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。
1 # start_nova_api() - Start the API process ahead of other things 2 function start_nova_api { 3 # Get right service port for testing 4 local service_port=$NOVA_SERVICE_PORT 5 local service_protocol=$NOVA_SERVICE_PROTOCOL 6 if is_service_enabled tls-proxy; then 7 service_port=$NOVA_SERVICE_PORT_INT 8 service_protocol="http" 9 fi 10 11 # Hack to set the path for rootwrap 12 local old_path=$PATH 13 export PATH=$NOVA_BIN_DIR:$PATH 14 15 # If the site is not enabled then we are in a grenade scenario 16 local enabled_site_file=$(apache_site_config_for nova-api) 17 if [ -f ${enabled_site_file} ] && [ "$NOVA_USE_MOD_WSGI" == "True" ]; then 18 enable_apache_site nova-api 19 enable_apache_site nova-ec2-api 20 restart_apache_server 21 tail_log nova-api /var/log/$APACHE_NAME/nova-api.log 22 tail_log nova-ec2-api /var/log/$APACHE_NAME/nova-ec2-api.log 23 else 24 run_process n-api "$NOVA_BIN_DIR/nova-api" 25 fi 26 27 echo "Waiting for nova-api to start..." 28 if ! wait_for_service $SERVICE_TIMEOUT $service_protocol://$SERVICE_HOST:$service_port; then 29 die $LINENO "nova-api did not start" 30 fi 31 32 # Start proxies if enabled 33 if is_service_enabled tls-proxy; then 34 start_tls_proxy '*' $NOVA_SERVICE_PORT $NOVA_SERVICE_HOST $NOVA_SERVICE_PORT_INT & 35 start_tls_proxy '*' $EC2_SERVICE_PORT $NOVA_SERVICE_HOST $EC2_SERVICE_PORT_INT & 36 fi 37 38 export PATH=$old_path 39 }
2. run_process函数，devstack/functions-common.
1 # Run a single service under screen or directly 2 # If the command includes shell metachatacters (;<>*) it must be run using a shell 3 # If an optional group is provided sg will be used to run the 4 # command as that group. 5 # run_process service "command-line" [group] 6 function run_process { 7 local service=$1 #此处的service就是n-api 8 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。 9 local group=$3 10 11 if is_service_enabled $service; then 12 if [[ "$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 13 screen_process "$service" "$command" "$group" 14 else 15 # Spawn directly without screen 16 _run_process "$service" "$command" "$group" & 17 fi 18 fi 19 }
3. screen_process函数，在devstack/functions-common文件中
1 # Helper to launch a process in a named screen 2 # Uses globals ``CURRENT_LOG_TIME``, ```LOGDIR``, ``SCREEN_LOGDIR``, `SCREEN_NAME``, 3 # ``SERVICE_DIR``, ``USE_SCREEN`` 4 # screen_process name "command-line" [group] 5 # Run a command in a shell in a screen window, if an optional group 6 # is provided, use sg to set the group of the command. 7 function screen_process { 8 local name=$1 #要启动的service的名字，此处为n-api 9 local command="$2" #要执行的命令，此处为nova-api 10 local group=$3 11 12 SCREEN_NAME=${SCREEN_NAME:-stack} 13 SERVICE_DIR=${SERVICE_DIR:-${DEST}/status} 14 USE_SCREEN=$(trueorfalse True USE_SCREEN) 15 16 screen -S $SCREEN_NAME -X screen -t $name #启动一个名为stack的session，在这个session中执行screen命令（给当前screen窗口起名为n-api） 17 18 local real_logfile="${LOGDIR}/${name}.log.${CURRENT_LOG_TIME}" 19 echo "LOGDIR: $LOGDIR" 20 echo "SCREEN_LOGDIR: $SCREEN_LOGDIR" 21 echo "log: $real_logfile" 22 if [[ -n ${LOGDIR} ]]; then 23 screen -S $SCREEN_NAME -p $name -X logfile "$real_logfile" 24 screen -S $SCREEN_NAME -p $name -X log on 25 ln -sf "$real_logfile" ${LOGDIR}/${name}.log 26 if [[ -n ${SCREEN_LOGDIR} ]]; then 27 # Drop the backward-compat symlink 28 ln -sf "$real_logfile" ${SCREEN_LOGDIR}/screen-${1}.log 29 fi 30 fi 31 32 # sleep to allow bash to be ready to be send the command - we are 33 # creating a new window in screen and then sends characters, so if 34 # bash isn't running by the time we send the command, nothing 35 # happens. This sleep was added originally to handle gate runs 36 # where we needed this to be at least 3 seconds to pass 37 # consistently on slow clouds. Now this is configurable so that we 38 # can determine a reasonable value for the local case which should 39 # be much smaller. 