s6-linux-init (package)

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s6-linux-init is a package that provides a set of tools to help create an s6-based init system, with optional s6-rc support, on top of the Linux kernel. The package's documentation is provided in HTML format, and can be read on a text user interface using for example www-client/links.

Installation

USE flags

USE flags for sys-apps/s6-linux-init Generates an init binary for s6-based init systems

sysv-utils Install sysvinit compatibility symlinks for telinit, halt, poweroff, reboot, and shutdown
Data provided by the Gentoo Package Database · Last update: 2024-01-17 10:38 More information about USE flags

Important
The sysv-utils USE flag is set by default for this package, causing a blocker with sysvinit. Because s6-based init systems are not officially supported by Gentoo yet, it is recommended to unset this flag in /etc/portage/package.use.

Emerge

root #emerge --ask sys-apps/s6-linux-init
Important
s6-linux-init is currently on the testing branch. Users with systems on the stable branch will need to add the package to /etc/portage/package.accept_keywords to be able to install it, and also add dev-libs/skalibs, as well as its runtime dependencies: dev-lang/execline and sys-apps/s6. While it is generally not advised to mix packages of stable and testing branches, the skarnet.org software stack only depends on the libc, so in this case it should be safe.

Configuration

Files

  • /etc/s6-linux-init/current — Default base directory of s6-linux-init and s6-linux-init-shutdownd.
  • /etc/shutdown.allow — List of account names used by s6-linux-init-shutdown when invoked with an -a option, to decide whether the machine shutdown request should be allowed to proceed or not.

Usage

The package provides the s6-linux-init-maker program, which helps create several components of an s6-based init system, programs for booting and shutting down the machine when such components are used, and other support programs. It also provides tools for implementing runlevel-like functionality similar to that of sysvinit.

s6-linux-init-maker

The s6-linux-init-maker program must be run as root, so that it can create files and directories with appropriate owner, group and permissions, and must be invoked with a pathname as its argument. It creates a directory at the specified pathname, which must not exist beforehand, containing several init system components:

  • A subdirectory named run-image, which contains:
    • An s6 scan directory image.
    • An empty directory named uncaught-logs.
  • A subdirectory named env, that might be empty.
  • A subdirectory named scripts, containing shell scripts named rc.init, rc.shutdown and runlevel. For s6-linux-init version 1.0.3.0 and later, this subdirectory also a shell script named rc.shutdown.final.
  • A subdirectory named bin, containing sysvinit compatibility execline scripts.

To use these components to make an init system, they must be copied to appropriate filesystem locations. The init script contained in the bin subdirectory can be e.g. renamed and copied to a directory that matches an init= kernel parameter. For example, if the bootloader is configured to pass an init=/sbin/s6-imkr-init parameter to the kernel, then the init file generated by s6-linux-init-maker must be copied to /sbin and renamed as s6-imkr-init. The run-image, env and scripts subdirectories must be copied to the s6-linux-init program's base directory. For the sysvinit compatibility scripts, see here. Care must be taken to ensure that owner, group and permissions of files and subdirectories are preserved when cpoying them to their destination place, e.g. by using GNU or BusyBox cp with the -a option, or the s6-hiercopy program from s6-portable-utils (sys-apps/s6-portable-utils).

The generated init system needs at least three files that must be supplied by the user to complete it: a stage2 init, a shutdown file and a runlevel changer file, all of which must be executable. The stage2 init is used by the s6-linux-init program, the shutdown file is used by the shutdown daemon, and the runlevel changer file is used by the runlevel changer service. For s6-linux-init version 1.0.3.0 and later, a fourth executable file can be supplied by the user if needed, the final shutdown file. It is also used by the shutdown daemon if present.

Note
s6-linux-init-maker from s6-linux-init version 1.0.3.0 failed if the final shutdown file was not provided. The one from s6-linux-init version 1.0.4.0 and later doesn't have this problem.

s6-linux-init-maker accepts a set of options that customize all init system components. For the full description of s6-linux-init-maker, please consult the HTML documentation in the package's /usr/share/doc subdirectory. A summary of each init system component and some of s6-linux-init-maker's options that customize it follows.

