The JUK universal Screw Terminal Block series has the typical features which are decisive for practical applications:
l The universal foot allows the terminal blocks to be easily snapped onto the NS35 or NS32 DIN Rail with G shape.
l Closed screw guide holes ensure screwdriver operation perfect.
l For terminal block with different wire cross-sectional areas, complete accessories are available, such as end plates, partition plates, etc.
l Potential distribution achieved by fixed bridges in the terminal center or insertion bridges in the clamping space.
l Same shape and pitch Grounding Terminal Blocks as the JUK universal series.
l Adopt ZB marker strip system,achieve unified identification.
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Summary of 33 tips for common troubleshooting and processing of Linux operation and maintenance
As a Linux system administrator, it's common to encounter various issues and failures. Accumulating experience, identifying problems, and analyzing their causes is a crucial habit for any professional in this field. Every technological breakthrough comes with its challenges, but through perseverance and continuous learning, we gain valuable insights and practical skills that are rewarding.
In the following, I'll summarize some of the common faults and solutions I've encountered during my projects. I hope you find these insights helpful and relatable.
### 1. Shell Script Not Executing
A colleague once asked me to help debug a shell script he was having trouble running. The error message was: `: bad interpreter: No such file or directory`. After checking, I realized the script was written on Windows and then uploaded to a Linux server. The issue was that Windows uses `\r\n` as line endings, while Linux uses `\n`, causing the script to fail. The solution was either to rewrite the script on Linux or use `vi` to replace `\r` with nothing using `:%s/\r//g`.
### 2. Controlling Crontab Output
The `/var/spool/clientmqueue` directory grew unexpectedly large. It turned out that cron was generating output, which was being sent as email to the user, but `sendmail` wasn't running. To fix this, we added `> /dev/null 2>&1` to the crontab entries to suppress unnecessary output.
### 3. Slow Telnet/SSH Connections
A colleague reported slow telnet connections between two machines. Upon investigation, we found that DNS resolution was failing. The solution was to update `/etc/hosts` or adjust `/etc/resolv.conf` to ensure proper DNS settings.
### 4. Read-Only Filesystem
MySQL failed to create a table with an error indicating a read-only filesystem. After checking permissions, we discovered the filesystem was mounted as read-only due to possible corruption or misconfiguration in `/etc/fstab`. Restarting the machine resolved the issue temporarily.
### 5. Deleted Files Not Freeing Disk Space
A disk space issue occurred when a file was deleted but the space wasn't released. This often happens when a file is open by a process. Using `lsof` to identify the process and then restarting it or sending a signal to release the file helped reclaim the space.
### 6. Improving `find` Performance
Running `find` on a large number of files caused high system load. We replaced it with a more efficient method using `ls`, `grep`, and `xargs`, reducing resource usage significantly.
### 7. Unable to Get Gateway MAC Address
A network connectivity issue was traced back to ARP not resolving the gateway’s MAC address. Binding the MAC address manually using `arp -s` resolved the problem.
### 8. HTTP Service Fails to Start
An Apache server failed to start due to port conflicts. Checking the configuration files revealed duplicate `Listen` directives. Commenting out one of them allowed the service to start successfully.
### 9. Too Many Open Files
To resolve the "Too many open files" error, we adjusted the limits in `/etc/security/limits.conf` and set `ulimit` in the user's `.bash_profile`.
### 10. Large ibdata1 and MySQL Bin Logs
Large `ibdata1` and `mysql-bin` files caused disk space issues. For `ibdata1`, a full dump and re-import was necessary. For bin logs, we used `PURGE MASTER LOGS` or configured `expire_logs_days` in `my.cnf`.
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### Troubleshooting Summary Table
| Serial Number | Failure Point | Analysis & Resolution |
|---------------|----------------|------------------------|
| 1 | Hard drive not recognized during installation | Set hard disk to compatibility mode in BIOS |
| 2 | Installation fails after partitioning | Ensure root and swap partitions are created |
| 3 | Confusion in software package selection | Gain more experience through repeated installations |
| 4 | Proxy filtering rules not working | Check module loading, default policies, syntax, and rule order |
| 5 | DMZ services not accessible | Check iptables rules and connectivity |
| 6 | Iptables rules lost after restart | Save rules using `iptables-save` |
| 7 | VLAN cannot access external network | Configure correct gateway for VLAN |
| 8 | Named service fails to start | Ensure necessary files and permissions are set |
| 9 | DNS resolution issues | Check zone files, configuration, and `resolv.conf` |
| 10 | DHCP error: No subnet declaration | Ensure IP is within the correct scope |
| 11 | Only one DHCP scope assigned | Use multiple NICs for different scopes |
| 12 | MySQL installation dependencies | Install required packages in the correct order |
| 13 | Web page not displayed | Check `DocumentRoot` path in `httpd.conf` |
| 14 | Samba shared directory not accessible | Disable iptables temporarily |
| 15 | NT_STATUS_BAD_NETWORK_NAME | Verify shared directory exists |
| 16 | NT_STATUS_ACCESS_DENIED | Check credentials and firewall status |
| 17 | NT_STATUS_LOGON_FAILURE | Ensure user has access rights |
| 18 | FTP upload rejected | Check write permissions on the upload directory |
| 19 | Root account denied access to FTP | Disable SELinux if enabled |
| 20 | Mail not received | Ensure POP3 service is running |
| 21 | NFS mount hangs | Start `portmap` service |
| 22 | NFS mount fails on other clients | Test with iptables disabled |
These experiences highlight the importance of systematic troubleshooting and continuous learning in Linux administration. Each challenge provides an opportunity to grow and improve.