40 sleep ${SCREEN_SLEEP:-3} 41 42 NL=`echo -ne ' 15'` 43 # This fun command does the following: 44 # - the passed server command is backgrounded 45 # - the pid of the background process is saved in the usual place 46 # - the server process is brought back to the foreground 47 # - if the server process exits prematurely the fg command errors 48 # and a message is written to stdout and the process failure file 49 # 50 # The pid saved can be used in stop_process() as a process group 51 # id to kill off all child processes 52 if [[ -n "$group" ]]; then 53 command="sg $group '$command'" 54 fi 55 56 # Append the process to the screen rc file 57 screen_rc "$name" "$command" 58 59 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 60 }
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：
1 [root@CloudGame shelltest]# screen -help 2 Use: screen [-opts] [cmd [args]] 3 or: screen -r [host.tty] 4 5 Options: 6 -4 Use IPv4. 7 -6 Use IPv6. 8 -a Force all capabilities into each window's termcap. 9 -A -[r|R] Adapt all windows to the new display width & height. 10 -c file Read configuration file instead of '.screenrc'. 11 -d (-r) Detach the elsewhere running screen (and reattach here). 12 -dmS name Start as daemon: Screen session in detached mode. 13 -D (-r) Detach and logout remote (and reattach here). 14 -D -RR Do whatever is needed to get a screen session. 15 -e xy Change command characters. 16 -f Flow control on, -fn = off, -fa = auto. 17 -h lines Set the size of the scrollback history buffer. 18 -i Interrupt output sooner when flow control is on. 19 -l Login mode on (update /var/run/utmp), -ln = off. 20 -list or -ls. Do nothing, just list our SockDir. 21 -L Turn on output logging. 22 -m ignore $STY variable, do create a new screen session. 23 -O Choose optimal output rather than exact vt100 emulation. 24 -p window Preselect the named window if it exists. 25 -q Quiet startup. Exits with non-zero return code if unsuccessful. 26 -r Reattach to a detached screen process. 27 -R Reattach if possible, otherwise start a new session. 28 -s shell Shell to execute rather than $SHELL. 29 -S sockname Name this session <pid>.sockname instead of <pid>.<tty>.<host>. 30 -t title Set title. (window's name). 31 -T term Use term as $TERM for windows, rather than "screen". 32 -U Tell screen to use UTF-8 encoding. 33 -v Print "Screen version 4.00.03 (FAU) 23-Oct-06". 34 -wipe Do nothing, just clean up SockDir. 35 -x Attach to a not detached screen. (Multi display mode). 36 -X Execute <cmd> as a screen command in the specified session.
在devstack环境下，相关的操作指令主要有：
screen -ls: 列出当前sockdir。
1 [stack@ip-10-121-4-176 devstack]$ screen -ls 2 There is a screen on: 3 6130.stack (Detached) 4 1 Socket in /var/run/screen/S-stack.
attach screen：
1 [stack@ip-10-121-4-176 devstack]$ screen -r 6130.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：
1 。。。。。。。。。。。。。。。 2 4-12-03T00:00:00Z", "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 3 2016-01-25 04:41:05.346975 4 :q! 5 6 0-$ stack@ip-10-121-4-17 1$(L) dstat 2$(L) key 3$(L) key-access 4$(L) horizon* (ip-10-121-4-176/0.29 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！