RHCSA
862 incident response problems that help with RHCSA prep.
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How to split Linux failures by systemd, permission, and filesystem signalsAn InfraTree guide that lays out the first signals to check, the CLI verification order, common misdiagnoses, and a safe recovery path when a service fails but you need to isolate whether the cause is a systemd unit, permission, inode, mount, or process.Linux3 min readWhen sudo works in the shell but fails under automationAn InfraTree guide that lays out the first signals to check, the CLI verification order, common misdiagnoses, and a safe recovery path when only automation jobs fail because of TTY, sudoers, environment reset, or service user differences.Linux3 min readWhen permissions are fixed but only new directories are created with the wrong groupAn InfraTree guide that lays out the first signals to check, the CLI verification order, common misdiagnoses, and a safe recovery path when existing file permissions are correct but group inheritance breaks for newly created files and directories.Linux3 min read
Recommended problems
Reviewed problems first, then problems with detailed scenarios.
LINUX-001An inode problem where files cannot be created even though disk space is freeA problem for identifying and organizing inode exhaustion, which is easy to miss when looking only at df output.ReviewedLinuxBeginner16 minFreeLINUX-005A missing WorkingDirectory problem that puts a systemd service into a restart loopA situation where the application itself is fine but a wrong unit-file path setting kills the service.ReviewedLinuxIntermediate20 minFreeLINUX-006The application cannot read filesA scenario that narrows down the root cause, centered on checking permissions, owner, and the execution account together, in the situation of the application being unable to read files because permissions changed after a deploy script.ReviewedLinuxIntermediate19 minFreeLINUX-010A missing environment variable that fails under cron but works when run manuallyA problem for checking which defaults are dropped when the same script runs in a different execution environment.ReviewedLinuxBeginner15 minFreeLINUX-031SELinux context breaks web content after rsync-based restoreFile ownership and permissions look correct after a restore, but the service still cannot read content because the restored paths lost the expected SELinux labels.ReviewedLinuxIntermediate21 minFreeLINUX-033Deleted log file keeps disk fullOperators remove the giant log file, but the filesystem usage never drops because the service keeps writing to an already deleted file descriptor.ReviewedLinuxIntermediate18 minFree
All problems (862)
No space left on device: the disk is full and files cannot be writtenNo space left on device: the disk is full and files cannot be written is a hands-on troubleshooting drill. Use df and du to find what filled the disk. Linux Storage and Filesystems needs to be checked by narrowing scope, recent change, and the current live signal before rollba...ReviewedLinuxBeginner3 minFreenginx 403 with (13: Permission denied): only the newly uploaded filenginx 403 with (13: Permission denied): only the newly uploaded file is a hands-on troubleshooting drill. Read file permissions to see why nginx cannot open a file. NGINX file-permissions needs to be checked by narrowing scope, recent change, and the current live signal before...ReviewedLinuxBeginner3 minFreeToo many open files: new connections fail under loadToo many open files: new connections fail (Resource Exhaustion) is a hands-on troubleshooting drill. Check a process's open-file limit against its open descriptors. resource-management needs to be checked by narrowing scope, recent change, and the current live signal before ro...ReviewedLinuxBeginner3 minFreeEADDRINUSE (address already in use): the new version will not startEADDRINUSE (address already in use): the new version will not start is a hands-on troubleshooting drill. Find which process already holds the port. Linux Service Operations needs to be checked by narrowing scope, recent change, and the current live signal before rollback. 실무에서...ReviewedLinuxBeginner3 minFreeSSH Permission denied (publickey): cannot log in to a new EC2 instanceSSH Permission denied (publickey): cannot log in to a new EC2 instance is a hands-on troubleshooting drill. Know each AMI's default login user. AWS remote-access needs to be checked by narrowing scope, recent change, and the current live signal before rollback. 실무에서는 remote-ac...ReviewedLinuxBeginner3 minFreecommand not found only under cron: the script works when run by handcommand not found only (404 and Rewrite Mismatch) is a hands-on troubleshooting drill. cron runs jobs with a minimal PATH. AWS scheduled-jobs needs to be checked by narrowing scope, recent change, and the current live signal before rollback. 실무에서는 scheduled-jobs 문제를 볼 때 서비스 로그...ReviewedLinuxBeginner3 minFreeLINUX-096NTP is synchronized but the hardware clock drifts back after every reboot and breaks certificatesThe running host time looks correct, but restarts reintroduce skew because the persistent hardware clock was never aligned.ReviewedLinuxIntermediate14 minFreeLINUX-006The application cannot read filesA scenario that narrows down the root cause, centered on checking permissions, owner, and the execution account together, in the situation of the application being unable to read files because permissions changed after a deploy script.ReviewedLinuxIntermediate19 minFreeLINUX-005A missing WorkingDirectory problem that puts a systemd service into a restart loopA situation where the application itself is fine but a wrong unit-file path setting kills the service.ReviewedLinuxIntermediate20 minFreeLINUX-031SELinux context breaks web content after rsync-based restoreFile ownership and permissions look correct after a restore, but the service still cannot read content because the restored paths lost the expected SELinux labels.ReviewedLinuxIntermediate21 minFreeLINUX-010A missing environment variable that fails under cron but works when run manuallyA problem for checking which defaults are dropped when the same script runs in a different execution environment.ReviewedLinuxBeginner15 minFreeLINUX-001An inode problem where files cannot be created even though disk space is freeA problem for identifying and organizing inode exhaustion, which is easy to miss when looking only at df output.ReviewedLinuxBeginner16 minFreeLINUX-056SSH key is valid but included config disables PubkeyAuthentication later in the chainAuthorized keys and file permissions look healthy, yet login falls back to password because a later include file overrides the expected sshd setting.ReviewedLinuxIntermediate17 minFreeLINUX-033Deleted log file keeps disk fullOperators remove the giant log file, but the filesystem usage never drops because the service keeps writing to an already deleted file descriptor.ReviewedLinuxIntermediate18 minFreeLINUX-1377A cron-triggered deploy script works manually and fails overnightA maintenance script is promoted to cron and later the scheduled run fails with a binary not found error no one can reproduce manually.ReviewedLinuxBeginner10 minFreeLINUX-063sudoers include file is ignoredThe rule is written correctly, yet sudo behavior never changes because the included file fails the safety checks and is silently skipped.ReviewedLinuxBeginner13 minFreeLINUX-1611A systemd service still failsA service still fails immediately after a package layout change.ReviewedLinuxBeginner6 minFreeLINUX-1625An sshd Match block is updated and one admin still misses accessOne admin still misses SSH access after a group rename.ReviewedLinuxBeginner7 minFreeLoad average is 40 but the CPU is mostly idle: I/O waitLoad average is 40 but the CPU is mostly idle: I/O wait is a hands-on troubleshooting drill. Separate CPU saturation from I/O wait when load average spikes. linux-performance-and-observability needs to be checked by narrowing scope, recent change, and the current live signal b...ReviewedLinuxIntermediate16 minFreeThe OOM killer terminates the database during a nightly batchThe OOM killer terminates the database (Resource Exhaustion) is a hands-on troubleshooting drill. Read kernel OOM logs to find which process caused memory exhaustion and why the database was chosen. linux-performance-and-observability needs to be checked by narrowing scope, re...ReviewedLinuxIntermediate18 minFree