The stage1 init

The init script contained in the bin subdirectory of the directory created by s6-linux-init-maker is a stage1 init. s6-linux-init-maker's stage1 init runs as process 1 during the machine's boot sequence, and is an execline script that wraps an invocation of a program supplied by the package, also named s6-linux-init.

s6-linux-init expects to access the contents of a certain base directory, and performs these actions:

  • It sets the PATH environment variable to a specified or default value.
  • It sets the working directory to /.
  • It sets the file mode creation mask to a specified or default value using a POSIX umask() call.
  • It makes itself the leader of a new process group using a POSIX setpgid(0, 0) call.
  • It unmounts any filesystem that might be mounted at /run, and mounts a read-write tmpfs with options mode=0755, nodev and nosuid.
    • If an -n option was passed to s6-linux-init, it assumes that a tmpfs is already mounted at /run, and remounts it with the previously mentioned options. This is useful when an initramfs is used to mount the tmpfs, and writes data to it that must be preserved.
    • If an -N option was passed to s6-linux-init, it assumes that a tmpfs is already mounted at /run and does nothing. This is useful when an initramfs is used to mount the tmpfs, and it should remain as the (read-write) rootfs of the machine.
  • If a -d option followed by a pathname was passed to s6-linux-init, it mounts a devtmpfs at the supplied pathname.
  • If an -s option followed by a pathname was passed to s6-linux-init, it dumps the enviroment set up by the kernel to a specified enviroment store. The environment store is an environment directory suitable for s6-envdir.
  • It copies the run-image subdirectory of the base directory to the tmpfs, preserving owner, group and permissions of files and subdirectories.
  • It unsets all enviroment variables except PATH.
  • It adjusts the enviroment according to the env subdirectory of the base directory, as if by an s6-envdir invocation.
  • It redirects its standard input to /dev/null.
  • It redirects its standard output and error to the catch-all logger's FIFO.
    • (version 1.0.4.0 and later) If a -B option was passed to s6-linux-init, it will instead create a pipe, close its reading end, and pass the file descriptor number of its writing end to s6-svscan after a -d (notify readiness) option. The child process that will execute the stage2 init is spawned before closing the reading end of the pipe; the process will therefore inherit the corresponding file descriptor, so that it can use it to wait for s6-svscan's readiness notification. This allows setting up the resulting init system without a catch-all logger.
  • It spawns the stage2 init as a session leader using a POSIX setsid() call.
  • (version 1.0.3.0 and later) It uses a Linux reboot() system call with an RB_DISABLE_CAD command, to ask the kernel to send a SIGINT signal to process 1 when the Ctrl+Alt+Del key combination is pressed.
  • (version 1.0.4.0 and later) It uses a Linux ioctl() system call with a KDSIGACCEPT request, to ask the kernel to send a SIGWINCH signal to process 1 when the key combination that corresponds to keyboard signal is pressed (usually configured to be Alt+Up Arrow).
  • It replaces itself with s6-svscan using a POSIX execve() call, passing it the -s (divert signals) and -t 0 (do not periodically scan the scan directory) options, and passing /run/service as the pathname of the scan directory.

If s6-linux-init finds that its process ID is not 1, it replaces itself with s6-linux-init-telinit.

Aspects of the stage1 init that can be customized via s6-linux-init-maker options are:

  • The pathname of s6-linux-init's base directory: -c. Default: /etc/s6-linux-init/current.
  • The value the PATH environment variable is initially set to: -p. This value affects all programs invoked by the stage2 init, and s6-svscan, and will be the initial value of PATH for all the init system's supervision tree processes. Default: /usr/bin:/bin.
  • The initial value of the file mode creation mask for all the starting processes: -m. Default: 0022.
  • Whether a devtmpfs filesystem will be mounted: -d. If this option is not specified, no devtmpfs is mounted by s6-linux-init. Otherwise, it must be followed by the pathname of the filesystem's mountpoint.
  • What to do about the read-write tmpfs: -N or -n.
  • (version 1.0.4.0 and later) Whether a catch-all logger will be used: -B.
  • The pathname of a read-write directory for storing the kernel's environment variables (i.e. the environment store): -s. If this option is not specified, the environment set up by the kernel is discarded before s6-linux-init replaces itself with s6-svscan.

