openstack(liberty): devstack之screen

在devstack的stack.sh文件中，可以看到所有配置的service启动方式有两种，根据是否USE_SCREEN进行是在screen window中启动，还是直接起。

默认情况，USE_SCREEN是true的，这个可以在devstack/stackrc中找到，这个文件默认将USE_SCREEN设置为True。 即，所有的service将在screen window中启动。即每一个service对应一个screen window。下面，我就拿nova api这个service的启动代码进行分析这个是如何在SCREEN窗口中启动的。

1. 下面的代码在devstack/lib/nova中。红色部分就是启动的第一级函数调用。

 # start_nova_api() - Start the API process ahead of other things
 function start_nova_api {
     # Get right service port for testing
     local service_port=$NOVA_SERVICE_PORT
     local service_protocol=$NOVA_SERVICE_PROTOCOL
     if is_service_enabled tls-proxy; then
         service_port=$NOVA_SERVICE_PORT_INT
         service_protocol="http"
     fi

     # Hack to set the path for rootwrap
     local old_path=$PATH
     export PATH=$NOVA_BIN_DIR:$PATH

     # If the site is not enabled then we are in a grenade scenario
     local enabled_site_file=$(apache_site_config_for nova-api)
     if [ -f ${enabled_site_file} ] && [ "$NOVA_USE_MOD_WSGI" == "True" ]; then
         enable_apache_site nova-api
         enable_apache_site nova-ec2-api
         restart_apache_server
         tail_log nova-api /var/log/$APACHE_NAME/nova-api.log
         tail_log nova-ec2-api /var/log/$APACHE_NAME/nova-ec2-api.log
     else
         run_process n-api "$NOVA_BIN_DIR/nova-api"
     fi

     echo "Waiting for nova-api to start..."
     if ! wait_for_service $SERVICE_TIMEOUT $service_protocol://$SERVICE_HOST:$service_port; then
         die $LINENO "nova-api did not start"
     fi

     # Start proxies if enabled
     if is_service_enabled tls-proxy; then
         start_tls_proxy '*' $NOVA_SERVICE_PORT $NOVA_SERVICE_HOST $NOVA_SERVICE_PORT_INT &
         start_tls_proxy '*' $EC2_SERVICE_PORT $NOVA_SERVICE_HOST $EC2_SERVICE_PORT_INT &
     fi

     export PATH=$old_path
 }

2. run_process函数，devstack/functions-common.

 # Run a single service under screen or directly
 # If the command includes shell metachatacters (;<>*) it must be run using a shell
 # If an optional group is provided sg will be used to run the
 # command as that group.
 # run_process service "command-line" [group]
 function run_process {
     local service=$1　　　　　　　　　　　　#此处的service就是n-api
     local command="$2"　　　　　　　　　　  #此处的command就是/usr/bin/nova-api,其他的nova相关的指令都在/usr/bin目录下。
     local group=$

     if is_service_enabled $service; then
         if [[ "$USE_SCREEN" = "True" ]]; then　　　　#由于前面分析了USE_SCREEN的值为True，所以，进入到screen_process处理
             screen_process "$service" "$command" "$group"
         else
             # Spawn directly without screen
             _run_process "$service" "$command" "$group" &
         fi
     fi
 }

3. screen_process函数，在devstack/functions-common文件中

 # Helper to launch a process in a named screen
 # Uses globals ``CURRENT_LOG_TIME``, ```LOGDIR``, ``SCREEN_LOGDIR``, `SCREEN_NAME``,
 # ``SERVICE_DIR``, ``USE_SCREEN``
 # screen_process name "command-line" [group]
 # Run a command in a shell in a screen window, if an optional group
 # is provided, use sg to set the group of the command.
 function screen_process {
     local name=$1　　　　　　#要启动的service的名字，此处为n-api
     local command="$2"　　　#要执行的命令，此处为nova-api
     local group=$

