Elastic Compute Service:Differences between running commands remotely by using Cloud Assistant and locally on Linux instances

Solution

In Cloud Assistant, load required startup files and add variables that your command or relevant programs depend on to the files. Then, use Cloud Assistant to rerun the command.

1. Check whether the dependency variables of the command or relevant programs are defined in Cloud Assistant.

   For information about how to create a Cloud Assistant command, see Create a command.

   Run the following command to check whether a variable is defined and take a specific action if the variable does not exist:

   if [ -z "${<Dependency variable name>+x}" ]; then
     <Action to take if the dependency variable does not exist...>
   fi

   In the following sample Cloud Assistant command, the $TERM variable is used:

   #!/bin/sh
   if [ -z "${TERM+x}" ]; then
     echo "\$TERM is not set when script executed via cloud assistant"
   else
     echo "\$TERM is set to '${NOT_SET_VARAIABLE}'"
   fi

   If the $TERM is not set when script executed via cloud assistant error message is returned in the command output, the $TERM variable does not exist.

2. Use one of the following methods to load environment configuration-related and initialization-related startup files, such as /etc/profile, ~/.bash_profile, and ~/.bashrc, into the command:

   • Method 1: Explicitly load startup files into the command.

     Place . commands or Bash-specific source commands in the front of the shell script to load specified environment configuration-related and initialization-related startup files. This eliminates the need to process each nonexistent variable. Sample commands:

     • . command:

       #!/bin/sh
       . /etc/profile

     • source command:

       #!/bin/bash
       source $HOME/.bashrc

   • Method 2: On the line that starts with the #! symbols at the beginning of the command, use parameters to specify the shell interpreter to load startup files.

     Note

     If the command output is to be parsed and processed, we recommend that you do not use this method.

     If you do not want to add statements at the beginning of the command but you want to enable interactive mode and logon mode for the shell interpreter to load the required startup files, specify the -i parameter to enable interactive mode and specify the -I parameter to enable logon mode for the shell interpreter. These parameters are combined to form the -iI parameter. Run the following command to enable interactive logon mode for the shell interpreter, which is similar to SSH logon:

     #!/bin/sh -il

     If you use the Bash interpreter, you can add #!/bin/bash -il at the beginning of the command. For more information about the parameters, see the documentation about the shell interpreter that you use. When you specify the -i parameter to enable interactive mode, the shell interpreter checks that the standard input (stdin) device is an interactive device before the interpreter provides interactive features such as task control. When you use Cloud Assistant to run a command, the stdin device is a null device, which is written as /dev/null. As a result, the stdin device check fails and the shell interpreter reports the following errors:

     sh: cannot set terminal process group (-1): Inappropriate ioctl for device
     sh: no job control in this shell

     Or

     /usr/local/share/aliyun-assist/work/script/t-xx.sh: 0: /usr/local/share/aliyun-assist/work/script/t-hz02vmfqb9vxxq8.sh: can't access tty; job control turned off
     stdin: is not a tty

Solution

On your Linux instance, you can set resource limits for Cloud Assistant to 65535 based on the system startup method.

1. Check the system startup method of your ECS instance.

   1. Connect to the Linux instance.

      For more information, see Methods for connecting to an ECS instance.

   2. Run the following commands to check the system startup method.

      • systemd

        strings $(which init) |grep "systemd"

        If systemd is displayed in the command output, the system startup method is systemd.

      • upstart

        strings $(which init) | grep "upstart"

        If upstart is displayed in the command output, the system startup method is upstart.

      • sysvinit

        strings $(which init) | grep "sysvinit"

        If sysvinit is displayed in the command output, the system startup method is sysvinit.

2. Configure resource limits for Cloud Assistant.

   Configure resource limits for Cloud Assistant on a Linux instance based on the system startup method of the instance.

   systemd startup method

   1. Configure resource limits for all services

      1. Connect to the Linux instance.

         For more information, see Methods for connecting to an ECS instance.

      2. Run the following commands to add the DefaultLimitNOFILE field to the Manager parameter in the /etc/systemd/system.conf configuration file and set the field to 65535:

         vim /etc/systemd/system.conf
         
         [Manager]
         DefaultLimitNOFILE=65535

      3. Run the following command to rerun the systemd manager and reread system configuration files:

         systemctl  daemon-reexec

      4. Run the following command to restart Cloud Assistant:

         systemctl restart aliyun.service

         After you restart Cloud Assistant, the resource limits for Cloud Assistant are set to 65535.

   2. Configure resource limits for Cloud Assistant

      1. Connect to the Linux instance.

         For more information, see Methods for connecting to an ECS instance.

