ok, _} = file:position(File, 512 * 1024 * 1024 - 1),
    ok = file:write(File, " "),
    ok = file:close(File),

    % Set the trace flag so that we can see each libguestfs call.
    ok = guestfs:set_trace(G, true),

    % Attach the disk image to libguestfs.
    ok = guestfs:add_drive_opts(G, Output,
                                [{format, "raw"}, {readonly, false}]),

    % Run the libguestfs back-end.
    ok = guestfs:launch(G),

    % Get the list of devices.  Because we only added one drive
    % above, we expect that this list should contain a single
    % element.
    [Device] = guestfs:list_devices(G),

    % Partition the disk as one single MBR partition.
    ok = guestfs:part_disk(G, Device, "mbr"),

    % Get the list of partitions.  We expect a single element, which
    % is the partition we have just created.
    [Partition] = guestfs:list_partitions(G),

    % Create a filesystem on the partition.
    ok = guestfs:mkfs(G, "ext4", Partition),

    % Now mount the filesystem so that we can add files. *)
    ok = guestfs:mount(G, Partition, "/"),

    % Create some files and directories. *)
    ok = guestfs:touch(G, "/empty"),
    Message = "Hello, world\n",
    ok = guestfs:write(G, "/hello", Message),
    ok = guestfs:mkdir(G, "/foo"),

    % This one uploads the local file /etc/resolv.conf into
    % the disk image.
    ok = guestfs:upload(G, "/etc/resolv.conf", "/foo/resolv.conf"),

    % Because we wrote to the disk and we want to detect write
    % errors, call guestfs:shutdown.  You don't need to do this:
    % guestfs:close will do it implicitly.
    ok = guestfs:shutdown(G),

    % Note also that handles are automatically closed if they are
    % reaped by the garbage collector.  You only need to call close
    % if you want to close the handle right away.
    ok = guestfs:close(G).

#!/usr/bin/env escript
%%! -smp enable -sname inspect_vm debug verbose
% Example showing how to inspect a virtual machine disk.

main([Disk]) ->
    {ok, G} = guestfs:create(),

    % Attach the disk image read-only to libguestfs.
    ok = guestfs:add_drive_opts(G, Disk, [{readonly, true}]),

    % Run the libguestfs back-end.
    ok = guestfs:launch(G),

    % Ask libguestfs to inspect for operating systems.
    case guestfs:inspect_os(G) of
        [] ->
            io:fwrite("inspect_vm: no operating systems found~n"),
        Roots ->
            list_os(G, Roots)

list_os(_, []) ->
list_os(G, [Root|Roots]) ->
    io:fwrite("Root device: ~s~n", [Root]),

    % Print basic information about the operating system.
    Product_name = guestfs:inspect_get_product_name(G, Root),
    io:fwrite("  Product name: ~s~n", [Product_name]),
    Major = guestfs:inspect_get_major_version(G, Root),
    Minor = guestfs:inspect_get_minor_version(G, Root),
    io:fwrite("  Version:      ~w.~w~n", [Major, Minor]),
    Type = guestfs:inspect_get_type(G, Root),
    io:fwrite("  Type:         ~s~n", [Type]),
    Distro = guestfs:inspect_get_distro(G, Root),
    io:fwrite("  Distro:       ~s~n", [Distro]),

    % Mount up the disks, like guestfish -i.
    Mps = sort_mps(guestfs:inspect_get_mountpoints(G, Root)),
    mount_mps(G, Mps),

    % If /etc/ file exists, print up to 3 lines. *)
    Filename = "/etc/",
    Is_file = guestfs:is_file(G, Filename),
    if Is_file ->
            io:fwrite("--- ~s ---~n", [Filename]),
            Lines = guestfs:head_n(G, 3, Filename),
       true -> ok

    % Unmount everything.
    ok = guestfs:umount_all(G),

    list_os(G, Roots).

% Sort keys by length, shortest first, so that we end up
% mounting the filesystems in the correct order.
sort_mps(Mps) ->
    Cmp = fun ({A,_}, {B,_}) ->
                  length(A) =< length(B) end,
    lists:sort(Cmp, Mps).

mount_mps(_, []) ->
mount_mps(G, [{Mp, Dev}|Mps]) ->
    case guestfs:mount_ro(G, Dev, Mp) of
        ok -> ok;
        { error, Msg, _ } ->
            io:fwrite("~s (ignored)~n", [Msg])
    mount_mps(G, Mps).

write_lines([]) ->
write_lines([Line|Lines]) ->
    io:fwrite("~s~n", [Line]),

guestfs(3), guestfs-examples(3), guestfs-gobject(3), guestfs-golang(3), guestfs-java(3), guestfs-lua(3), guestfs-ocaml(3), guestfs-perl(3), guestfs-python(3), guestfs-recipes(1), guestfs-ruby(3),

Richard W.M. Jones ("rjones at redhat dot com")

Copyright (C) 2011-2023 Red Hat Inc.

This manual page contains examples which we hope you will use in your programs. The examples may be freely copied, modified and distributed for any purpose without any restrictions.

To get a list of bugs against libguestfs, use this link:

To report a new bug against libguestfs, use this link:

When reporting a bug, please supply:

  • The version of libguestfs.
  • Where you got libguestfs (eg. which Linux distro, compiled from source, etc)
  • Describe the bug accurately and give a way to reproduce it.
  • Run libguestfs-test-tool(1) and paste the complete, unedited output into the bug report.
2024-01-05 libguestfs-1.52.0