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Common Linux Errors and Solutions

System administrators encounter specific errors repeatedly. Knowing how to quickly diagnose and fix these common issues is vital.

1. "No space left on device"​

Symptoms: Services crash, files cannot be saved, database writes fail.

Diagnosis: This can happen for two reasons: out of disk blocks, or out of inodes (file metadata structures).

  • Check block space: df -h
  • Check inode space: df -i

Solutions:

  • If disk space is 100%, find large files using du -sh /* or find / -type f -size +1G.
  • Clear old logs, temp files, or package caches.
  • If inode usage is 100%, you have too many tiny files. Look for directories with millions of small files (often session files or old mail queues).

2. "Permission denied"​

Symptoms: Unable to read/write a file, execute a script, or bind to a port.

Diagnosis & Solutions:

  • File Permissions: Check ownership and mode with ls -l. Fix with chmod and chown.
  • Execution: Scripts must have the execute bit set (chmod +x script.sh).
  • Low Ports: Binding to ports under 1024 requires root privileges.
  • SELinux / AppArmor: If standard permissions look correct but access is still denied, security modules might be blocking it. Check /var/log/audit/audit.log (SELinux) or /var/log/syslog (AppArmor).

3. "Connection refused"​

Symptoms: Cannot connect to a service via network (e.g., SSH, HTTP).

Diagnosis & Solutions:

  • Is the service running? Check with systemctl status [service] or ps aux.
  • Is it listening on the correct IP/Port? Verify with ss -tlnp or lsof -i tcp:port. It might be listening only on 127.0.0.1 instead of 0.0.0.0.
  • Is a firewall blocking it? Check iptables -L, ufw status, or firewalld-cmd --list-all.

4. "Command not found"​

Symptoms: Typing a command results in an error, even if the software is installed.

Diagnosis & Solutions:

  • The binary is not in your $PATH environment variable. Check echo $PATH.
  • Find the binary manually (e.g., find / -name mycommand) and use the absolute path (e.g., /opt/myapp/bin/mycommand).
  • The package isn't actually installed. Reinstall or verify using your package manager (apt, yum, etc.).

5. "Read-only file system"​

Symptoms: Cannot create or modify files; the filesystem behaves as if it's locked.

Diagnosis & Solutions:

  • The kernel remounts filesystems as read-only when it detects disk corruption or hardware failure to prevent further damage.
  • Check dmesg -T immediately for I/O errors or SCSI layer issues.
  • You will likely need to run fsck (filesystem check) on the partition from a live CD or rescue mode, and potentially replace failing hardware.