如何在超时bash -c命令中使用退出状态

Question

我正在运行一个小脚本，以实质上轮询新创建的AWS实例以进行SSH访问。 我希望它轮询最多60秒，这就是为什么我使用linux timeout命令的原因。

我有一个小的脚本，可以在超时命令中运行while循环。

“完整”脚本供参考。 您可以假设IP地址正确

  # poll for SSH access
  timeout_threshold=60
  INSTANCE_IP=199.199.199.199
  timeout $timeout_threshold bash -c "while true; do
    ssh -oConnectTimeout=2 -oStrictHostKeyChecking=no -q ${INSTANCE_IP} exit
    response_code=$?
    if (( response_code == 0 )); then
      echo \"Successfully connected to instance via SSH.\"
      exit
    else
      echo \"Failed to connect by ssh. Trying again in 5 seconds...\"
      sleep 5
    fi
  done"

轮询的关键部分是

    ssh -oConnectTimeout=2 -oStrictHostKeyChecking=no -q ${INSTANCE_IP} exit
    response_code=$?

问题在于退出状态（即$？）始终为空，从而导致以下输出：

line 4: ((: == 0 : syntax error: operand expected (error token is "== 0 ")
Failed to connect by ssh. Trying again in 5 seconds...

用bash -c命令执行命令时如何使用退出状态？

Answer 1

您的脚本中会发生什么，那是$? 甚至在 bash运行之前就被扩展了。 它总是为零或为空。

你可以做改变报价，从"到'记住扩大要适当扩大变量。另外，你可以只改变逃脱$?为\\$?

timeout "$timeout_threshold" bash -c 'while true; do
    ssh -oConnectTimeout=2 -oStrictHostKeyChecking=no -q '"${INSTANCE_IP}"' exit
    response_code=$?
    if (( response_code == 0 )); then
      echo "Successfully connected to instance via SSH."
      exit
    else
      echo "Failed to connect by ssh. Trying again in 5 seconds..."
      sleep 5
    fi
  done'

或使用功能：

connect() {
    # I pass the instance as the first argument
    # alternatively you could export INSTANCE_IP from parent shell
    INSTANCE_IP="$1"
    while true; do
       ssh -oConnectTimeout=2 -oStrictHostKeyChecking=no -q "$INSTANCE_IP" exit
       response_code=$?
       if (( response_code == 0 )); then
          echo "Successfully connected to instance via SSH."
          exit
       else
          echo "Failed to connect by ssh. Trying again in 5 seconds..."
          sleep 5
       fi
    done
}

timeout_threshold=60
INSTANCE_IP=199.199.199.199

# function needs to be exprted to be used inside child bashs
export -f connect

timeout "$timeout_threshold" bash -c 'connect "$@"' -- "$INSTANCE_IP"