All these options, if specified, are passed as-is to s6-linux-init (via the generated init script). s6-linux-init-maker checks that the pathname specified after options -c, -d and -s is absolute.

The stage2 init

The stage2 init must be provided by the user, must be an executable file named rc.init, and located in the scripts subdirectory of s6-linux-init's base directory. Unless s6-linux-init has been invoked with the -B option, rc.init runs with its standard output and error redirected to the catch-all logger's FIFO, i.e. all messages will be logged to the logging directory. Otherwise, its standard output and error are redirected to the machine's console.

rc.init is invoked with at least one argument: an initial runlevel name. All arguments of the stage1 init are also passed verbatim to rc.init, after the initial runlevel name. s6-linux-init scans its argument list, and if one of the arguments is the word default, 2, 3, 4 or 5, then it is interpreted as the initial runlevel name and passed to rc.init as the first argument —which means that it will be repeated later in the argument list—. Otherwise, if a -D option followed by a runlevel name was passed to s6-linux-init, then it is interpreted as the initial runlevel name and passed to rc.init as the first argument. Otherwise, the word default is passed as the first argument.

Note
Every word in the space-separated kernel command line after a "--" (double hyphen) marker is passed as an argument to process 1, as is every word before the marker, if present, that does not contain an equals sign ('=') or dot ('.'), and that is not recognized as a kernel parameter. Therefore, arguments can be passed to the stage1 init by specifying them in the kernel's command line (e.g. using the bootloader's available mechanisms).

s6-linux-init-maker copies the file named rc.init from a directory that can be specified with the -f option, to the scripts subdirectory of its generated directory as the stage2 init. If no -f option is specified, directory /etc/s6-linux-init/skel is used on Gentoo. The package installs an example stage2 init in /etc/s6-linux-init/skel, that is a shell script containing only comments.

Aspects of the stage2 init that can be customized via s6-linux-init-maker options are:

  • The name of the initial rulnevel, if it was not specified in the kernel command line: -D. Default: the word default.

This option, if specified, is passed as-is to s6-linux-init (via the generated init script).

The scan directory image

The init system's scan directory image is the run-image/service subdirectory of of s6-linux-init's base directory, which is copied by s6-linux-init to the read-write tmpfs mounted at /run. This copy then becomes s6-svscan's scan directory when it starts running as process 1. Therefore, the init system's scan directory's absolute pathname will be /run/service.

The scan directory image must contain at least these service directories:

  • One for the shutdown daemon, which must be named s6-linux-init-shutdownd.
  • One for the catch-all logger, if one is used. For s6-linux-init version 1.0.4.0, if present, it must be named s6-svscan-log.
  • One for the runlevel changer service, if the s6-linux-init-telinit program is going to be used. If present, it must be named s6-linux-init-runleveld.

The scan directory image generated by s6-linux-init-maker satisfies this requirements, and can optionally contain a service directory for an early getty service, named s6-linux-init-early-getty. This service, if present, allows logging in to the machine from its console as early as possible. The scan directory image also contains s6-svscan diverted signal handlers, as well as the .s6-svscan/crash and .s6-svscan/finish files used for s6-svscan's finish procedure. These two files simply print an "s6-svscan crashed. Rebooting." or "s6-svscan exited. Rebooting." message, respectively, and perform a forced reboot of the machine with an s6-linux-init-hpr -r -f command.

Warning
This means that rc.shutdown, s6-linux-init-umountall and rc.shutdown.final won't be executed, which might result in s6-rc oneshots not stopping properly and filesystems not being cleanly unmounted. Therefore, the s6-svscanctl program should not be run with /run/service and an option that makes s6-svscan initiate its finish procedure.

Aspects of the scan directory image that can be customized via s6-linux-init-maker options are:

  • Whether an early getty service will be present: -G. If this option is not specified, there will be no early getty service, and any getty setup must be done by the user-provided stage2 init. Otherwise, it must be followed by a suitable invocation (program name and arguments) of some getty implementation, such as, for example, -G '/sbin/agetty 38400 tty1 linux' for Gentoo. The generated service directory will contain a run execline script with the specified invocation.