      2. Run the following commands to add the LimitNOFILE field to the Service parameter in the service configuration file of Cloud Assistant and set the field to 65535:

         [root@localhost ~]# vim /etc/systemd/system/aliyun.service
         ....
         [Service]
         LimitNOFILE=65535
         ....

      3. Run the following command to reload system configuration files:

         [root@localhost ~]# systemctl daemon-reload

      4. Run the following command to restart Cloud Assistant:

         [root@localhost ~]# systemctl restart aliyun

         After you restart Cloud Assistant, the resource limits for Cloud Assistant are set to 65535.

   Non-systemd startup method

   • Modify the Cloud Assistant startup script

     1. Connect to the Linux instance.

        For more information, see Methods for connecting to an ECS instance.

     2. Run the following commands to add the ulimit -n field at the beginning of the Cloud Assistant process file and set the field to 65535:

         [root@localhost ~]# vim /etc/init.d/aliyun-service
        
        ..................
         start() {
            if [ -f $service_lock ]; then
                ps aux | grep "$start_cmd" | grep -v grep 1>/dev/null 2>&1
                if [ $? -eq 0 ]; then
                    echo "$aliyun_service_name is already started!" 1>/dev/null 2>&1
                    return 0
                else
                    rm -rf $service_lock
                fi
            fi
            ulimit -n 65535
            $start_cmd
        
            ### Create the lock file ###
            touch $service_lock
        }
        ....................

     3. Run the following command to restart Cloud Assistant:

        [root@localhost ~]# /etc/init.d/aliyun-service restart

        After you restart Cloud Assistant, the resource limits for Cloud Assistant are set to 65535.

   • Modify the Cloud Assistant configuration file

     Note

     You can configure resource limits by modifying the Cloud Assistant configuration file only if the system startup method is upstart.

     1. Connect to the Linux instance.

        For more information, see Methods for connecting to an ECS instance.

     2. Run the following commands to set the limit nofile field in the Cloud Assistant configuration file to 65535:

        [root@localhost ~]# cat /etc/init/aliyun-service.conf
        description     "Aliyun Assist Tool"
        author          "aliyun.com"
        
        start on runlevel [2345]
        stop on runlevel [!2345]
        
        limit nofile 65535 65535
        
        respawn
        
        exec /usr/sbin/aliyun-service
        post-stop exec sleep 1
        
        [root@localhost ~]# service aliyun-service restart

     3. Run the following command to restart Cloud Assistant:

        [root@localhost ~]# service aliyun-service restart

        After you restart Cloud Assistant, the resource limits for Cloud Assistant are set to 65535.

Cause

The command output size is limited to 16 KB for Cloud Assistant. If you use Cloud Assistant to run a command remotely on ECS instances and the command output exceeds 16 KB in size, an incomplete command output is returned.

Solution

We recommend that you store Cloud Assistant command outputs in local logs or Object Storage Service (OSS).

Causes

• Possible cause 1:

  When you run a command remotely on ECS instances by using Cloud Assistant or run the command locally on the instances, the exit code of the command script is the exit code returned by the last command that is included in the script. If the exit code is not zero, an error is returned indicating that the command failed to be run. Some commands return error-free command outputs and non-zero exit codes when the commands are run locally on ECS instances. If you use Cloud Assistant to run such a command remotely on an ECS instance, an error is returned indicating that the command failed to be run remotely on the instance.

• Possible cause 2:

  When you use Cloud Assistant to run a command remotely on ECS instances, Cloud Assistant saves the command as a script and then uses sh -c "./<Script name>" to run the command in the sh interpreter. When you run a command locally on ECS instances, the command is run in the Bash interpreter. The sh interpreter differs from the Bash interpreter. For example, the sh interpreter does not support the process replacement syntax (<(cmd)). However, the Bash interpreter supports the syntax. As a result, commands may fail to be run remotely on ECS instances by using Cloud Assistant but can be run locally on ECS instances.

Solution

Before you use Cloud Assistant to run a command, add #!/bin/bash to the first line of the command to use the Bash interpreter.

Cause

After you log on to a Debian, Ubuntu, or UOS instance in an interactive manner, the /bin/bash Bash environment is allocated by default. The default settings of this environment are defined in the /etc/adduser.conf file. After you log on to a Debian, Ubuntu, or UOS instance in a non-interactive manner and run a script in which no interpreters are specified, /bin/sh is used by default.

In Debian, Ubuntu, or UOS instances, /bin/sh is linked to dash. As result, Cloud Assistant commands may fail to be run due to syntax incompatibility, such as commands whose scripts include Bash-specific source commands or functions that are defined by using the function keyword. For more information, visit the Shell webpage.

Solution

Before you use Cloud Assistant to run a command, add #!/bin/bash to the first line of the command to use the Bash interpreter.