linux - 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来轮询新创建的 AWS 实例以进行 SSH 访问。我希望它最多轮询 60 秒，这就是我使用 linux timeout 命令的原因。

我有一个在超时命令中运行 while 循环的小脚本。

“完整”脚本供引用。您可以假设 IP 地址是正确的

  # 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，导致此输出:

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

当使用 bash -c 命令执行命令时，如何使用退出状态？

Answer 1

在您的脚本中发生了什么，$? 在 bash 运行之前被展开。它始终为零或为空。

您可以更改引号，从 " 到 '。记住要正确扩展您要扩展的变量。或者，您可以只更改转义符 $? 到 \$?。

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'

或者使用函数:

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"