The catch-all logger

The catch-all logger generated by s6-linux-init-maker is an s6-log process that, on Gentoo, logs to (automatically rotated) logging directory /run/uncaught-logs. This means that the logdir is placed in the read-write tmpfs mounted by s6-linux-init. The standard output of the s6-log process is redirected to /dev/null, and s6-svscan redirects s6-log's standard error to the machine's console, using an -X option (console holder) passed by s6-linux-init.

The run file of the logger's s6 service directory opens the corresponding FIFO for reading, and then invokes s6-log with the -p option, so that it cannot be killed by the SIGTERM signal, with a -d 3 option, which makes it signal readiness to its s6-supervise parent process using file descriptor 3 for the notification channel, and with the -b (blocking) option, which makes s6-log stop reading from the FIFO while it has unflushed buffers. This avoids unbound memory use if there is a lot of output to write.

For s6-linux-init version 1.0.4.0 and later, the catch-all logger is optional. If one is not used, all messages that would be logged are printed to the machine's console instead.

Aspects of the catch-all logger that can be customized via s6-linux-init-maker options are:

  • (version 1.0.4.0 and later) Whether a catch-all logger will be used: -B. If this option is specifed, no s6 service directory is generated for the catch-all logger in the scan directory image.
  • The s6-log process' effective user and group: -u. Default: s6-log runs as root. If this option is specified, it must be followed by an account database username. The s6-setuidgid program from the s6 package is used in the generated service directory's run script to change to a non-privileged user.
  • How logged lines of text are timestamped: -t. Default: timestamps in external TAI64N format are used, as generated by s6-log's t directive.
  • Whether the catch-all logger also prints logged lines to the machine's console: -1 (the number one). This is generally useful to debug a system at a glance, but if a failing program keeps sending error messages, it may interfere with comfortable usage of kernel virtual terminals. This can be avoided by not using /dev/tty1 for any getty process. If this option is used, the standard output of the s6-log process is redirected to the console instead of /dev/null, and a "1" directive is used for printing logged lines.

s6-linux-init-maker creates an uncaught-logs subdirectory in the run-image subdirectory of its generated directory, with an owner and group that are suitable for s6-log's effective user and group, and appropriate permissions.

The runlevel changer service

The runlevel changer service generated by s6-linux-init-maker is named s6-linux-init-runleveld, and is an s6-sudod process that executes a user-provided file. The s6-sudoc - s6-sudod mechanism allows executing the file in a controlled, reproducible environment, i.e. the one set up for the init system's supervision tree processes. This is similar to the s6rc-oneshot-runner service that s6-rc uses for oneshots. The file runs with its standard input and output redirected to /dev/null, and its standard error logged by the catch-all logger, if one is used.

This service is used by the s6-linux-init-telinit program.

Aspects of the runlevel changer service that can be customized via s6-linux-init-maker options are:

  • The pathname of the base directory, where the user-provided script is expected: -c. Default: /etc/s6-linux-init/current. It is used to construct the argument list for s6-sudod.

The shutdown service and daemon

The shutdown service generated by s6-linux-init-maker is a supervised s6-linux-init-shutdownd process. It is used by the s6-linux-init-shutdown and s6-linux-init-hpr programs. The s6-linux-init-sutdownd program is a shutdown daemon that takes care of performing the shutdown sequence. It runs as a long-lived process, expects to access the contents of a certain base directory, and performs these actions:

  • It reads from a control FIFO, waiting for either machine shutdown requests, or shutdown cancellation requests. A shutdown request consists of an action that can be halt, poweroff, reboot or the special action S, a time value that specifies a delay until actual shutdown, and another optional time value that specifies a grace period.
  • If a shutdown request is received, it waits until the specified shutdown delay expires, while it keeps reading from the control FIFO.
  • If a shutdown cancellation request is received, it cancels any pending machine shutdown that has been previously requested.
  • If the shutdown delay expires and the shutdown request has not been canceled, it starts the shutdown sequence by executing a shutdown file as a child process, and waits until it terminates. The shutdown file is an executable file that must be provided by the user, must be named rc.shutdown, and must be located in the scripts subdirectory of the base directory. Because s6-linux-init-sutdownd is part of the init system's supervision tree, rc.shutdown runs with the same environment that was set up for s6-linux-init-sutdownd, with its standard input redirected to /dev/null, and its standard output and error logged by the catch-all logger, if one is used.
  • If the action specified in the shutdown request was S, it does nothing and starts reading again from the control FIFO. This allows the implementation of a behaviour that is similar to sysvinit's single user mode. Otherwise:
  • It redirects its standard output and error to /dev/console, i.e. the machine's console.
  • It creates an execline script named stage 4 in its s6 service directory.
  • It stops all processes from the supervision tree except the catch-all logger, if one is used, and itself, by renaming the corresponding service directories with a name that starts with a dot ('.'), and performing the equivalent of an s6-svc -dx command. The renaming of the servicedirs prevents s6-svscan from restarting their corresponding s6-supervise process after they exit.
  • It syncs all disks, flushing all the dirty system buffers, using a POSIX sync() call.
  • It kills all remaining processes by sending them a SIGTERM signal followed by a SIGCONT signal, using POSIX kill(-1, SIGTERM) and kill(-1, SIGCONT) calls, and waits for the expiry of the grace period specified in the shutdown request.
    • If the shutdown request specified a duration for the grace period, that value is used.
    • Otherwise, if a -g option followed by time value in milliseconds was passed to s6-linux-init-shutdownd, the specified value is used as the duration of the grace period.
    • Otherwise, the duration of the grace period is 3 seconds.
  • It kills all remaining processes by sending them a SIGKILL signal using a kill(-1, SIGKILL) call. Then, if s6-linux-init-sutdownd did not get killed by the signal, it exits.
  • The shutdown service's corresponding s6-supervise process restarts s6-linux-init-sutdownd.
  • s6-linux-init-sutdownd notes that there is a file named stage 4 in its service directory, redirects its standard output and error to /dev/console, and replaces itself with it using a POSIX execve() call.
  • The execline script unmounts all mounted filesystem according to /proc/mounts, and remounts the rootfs read-only, using auxiliary program s6-linux-init-umountall.
  • (version 1.0.3.0 and later) The execline script executes a final shutdown file as a child process, and waits until it terminates. The final shutdown file is an executable file that must be provided by the user, must named rc.shutdown.final, and must be located in the scripts subdirectory of the base directory. Because s6-linux-init-sutdownd is part of the init system's supervision tree, rc.shutdown.final runs with the same environment that was set up for s6-linux-init-sutdownd, with its standard input redirected to /dev/null, and its standard output and error redirected to the machine's console.
  • The execline script invokes the s6-linux-init-hpr program with an -f option (force), and an -h, -p or -r option, according to the action specified in the shutdown request (i.e. halt, poweroff or reboot).

s6-linux-init-sutdownd's control FIFO must be located in the s6 service directory, and must be named fifo, owned by root, and have permissions 0600 (i.e. the output of ls -l should display prw-------) to restrict unprivileged users from shutting down the machine. The service directory generated by s6-linux-init-maker satisfies all these conditions.

For further information about s6-linux-init-shutdownd, please consult the HTML documentation in the package's /usr/share/doc subdirectory. s6-linux-init-maker copies the files named rc.shutdown and rc.shutdown.final from a directory that can be specified with the -f option, to the scripts subdirectory of its generated directory as the shutdown and final shutdown files. If no -f option is specified, directory /etc/s6-linux-init/skel is used on Gentoo. The package installs example shutdown and final shutdown files in /etc/s6-linux-init/skel, that are shell scripts containing only comments.

Aspects of the shutdown service that can be customized via s6-linux-init-maker options are:

  • The pathname of s6-linux-shutdownd's base directory: -c. Default: /etc/s6-linux-init/current.
  • (version 1.0.4.0 and later) Whether a catch-all logger is being used: -B. If this option is specified, s6-linux-shutdownd does not give special treatment to the service directory named s6-svscan-log during the shutdown sequence. Otherwise, this directory is assumed to be that of the catch-all logger, and s6-linux-init-shutdownd does not attempt to rename it and stop the service during the shutdown sequence.
  • The default grace period: -q. Default: 3 seconds. If this option is specified, it must be followed by a time value in milliseconds. Any value greater than 5 minutes is treated as 5000.