     SCREEN_NAME=${SCREEN_NAME:-stack}
     SERVICE_DIR=${SERVICE_DIR:-${DEST}/status}
     USE_SCREEN=$(trueorfalse True USE_SCREEN)

     screen -S $SCREEN_NAME -X screen -t $name　　#启动一个名为stack的session，在这个session中执行screen命令（给当前screen窗口起名为n-api）

     local real_logfile="${LOGDIR}/${name}.log.${CURRENT_LOG_TIME}"
     echo "LOGDIR: $LOGDIR"
     echo "SCREEN_LOGDIR: $SCREEN_LOGDIR"
     echo "log: $real_logfile"
     if [[ -n ${LOGDIR} ]]; then
         screen -S $SCREEN_NAME -p $name -X logfile "$real_logfile"
         screen -S $SCREEN_NAME -p $name -X log on
         ln -sf "$real_logfile" ${LOGDIR}/${name}.log
         if [[ -n ${SCREEN_LOGDIR} ]]; then
             # Drop the backward-compat symlink
             ln -sf "$real_logfile" ${SCREEN_LOGDIR}/screen-${}.log
         fi
     fi

     # sleep to allow bash to be ready to be send the command - we are
     # creating a new window in screen and then sends characters, so if
     # bash isn't running by the time we send the command, nothing
     # happens.  This sleep was added originally to handle gate runs
     # where we needed this to be at least  seconds to pass
     # consistently on slow clouds. Now this is configurable so that we
     # can determine a reasonable value for the local case which should
     # be much smaller.
     sleep ${SCREEN_SLEEP:-}

     NL=`echo -ne '\015'`
     # This fun command does the following:
     # - the passed server command is backgrounded
     # - the pid of the background process is saved in the usual place
     # - the server process is brought back to the foreground
     # - if the server process exits prematurely the fg command errors
     # and a message is written to stdout and the process failure file
     #
     # The pid saved can be used in stop_process() as a process group
     # id to kill off all child processes
     if [[ -n "$group" ]]; then
         command="sg $group '$command'"
     fi

     # Append the process to the screen rc file
     screen_rc "$name" "$command"

     screen -S $SCREEN_NAME -p $name -X stuff "$command & echo \$! >$SERVICE_DIR/$SCREEN_NAME/${name}.pid; fg || echo \"$name failed to start\" | tee \"$SERVICE_DIR/$SCREEN_NAME/${name}.failure\"$NL" #在指定name为nova-api的screen窗口里面执行启动nova-api的服务，并将pid写入对应文件。如果出错，将错误信息tee到指定格式的文件
 }

其中，上面标识红色的部分，是关键指令。通过上面的3个函数的分析，是不是对于devstack的各个service的启动比较熟悉了？或者说有个比较直观感性的认识了？

下面，介绍一下，screen相关的操作。首先看看screen都有些什么命令选项，或者说screen的帮助信息：

 [root@CloudGame shelltest]# screen -help
 Use: screen [-opts] [cmd [args]]
  or: screen -r [host.tty]

 Options:
 -            Use IPv4.
 -            Use IPv6.
 -a            Force all capabilities into each window's termcap.
 -A -[r|R]     Adapt all windows to the new display width & height.
 -c file       Read configuration file instead of '.screenrc'.
 -d (-r)       Detach the elsewhere running screen (and reattach here).
 -dmS name     Start as daemon: Screen session in detached mode.
 -D (-r)       Detach and logout remote (and reattach here).
 -D -RR        Do whatever is needed to get a screen session.
 -e xy         Change command characters.
 -f            Flow control on, -fn = off, -fa = auto.
 -h lines      Set the size of the scrollback history buffer.
 -i            Interrupt output sooner when flow control is on.
 -l            Login mode on (update /var/run/utmp), -ln = off.
 -list         or -ls. Do nothing, just list our SockDir.
 -L            Turn on output logging.
 -m            ignore $STY variable, do create a new screen session.
 -O            Choose optimal output rather than exact vt100 emulation.
 -p window     Preselect the named window if it exists.
 -q            Quiet startup. Exits with non-zero return code if unsuccessful.
 -r            Reattach to a detached screen process.
 -R            Reattach if possible, otherwise start a new session.
 -s shell      Shell to execute rather than $SHELL.
 -S sockname   Name this session <pid>.sockname instead of <pid>.<tty>.<host>.
 -t title      Set title. (window's name).
 -T term       Use term as $TERM for windows, rather than "screen".
 -U            Tell screen to use UTF- encoding.
 -v            Print "Screen version 4.00.03 (FAU) 23-Oct-06".
 -wipe         Do nothing, just clean up SockDir.
 -x            Attach to a not detached screen. (Multi display mode).
 -X            Execute <cmd> as a screen command in the specified session.

