可以编写一个BASH脚本，使用tail命令实时监测日志文件，并使用grep命令匹配关键字。脚本使用while循环持续监测日志文件，直到达到指定时间（30分钟）后退出。

以下是一个示例脚本：

#!/bin/bash

# Set the keyword you want to look for in the log file
keyword="error"

# Set the time in seconds to keep looking for the keyword in the log file
time_to_run=1800 # 30 minutes

# Set the log file you want to scan
logfile="/var/log/test.log"

# Start time
start_time=$(date +%s)

# Loop until the time to run has elapsed
while [ $(( $(date +%s) - $start_time )) -lt $time_to_run ]
do
    # Use tail command to monitor the log file and grep for the keyword
    tail -n0 -f $logfile | grep -q $keyword
    
    # Check if the keyword has been found. If it has, exit the loop
    if [ $? -eq 0 ]; then
        echo "Keyword found. Stopping script."
        break
    fi
done

echo "Script has finished running."

使用方法：

1. 将脚本保存为scan_log_file.sh
2. 在终端中执行以下命令：

chmod +x scan_log_file.sh
./scan_log_file.sh

3. 脚本将开始运行并扫描指定的日志文件，直到找到关键字或达到指定时间。