The -c and -B options, if specified, are passed as-is to s6-linux-shutdownd (via the generated run script in the s6 service directory). The -q option is converted to a -g option with the same value and also passed to s6-linux-shutdownd. s6-linux-init-maker checks that the pathname specified after option -c is absolute.

The diverted signal handlers

The s6-svscan diverted signal handlers generated by s6-linux-init-maker are execline scripts that make the init system react to signals sent to process 1. The effect of diverted signals is:

  • SIGHUP: does nothing.
  • SIGQUIT: does nothing.
  • SIGINT: reboots the machine using an s6-linux-init-shutdown -a -r -- now command.
  • SIGTERM: does nothing.
  • SIGUSR1: powers off the machine using an s6-linux-init-shutdown -a -p -- now command.
  • SIGUSR2: halts the machine using an s6-linux-init-shutdown -a -h -- now command.
  • (version 1.0.4.0 and later) SIGPWR: powers off the machine using an s6-linux-init-shutdown -a -p -- now command.
  • (version 1.0.4.0 and later) SIGWINCH: does nothing.
Note
This means that shutting down by sending signals to process 1 will take /etc/shutdown.allow into account, if present.

Runlevels

s6-linux-init allows the implementation of runlevel-like functionality similar to that of sysvinit. An s6-linux-init runlevel normally represents a service set, and entering a runlevel normally means to start all services in the corresponding set, and stopping all services not in it. Each runlevel has an alphanumerical identifier. The package uses two components to implement runlevels: the s6-linux-init-telinit program, and the runlevel changer service.

What happens exactly when a runlevel is entered is defined by a runlevel changer file that must be provided by the user. The runlevel changer service generated by s6-linux-init-maker expects this file to be an executable file named runlevel, and located in the scripts subdirectory of the service's base directory. It is normally an execline or shell script that relies on some service manager to actually perform the necessary service transitions.

The s6-linux-init-telinit program accepts a runlevel identifier as its argument, and uses an s6-sudo -e -T 3600000 command to connect to the runlevel changer service's s6-sudod socket (absolute pathname /run/service/s6-linux-init-runleveld/s on Gentoo), and pass the identifier to it. The -e option avoids passing variables from s6-linux-init-telinit's enviromnent to the the runlevel script, and the -T option prevents it from running for more than an hour. The service then executes the runlevel changer file with the received identifier as its argument.

For compatibility with sysvinit, if s6-linux-init-telinit is invoked with 0 or 6 as the runlevel identifier, after passing it to the runlevel changer and waiting for the changer file to terminate, it uses an s6-linux-init-hpr -h or s6-linux-init-hpr -p command, respectively, to initiate machine shutdown.

Integration of s6-linux-init-telinit with OpenRC can be achieved by having the script execute the openrc program when invoked. In this case, s6-linux-init runlevels are OpenRC runlevels.

For further information about s6-linux-init-telinit, please consult the HTML documentation in the package's /usr/share/doc subdirectory.

s6-linux-init-maker copies the file named runlevel from a directory that can be specified with the -f option, to the scripts subdirectory of its generated directory as the runlevel changer file. If no -f option is specified, directory /etc/s6-linux-init/skel is used on Gentoo. The package installs an example runlevel changer file in /etc/s6-linux-init/skel, that is a shell script containing only comments that illustrate integration with both OpenRC and s6-rc.

s6-rc support

See here.