在devstack环境下，相关的操作指令主要有：

screen -ls:　　列出当前sockdir。

 [stack@ip---- devstack]$ screen -ls
 There is a screen on:
     .stack    (Detached)
  Socket in /var/run/screen/S-stack.

attach screen：

 [stack@ip---- devstack]$ screen -r .stack

进入了screen后，通过执行ctrl+a + n或者p键，可以实现screen窗口的滚动切换。当前的服务窗口对应于服务名字上有个*号：

 。。。。。。。。。。。。。。。
 --03T00::00Z", "name": "CloudpipeUpdate", "links": [], "namespace": "http://docs.openstack.org/compute/ext/fake_xml", "alias": "os-cloudpipe-update", "description": ""}, {"updated": "2014-12-03T00:00:00Z", "name": "ConfigDrive", "links": [], "namespace": "http://docs.openstack.org/compute/ext/fake_xml", "alias": "os-config-drive", "description": "Config Drive Extension."}, {"updated": "2014-12-03T00:00:00Z", "name": "ConsoleAuthTokens", "links": [], "namespace": "http://docs.openstack.org/compute/ext/fake_xml", "alias": "os-console-auth-tokens", "description": "Console token authentication support."}, {"updated": "2014-12-03T00:00:00Z", "name": "ConsoleOutput", "links": [], "namespace": "http://docs.openstack.org/compute/ext/fake_xml", "alias": "os-console-output", "description": "Console log output support, with tailing ability."}, {"updated": "2014-12-03T00:00:00Z", "name": "Consoles", "links": [], "namespace": "http://docs.openstack.org/compute/ext/fake_xml", "alias": "os-consoles", "description": "Interactive Console support."}, {"updated": "2014-12-03T00:00:00Z", "name": "CreateBackup", "links": [], "namespace": "http://docs.openstack.org/compute/ext/fake_xml", "alias": "os-create-backup", "description": "Create a backup of a server."}, {"updated": "2014-12-03T00:00:00Z", "name": "Createserverext", "links": [], "namespace": "http://docs.openstack.org/compute/ext/fake_xml", "alias": "os-create-server-ext", "description": ""}, {"updated": "2014-12-03T00:00:00Z", "name": "DeferredDelete", "links": [], "namespace": "http://docs.openstack.org/compute/ext/fake_xml", "alias": "os-deferred-delete", "description": "Instance deferred delete."}, {"updated": "2014-12-03T00:00:0
 -- ::05.346975
 :q!

  -$ stack@ip----  $(L) dstat  $(L) key  $(L) key-access   4$(L) horizon*                         (ip----/0.29 0.30 0.74)

在当前窗口执行ctrl+c就是杀掉当前的process。

deatch screen（ctrl+a + d）：

注意，是ctrl+a指令输入后等一会，再输入d键。

进入服务列表模式（ctrl+a+双引号键,双引号键shift+,就是Enter键左边那个）：

在这种模式下，当前所在的process对应的服务名字高亮显示，例如上图中的horizon，此时，点击enter，会进入到常规模式。

其他一些相关的指令，自己以后慢慢摸索吧！