Topic: boot
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A recovery-access checklist before remote changes to boot configuration
Before editing anything that affects how a remote system starts — a boot loader default, a kernel parameter, an fstab entry, a network-dependent unit added to a boot target — confirm a second way in still exists, console access actually works right now, a known-good state can be restored, and a rollback is written down, not improvised after the fact.
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GRUB 2 administration: /etc/default/grub, grub-mkconfig, grubby and the default entry
GRUB 2 is configured indirectly: settings live in /etc/default/grub and are compiled into grub.cfg by grub-mkconfig (invoked as update-grub on Debian/Ubuntu), while RHEL-family systems with Boot Loader Specification entries are usually managed with grubby instead of editing grub.cfg by hand.
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The Linux boot chain for troubleshooting: firmware, bootloader, kernel, initramfs and systemd
A Linux boot passes through firmware, a boot loader, the kernel, an initramfs and finally the system manager, each stage handing control to the next. Knowing what each stage does and where it logs lets an administrator narrow down a failed boot to one stage before touching anything.
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systemd rescue.target and emergency.target: entering them and getting back to normal
rescue.target starts the base system and a single-user shell; emergency.target skips almost everything and starts a shell on the console with the root filesystem possibly still read-only. Both are reached with a systemd.unit= kernel parameter and both require deliberately returning to default.target afterward.
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Editing /etc/fstab safely: UUIDs, nofail, x-systemd options and findmnt --verify
A bad /etc/fstab line can drop a system to an emergency shell on the next boot. This methodology uses UUID= instead of device names, nofail and x-systemd.device-timeout= for optional mounts, and findmnt --verify plus mount -a to test the file before rebooting.
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Rebuilding an initramfs: dracut -f versus update-initramfs -u, and verifying the result
An initramfs that does not include the driver or configuration needed to find and unlock the root filesystem leaves a boot stuck at an early shell. Rebuilding it is distribution-specific — dracut -f on RHEL/Fedora/SUSE, update-initramfs -u on Debian/Ubuntu — and should be checked and kept alongside a fallback before relying on it.
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Managing AIX services with the System Resource Controller: lssrc, startsrc/stopsrc, and rc.tcpip
AIX manages long-running daemons through the System Resource Controller (SRC), a subsystem/group model configured in the ODM's SRCsubsys object class, not systemd units. lssrc/startsrc/stopsrc drive it interactively, while /etc/inittab and scripts like rc.tcpip control what starts automatically at boot.
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Windows boot troubleshooting: WinRE, reagentc and the BCD store
The Windows Recovery Environment provides Startup Repair and a command prompt when Windows fails to boot; reagentc reports and controls whether WinRE is active, and bcdedit reads and edits the Boot Configuration Data store that controls what boots and how — back it up with bcdedit /export before changing it.
Machine-readable: JSON