On Windows, Apache is normally run as a service on Windows NT, or as a console application on Windows 95. This does not apply in its full extend for the Cygwin platform. For details, see running Apache for Windows.
On Unix, the httpd program
is run as a daemon which executes continuously in the
background to handle requests. It is possible to have Apache
invoked by the Internet daemon inetd each time a
connection to the HTTP service is made using the ServerType directive, but
this is not recommended.
If the Port specified in
the configuration file is the default of 80 (or any other port
below 1024), then it is necessary to have root privileges in
order to start Apache, so that it can bind to this privileged
port. Once the server has started and completed a few
preliminary activities such as opening its log files, it will
launch several child processes which do the work of
listening for and answering requests from clients. The main
httpd process continues to run as the root user,
but the child processes run as a less privileged user. This is
controlled by Apache's process creation
directives.
The first thing that httpd does when it is
invoked is to locate and read the configuration file
httpd.conf. The location of this file is set at
compile-time, but it is possible to specify its location at run
time using the -f command-line option as in
/usr/local/apache/bin/httpd -f
/usr/local/apache/conf/httpd.conf
As an alternative to invoking the httpd binary
directly, a shell script called apachectl is provided which
can be used to control the daemon process with simple commands
such as apachectl start and apachectl
stop.
If all goes well during startup, the server will detach from the terminal and the command prompt will return almost immediately. This indicates that the server is up and running. You can then use your browser to connect to the server and view the test page in the DocumentRoot directory and the local copy of the documentation linked from that page.
If Apache suffers a fatal problem during startup, it will
write a message describing the problem either to the console or
to the ErrorLog before
exiting. One of the most common error messages is "Unable
to bind to Port ...". This message is usually caused by
either:
For further trouble-shooting instructions, consult the Apache FAQ.
If you want your server to continue running after a system
reboot, you should add a call to httpd or
apachectl to your system startup files (typically
rc.local or a file in an rc.N
directory). This will start Apache as root. Before doing this
ensure that your server is properly configured for security and
access restrictions. The apachectl script is
designed so that it can often be linked directly as an init
script, but be sure to check the exact requirements of your
system.
Additional information about the command-line options of httpd and apachectl as well as other support programs included with the server is available on the Server and Supporting Programs page. There is also documentation on all the modules included with the Apache distribution and the directives that they provide.