Linux logger Command

Linux 命令大全 Linux Command Reference

logger is a simple yet powerful command-line tool in Linux, used to add log entries to the system log (syslog). It allows users and script programs to write custom messages to system log files, making it an important tool for system administration and troubleshooting.

Typical Use Cases：

Record key operations during script execution
Track the status of system maintenance tasks
Log records for custom applications
Remote logging (with a syslog server)

Basic Syntax and Parameters

The basic syntax of the logger command is as follows:

logger [选项] [消息]

Common Option Parameters

Option	Description
`-p, --priority <优先级>`	Specify log priority (facility.level)
`-t, --tag <标签>`	Tag log entries with a specified label
`-i, --id`	Include process ID in each log line
`-f, --file <文件>`	Read log content from a specified file
`-s, --stderr`	Also output logs to standard error
`-n, --server <主机>`	Send logs to a remote syslog server
`-P, --port <端口>`	Specify the remote syslog server port
`-u, --socket <套接字>`	Specify a Unix domain socket instead of the default device
`-d, --udp`	Send logs using UDP protocol (default)
`-T, --tcp`	Send logs using TCP protocol
`-h, --help`	Display help information
`-V, --version`	Display version information

Priority Explained in Detail

Log priority consists of two parts: facility and level, formatted asfacility.level。

Facility Types

Facility Code	Description
auth	Security/authorization messages
authpriv	Private authorization messages
cron	Cron daemon
daemon	System daemon
kern	Kernel messages
lpr	Printing system
mail	Mail system
news	News system
syslog	syslogd internal messages
user	User-level messages (default)
local0-7	Custom local use

Log Levels

Level Code	Description	Value
emerg	System is unusable	0
alert	Action must be taken immediately	1
crit	Critical conditions	2
err	Error conditions	3
warning	Warning conditions	4
notice	Normal but significant conditions	5
info	Informational messages	6
debug	Debug-level messages	7

Practical Examples

Basic Usage Examples

Simple log recording：
```
logger "系统备份任务开始执行"
```
This will record a message in the system log, using the user.notice priority by default.
Log with a tag：
```
logger -t BACKUP "数据库备份完成"
```
The log entry will be displayed as:BACKUP: 数据库备份完成

Specify priority：

logger -p local0.notice "自定义应用程序启动"

Advanced Usage Examples

Read log content from a file：
```
logger -f /var/log/myapp.log
```

Record script execution status：

#!/bin/bash
logger -t SCRIPT -p user.info "脚本开始执行"
# 执行任务...
if [ $? -eq 0 ]; then
    logger -t SCRIPT -p user.info "任务执行成功"
else
    logger -t SCRIPT -p user.err "任务执行失败"
fi

Remote log recording：

logger -n 192.168.1.100 -P 514 "发送到远程服务器的日志"

Output to terminal and log simultaneously：

logger -s "这条消息会同时出现在终端和日志中"

Viewing and Managing Logs

After recording logs, you can use the following commands to view them:

View system logs：
journalctl # systems using systemd or tail -f /var/log/syslog # traditional systems
Filter logs by a specific tag：
```
journalctl -t BACKUP
```

View logs by time：

journalctl --since "2023-01-01" --until "2023-01-02"

Best Practices and Precautions

Log tag conventions：
- Use meaningful tags (such as application name or script name)
- Keep tags consistent for easier filtering and analysis later
Priority selection：
- Use different levels appropriately; avoid using the same level for all logs
- Use warning or higher for critical operations
Log content suggestions：
- Include sufficient context information
- Avoid recording sensitive information (such as passwords)
- Keep logs concise but complete
Performance considerations：
- High-frequency logging may affect performance
- Consider using buffered or asynchronous methods for logging large volumes
Log rotation：
- Regularly clean up old logs
- Configure logrotate to manage log file sizes

FAQ

Q1: Why does my log not appear in /var/log/syslog?

A1: This may be because:

The facility you are using is configured to write to another file
The log level is lower than the system-configured minimum recording level
The system uses journald instead of traditional syslog

Q2: How do I determine which logging system my system uses?

A2: Run the following command to check:

ps aux | grep -E "syslog|journald"

Q3: Can I use multiple options at the same time?

A3: Yes, for example:

logger -t MYAPP -p local0.err -i "关键错误发生"

Q4: How do I test the remote log server configuration?

A4: Use logger to send a test message:

logger -n 远程服务器IP -P 端口 "测试消息"

Through this article, you should have mastered the core usage of the logger command. This seemingly simple tool is very useful in practical system administration and application development, helping you establish a complete logging mechanism. It is recommended to practice more in your actual work and flexibly use various option parameters according to specific needs.