Linux error messages, explained and fixed

Find the exact message you're seeing. Each page explains the cause in plain words and gives the fix first.

Tip: use the search box at the top of the page and paste the part of the message that doesn't change between machines — for example Unable to locate package rather than the package name.

Package management

apt 404 Not Found

apt asked for a file the server no longer has. Usually stale package lists: run apt update and retry.

Could not get lock (dpkg lock-frontend)

Another apt/dpkg process holds the lock: find it, let it finish, then repair.

dnf: Failed to download metadata

dnf can't fetch a repo's index: fix network/DNS, clean the cache, or disable the broken repo.

dpkg returned an error code (1)

A package's install script failed. Find which one and why, then repair it.

E: dpkg was interrupted

An install was cut off: run dpkg --configure -a, then fix whatever still fails.

E: Unable to locate package

apt can't find the package: refresh lists, check the name, enable universe/contrib.

NO_PUBKEY / public key is not available

A repository's signing key is missing: install it as a keyring with signed-by, not apt-key.

pacman: invalid PGP signature

Outdated keyring or bad cache: update archlinux-keyring, then repopulate with pacman-key.

pacman: unable to lock database

A stale or active pacman lock: make sure nothing runs, remove db.lck, rerun the upgrade.

pip: externally-managed-environment

pip refuses to touch system Python: use a venv, pipx, or the distro package.

Repository does not have a Release file

apt can't find the repo's index: wrong release, unsupported PPA, or an end-of-life system.

snap: cannot communicate with server

The snap command cannot reach the snapd daemon. Start or install snapd, or check WSL, containers and Mint.

You have held broken packages

apt can't satisfy dependencies: find the real blocker, then fix PPAs, holds or mixed releases.

Disk & filesystem

No space left on device

Disk full, inodes exhausted or deleted-but-open files: find the cause and free space safely.

Read-only file system

Disk suddenly read-only? Read dmesg, repair with fsck, check the drive and remount read-write.

Structure needs cleaning

File system metadata is corrupt: unmount, check hardware, repair with xfs_repair or e2fsck.

umount: target is busy

Find which process holds the mount with lsof or fuser, stop it, then unmount cleanly.

unknown filesystem type

Missing driver or a container layer (LVM, LUKS, RAID) that must be opened before mounting.

wrong fs type, bad superblock

Usually the wrong device, a missing CIFS/NFS helper or a bad option, not corruption.

Boot & system

Cannot allocate memory

RAM and swap exhausted, strict overcommit, or a map/address-space limit. Diagnose and fix.

Kernel panic: unable to mount root fs

Usually a missing or broken initramfs after a kernel update. Boot an older kernel and rebuild it.

Out of memory: Killed process

Process killed by the OOM killer? Confirm it in the logs, find the memory hog, add swap and set limits.

Segmentation fault (core dumped)

Find why a program segfaults with coredumpctl and gdb, then fix packages, configs, RAM or your code.

You are in emergency mode

Booted into emergency mode? Read the journal, fix the bad fstab line or run fsck, then reboot.

SSH & networking

Host key verification failed

ssh could not confirm the server's identity and had no way to ask you. Add the verified key to known_hosts.

Network is unreachable

Your own machine has no route for the address. Usually no default gateway, a down link, or IPv6 without IPv6.

No route to host

Either the host does not answer on the local network or a firewall rejects you. Tell them apart in two steps.

Permission denied (publickey)

SSH rejected every key you offered. Find out which key, which user and which file is wrong.

Remote host identification changed

The server's host key no longer matches known_hosts. Verify it, then remove the stale entry.

ssh Connection refused

Nothing accepted the connection on that port. Install or start sshd, then check port and firewall.

ssh Connection timed out

Your SYN got no answer at all. Find where packets are dropped: wrong IP, host down, firewall or NAT.

sudo: unable to resolve host

Your machine's own name is not in /etc/hosts. Add one line and the warning and the delay go away.

Temporary failure in name resolution

Your system can't reach a DNS server. Check the network, resolv.conf and systemd-resolved.

Permissions & users

bash: Permission denied

The script lacks the execute bit or lives on a noexec mount. chmod +x fixes most cases.

User is not in the sudoers file

Your account has no sudo rights. Add it to the sudo or wheel group from a root shell.

Shell & commands

/bin/bash^M: bad interpreter

Windows CRLF line endings broke the shebang: strip them with sed or dos2unix.

Argument list too long

A glob expanded to more arguments than the kernel accepts. Use find or xargs instead.

bash: command not found

The shell can't find the program. Install the right package or add its directory to PATH.

error while loading shared libraries

A program needs a .so library the loader can't find: install it or register its path.

Exec format error

Wrong CPU architecture, a non-Linux binary or a script without a shebang.

GLIBC_2.xx not found

The program was built against a newer glibc than yours. Never replace system glibc.

Operation not permitted

EPERM even as root: immutable attribute, mount options, sticky bit or missing capabilities.

Too many open files

A process hit its file descriptor limit. Raise the right limit, then check for a leak.