Shutdown and reboot

s6-linux-init provides the s6-linux-init-shutdown and s6-linux-init-hpr programs, that can be used in combination with the init system components generated by s6-linux-init-maker to shut down the machine. The former conforms to the LSB shutdown interface[1], modeled after sysvinit's shutdown, and the latter provide equivalent functionality to that of sysvinit's halt, poweroff and reboot.

s6-linux-init-shutdown

s6-linux-init-shutdown accepts a time specification, an optional message to be shown to logged in users, and the -h (halt after shutdown), -r (reboot after shutdown), -c (cancel shutdown), -k (do not really shutdown; only warn), -t (grace period between SIGTERM and SIGKILL) and -a (access control) options, with the same meaning as for sysvinit's shutdown. For compatibility with the LSB, it accepts -f (ask the rc subsystem to skip checking filesystems with fsck after next boot) and -F (ask the rc subsystem to make fsck check filesystems after next boot, even if they are marked clean) options, that it ignores. It also accepts -H (halt), -p and -P (poweroff) options, that combine with -h according to the following table:

s6-linux-init-shutdown option Results in Comments
-h Halting Note that Gentoo's patched shutdown powers off in this case.
-H Halting Same as Gentoo's patched shutdown
-P Powering off Same as Gentoo's patched shutdown
-h -H Halting Compatible with upstream sysvinit
-h -P Powering off Compatible with upstream sysvinit


Unless it was invoked with the -k option, s6-linux-init-shutdown connects to the shutdown service's FIFO and sends a shutdown request, or shutdown cancellation request if the -c option has been specified, to s6-linux-init-shutdownd. If no -h, -p, -H or -H option was specified, it sends a special 'S' shutdown request, that only makes s6-linux-init-shutdownd execute the shutdown script, but not actually shut down the machine. This allows the implementation of a behaviour that is similar to sysvinit's single user mode.

If it was invoked with the -a option, s6-linux-init-shutdown performs access control using file /etc/shutdown.allow just like sysvinit's shutdown. POSIX setutxent(), getutxent() and endutxent() calls are used to read user process records (i.e. struct utmpx objects with ut_type == USER_PROCESS) from the user accounting database (/var/run/utmp for GNU libc), and their ut_user user field is matched against account usernames in /etc/shutdown.allow. If the duration of the grace time specified with the -t option is greater that 5 seconds, it is capped to that value.

For further information about s6-linux-init-shutdown, please consult the HTML documentation in the package's /usr/share/doc subdirectory.

s6-linux-init-hpr

s6-linux-init-hpr must be invoked either with an -h option, a -p option, or an -r option, to halt, poweroff, or reboot the machine, respectively. Without other options, it first writes a shutdown record to the user accounting log file (/var/log/wtmp for GNU libc), using the libc's updwtmpx() function. Then, it prints a warning about the system going down to logged in users. And finally, it sends a shutdown request to s6-linux-init-shutdownd, specifying that machine shutdown should be initiated immediately. That is, s6-linux-init-hpr -h is equivalent to s6-linux-init-shutdown -h now, s6-linux-init-hpr -p, to s6-linux-init-shutdown -p now, and s6-linux-init-hpr -r, to s6-linux-init-shutdown -r now.

The shutdown record is a runlevel record (i.e. a struct utmpx object with ut_type == RUN_LVL) that has ut_user set to "shutdown" (or "reboot" if s6-linux-init-hpr was invoked with the -r option), ut_id set to the empty string, ut_line set to "~", ut_pid set to s6-linux-init-hpr's process ID, ut_session set to s6-linux-init-hpr's session ID (as returned by a POSIX getsid(0) call), ut_tv set to the current time, and ut_host set to the name returned by the POSIX gethostname() call (or the empty string if the call fails).

If s6-linux-init-hpr is invoked with a -W (capital 'w') option, it does not print the warning to logged in users. And, just like sysvinit's halt, poweroff and reboot, if it is invoked with a -w (small 'w') option, it only writes the shutdown record to the user accounting log file, without actually shutting down the machine, and if it is invoked with a -d option, no shutdown record is written to the user accounting log file.

If s6-linux-init-hpr is invoked with an -f (force) option, it syncs all disks, flushing all the dirty system buffers, using the POSIX sync() call, and directly performs the shutdown action requested by the -h, -p or -r option, using the Linux reboot() system call. No shutdown record is written to the user accounting log file, no warning is printed a to logged in users, and options -W, -w and -d are ignored. If s6-linux-init-hpr is also invoked with an -n option, the syncing of disks is skipped.

