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A quick-reference glossary of the Linux error messages you're most likely to encounter, what they actually mean, and where to go for the full fix.

"Command Not Found"

The program isn't installed, or isn't in your PATH.

which COMMAND_NAME
sudo apt install PACKAGE_NAME

"Permission Denied"

You lack the required file, execution, or sudo permission for the action. See How to Fix "Permission Denied" Errors on Linux.

"No Such File or Directory"

The path you referenced doesn't exist — check for typos, and confirm you're in the expected working directory:

pwd
ls -la

"Address Already in Use"

Another process is already listening on the port you're trying to bind:

sudo ss -tulpn | grep :PORT

"Connection Refused"

Nothing is listening on the target port, or a firewall is actively rejecting the connection. See How to Fix "Connection Timed Out" Errors for the related timeout case.

"No Space Left on Device"

The disk (or inodes) are full. See How to Fix "No Space Left on Device" Errors.

"Too Many Open Files"

The process has hit its file descriptor limit:

ulimit -n

Increase it in /etc/security/limits.conf for the affected user/service if the application genuinely needs more concurrent connections.

"Segmentation Fault"

A program tried to access memory it shouldn't have — usually a bug in the application itself or a corrupted installation. Check for updates to the affected software, or reinstall it.

"Kernel Panic"

A fatal error the kernel couldn't recover from, forcing a halt or reboot. See How to Fix a VPS Stuck in a Reboot Loop if this is causing a boot loop.

"Read-Only File System"

The filesystem was remounted read-only, often after a disk error was detected:

dmesg | grep -i "read-only"

Attempt to remount and check the filesystem (from rescue mode if the root filesystem is affected):

sudo mount -o remount,rw /
sudo fsck /dev/sda1

"Broken Pipe"

The process on the other end of a connection closed unexpectedly — often harmless (e.g. a client disconnecting mid-transfer), but can indicate a network or timeout issue if persistent.

"Cannot Allocate Memory"

The system is out of available RAM. See How to Diagnose and Fix Out of Memory (OOM) Errors.

"Operation Not Permitted"

Similar to "Permission denied" but often indicates a kernel-level restriction (like trying to modify an immutable file, or an operation requiring capabilities beyond standard root):

lsattr filename

"Bad Gateway" / "Gateway Timeout" (Web Context)

Nginx/Apache couldn't get a valid or timely response from a backend application. See How to Fix Common Nginx Errors (502/504/403).

"Illegal Instruction"

The binary was compiled for a different CPU architecture than the server is running — verify you downloaded the correct architecture (e.g. ARM vs x86_64) for your VPS.

Quick Reference Table

Related Articles

• How to Fix "Permission Denied" Errors on Linux
• How to Fix "No Space Left on Device" Errors
• How to Diagnose and Fix Out of Memory (OOM) Errors