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110 lines
3.6 KiB
Markdown
110 lines
3.6 KiB
Markdown
# Service Files
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Services that can be started and managed by init are described by service
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description files stored in `/usr/share/init`.
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The init process actually reads from `/etc/init.d` which contains symlinks to
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the actual service files.
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Enabling a service means adding a symlink, disabling means removing a symlink.
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Service descriptions can be parameterized. The arguments are extracted from the
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name of the symlink. Currently only 1 parameter is supported. The argument
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value is separated from the service name by an '@' character in the symlink
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name.
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Below is an annotated example for a simple, service description for a
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generic, parameterized agetty service:
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#
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# The text that init should print out when the status of the
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# service changes.
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#
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# The '%0' is replaced with the first argument extracted from the
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# symlink name.
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#
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description "agetty on %0"
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#
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# How to run the service. 'respawn' means restart the service when it
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# terminates, 'once' means run it only once and continue with other
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# services in the mean while, 'wait' means run it once, but block until
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# it exits.
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#
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type respawn
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#
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# When to start the service. 'boot' means when booting the system. Other
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# options are 'reboot', 'shutdown' and 'ctrlaltdel'. The system always
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# starts into the 'boot' target and then later transitions to one of the
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# others.
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#
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target boot
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#
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# A list of service names that must be started before this service can
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# be run, i.e. this services needs to be started after those.
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#
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# This can only refer to generic names, not specific instances. For
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# instance, you can say "after getty" to make sure a service comes up after
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# all gettys are started, but you cannot specify "after agetty@tty1".
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#
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# Similar to 'after', there is also a 'before' keyword for specifying
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# dependencies.
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#
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after sysinit
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#
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# The 'tty' directive specifies a file to which all I/O of the process is
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# redirected. The specified device file is used as a controlling tty for
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# the process and a new session is created with the service process as
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# session leader.
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#
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# In this example, we derive the controlling tty from the service
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# description argument.
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#
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tty "/dev/%0"
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#
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# The 'exec' directive specifies the command to execute in order to start
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# the service. See in the example below on how to run multiple commands.
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#
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# Again we use the argument to specify what terminal our getty
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# should run on.
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#
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exec agetty %0 linux
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As can be seen in this simple example, each line in a service description is
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made up of a keyword, followed by one or more arguments and terminated by a
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line break.
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Blank lines are ignored and shell-style comments can be used.
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Arguments are separated by space. Quotation marks can be used to treat
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something containing spaces or comment character as a single argument.
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In between quotation marks, C-style escape sequences can be used.
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Argument substitution (arguments derived from the symlink name) can be
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done using a '%' sign, followed by the argument index. A '%' sign can be
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escaped by writing '%%'.
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If a service should sequentially run multiple commands, they can be grouped
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inside braces as can be seen in the following, abbreviated example:
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description "mount /var"
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type wait
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target boot
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before vfs
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exec {
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mount -t tmpfs none /var
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mkdir /var/log -m 0755
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mkdir /var/spool -m 0755
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mkdir /var/lib -m 0755
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mkdir /var/tmp -m 0755
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mount --bind /cfg/preserve/var_lib /var/lib
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}
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