For further information about s6-linux-init-hpr, please consult the HTML documentation in the package's /usr/share/doc subdirectory.

sysvinit compatibility scripts

s6-linux-init-maker creates the following execline scripts in the bin subdirectory of its generated directory:

  • init, which calls the s6-linux-init program with at least the -c (base directory) and -m (file mode creation mask) options.
  • halt, which calls the s6-linux-init-hpr program with the -h option (halt) and all the supplied arguments.
  • poweroff, which calls the s6-linux-init-hpr program with the -p option (poweroff) and all the supplied arguments.
  • reboot, which calls the s6-linux-init-hpr program with the -r option (reboot) and all the supplied arguments.
  • telinit, which calls the s6-linux-init-telinit program with all the supplied arguments.
  • shutdown, which calls the s6-linux-init-shutdown program with all the supplied arguments.

These compatibility scripts are provided as replacements of the sysvinit programs of the same name in pure s6-based setups, and are installed by the Gentoo ebuild in /sbin if the sysv-utils USE flag is set. On Gentoo, however, this does not allow having both sys-apps/sysvinit and sys-apps/s6-linux-init installed, and therefore, switching between init systems with a reboot. In fact, installing sys-apps/s6-linux-init with this USE flag set on a machine with sys-apps/openrc installed, or with sys-apps/systemd with sysv-utils also set, results in a blocker. So generally sys-apps/s6-linux-init should be installed with this USE flag unset. The bin subdirectory of s6-linux-init-maker's generated directory could be copied to /etc/s6-linux-init/current using, for example, cp -a, and then scripts could be run using absolute pathname /etc/s6-linux-init/current/bin/${script_name} (replacing ${script_name} with the actual the script name).

Support programs

s6-linux-init provides two support programs for init system components created by s6-linux-init-maker.

The s6-linux-init-echo program is used by s6-svscan's finish and crash files for printing messages, and is an exact duplicate of s6-echo from sys-apps/s6-portable-utils. Its inclusion in s6-linux-init avoids creating a runtime dependency on that package.

The s6-linux-init-umountall program is used by the stage 4 execline script created by the shutdown daemon, and it unmounts all filesystems, processing /proc/mounts in reverse order, starting with the most recently mounted partition and ending with the rootfs. It behaves in a similar way to that of the s6-umount program from sys-apps/s6-linux-utils when the -a option is specified. Its inclusion in s6-linux-init again avoids creating a runtime dependency on that package. For s6-linux-init version 1.0.3.0 and later, s6-linux-init-umountall skips the unmounting of the first instance of a devtmpfs, sysfs and proc filesystem it finds in /proc/mounts, which would usually correspond to /dev, /sys and /proc, respectively, so that they are available for the final shutdown file, rc.shutdown.final.

Note
Before s6-linux-init version 1.0.4.0, s6-linux-init-umountall would attempt the unmounting of the tmpfs mounted by s6-linux-init, which would fail with a warning, but would not otherwise affect the shutdown sequence.

Historical notes

Before version 1.0.0.0, the s6-linux-init package could be used to create an s6 and s6-rc-based init system with a different design, see here for a summary. s6-linux-init-maker accepted a different set of options that the one described in this article, and its generated directory had a different structure. That design has been deprecated by the release of version 1.0.0.0, and became unsupported with upstream's release of version 2.10.0.0 of the s6 package.

Removal

Unmerge

root #emerge --ask --depclean --verbose sys-apps/s6-linux-init

If s6-linux-init's programs and components generated by s6-linux-init-maker are being used as the init system, an alternative one must be installed in parallel, and the machine rebooted to use it (possibly by reconfiguring the bootloader), before the package is removed, or otherwise the machine will become unbootable.

See also

External resources

  • Adélie Linux, a distribution of Linux and musl that supports using s6-linux-init + OpenRC as its init system, as an alternative to sysvinit + OpenRC (currently in beta stage).

References

  1. ↑ Linux Standard Base Core Specification 5.0.0, Generic Part, Chapter 17, "Commands and Utilities", 17.2, "Command Behavior", shutdown. Retrieved on September 9th, 2019.
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