Tag: Linux

Recreating Grub Bootloader After the Windows Install Wipes It

Setting up the dual-boot Windows/Fedora system was straight-forward on my laptop. I installed Windows, then installed Linux and grub mkconfig found Windows and included it in the menu. Scott already had Fedora, and we needed to repair his Windows installation. Which, of course, blew away grub. Easy enough to get back, provided you’ve got a Live USB installation from which to boot.

Boot the Live media and use fdisk to find the Linux partition (in our installations, /boot is contained within the root partition).

[root@fedora02 ~]# fdisk -l
Disk /dev/sda: 10 GiB, 10737418240 bytes, 20971520 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0xd847fbc2

Device Boot Start End Sectors Size Id Type
/dev/sda1 * 2048 1026047 1024000 500M 83 Linux
/dev/sda2 1026048 20971519 19945472 9.5G 8e Linux LVM

Mount that partition somewhere:

mkdir /mnt/mycomputer
mount /dev/sda2 /mnt/mycomputer

Add bind mounts so /dev and /proc are in there

mount –bind /dev /mnt/mycomputer/dev
mount –bind /proc /mnt/mycomputer/proc

Chroot yourself into the mount point

chroot /mnt/mycomputer

Now you can reinstall grub. You don’t want the partition (e.g. /dev/sda2) but the disk. The following commands install the grub2 bootloader and reboot.

grub2-install /dev/sda
reboot

If you forget to pay attention on boot and thus inadvertently end up in the default operating system (<G>), edit /etc/default/grub and increase “GRUB_TIMEOUT=5”. Build the grub config — this should identify your Windows partition and include it in the menu

grub2-mkconfig -o /boot/grub2/grub.cfg

Reboot again, and you’ll be able to select between Windows and Linux.

Using grub rescue to boot machine and repair MBR

Using grub rescue to boot machine and repair MBR

Use “ls” to find your partition list:
ls
(hd0) (hd0,msdos3) (hd0,msdos2) (hd0,msdos1)

Check the content of each to find your Linux partition:
ls (hd0,msdos3)/
ls (hd0,msdos2)/
ls (hd0,msdos1)/

You want the one with the /boot folder. In our case, this is (hd0,msdos3). The following commands will boot your Linux OS.

set prefix=(hd0,msdos3)/boot/grub2
set root=(hd0,msdos3)
insmod normal
normal

<root password for maintenance>

Use “df” or “mount” to figure out which disk holds the Linux partition. In our case, it is /dev/sdb. You don’t want the partition (e.g. /dev/sdb2) but the disk. The following commands install the grub2 bootloader and build a config file.

grub2-install /dev/sdb
grub2-mkconfig -o /boot/grub2/grub.cfg

Linux – Finding “Missing” Disk Space

You can have ‘stuff’ in a folder, mount a partition to that folder, and have disk space used for which you cannot account. Rather than randomly umounting partitions to see if there’s anything in the mount point folder, you can bind mount root to another location and check the disk utilization on the new mount.

 

[root@fedora123 ~]# mount -o bind / /mnt/fakeout/
[root@fedora123 ~]# du -sh `ls /mnt/fakeout | grep -v mnt`
0 bin
0 boot
280K ca
8.0K cacert.pem
4.0K careq.pem
0 certs
0 crl
0 dev
44M etc
52K home
0 index.txt
0 lib
0 lib64
0 media
0 newcerts
0 openhab
724K opt
4.0K private
0 proc
847M root
0 run
0 sbin
227M srv
0 sys
4.0K tmp
3.0G usr
4.0K var
[root@fedora123 ~]# umount fakeout

Updating ll alias

Fedora and Red Hat Enterprise Linux have an alias “ll” which uses the long listing format. It’s a quick little tweak that I love, but I generally want to list hidden files too. Seemed easy enough to tweak the alias … but I never had any luck overriding the system setting or finding the source of the alias. Typing “ll -a” gave me what I wanted, although that’s not appreciably easier than typing “ls -al” …

The ll alias is defined in /etc/profile.d/colorls.sh (or colorls.csh if you use the C shell). Add the ‘a’ and “ll” produces a long list format of all files.

lisa@fedora123 ~]# grep “alias ll” /etc/profile.d/colorls.sh
alias ll=’ls -la’ 2>/dev/null
alias ll=’ls -la –color=auto’ 2>/dev/null

Quicker Way To Set Up Key-Based Authentication

I’ve always added my public key to a remote host’s authorized_keys file manually, but happened across the “ssh-copy-id” command which does that for you.

[lisa@workstation-fedora .ssh]$ ssh-copy-id -o PreferredAuthentications=password -o PubkeyAuthentication=no lisa@fedora123.example.com
The authenticity of host ‘fedora123.example.com (10.1.2.3)’ can’t be established.
ECDSA key fingerprint is SHA256:5EuKd5LNRnx5sHgQNFb6HO6W/p0hQk4pEmShTgj3zyU.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed — if you are prompted now it is to install the new keys
lisa@fedora123.example.com’s password:

Number of key(s) added: 1

Now try logging into the machine, with: “ssh -o ‘PreferredAuthentications=password’ -o ‘PubkeyAuthentication=no’ ‘lisa@fedora123.example.com'”
and check to make sure that only the key(s) you wanted were added.

Omit the -o options when attempting to log in over the key-based authentication. This, of course, presupposes that you have a public/private key pair. To create one, use ssh-keygen -t rsa -b 2048

Linux Mounts Windows Volumes As Read Only

Since I’ve got a larger hard drive installed, I have both Fedora and Windows in a dual boot configuration. I have a shared NTFS partition for data, but it’s mounted as read-only under Fedora. Turns out that Fedora sees the file system as not cleanly shut down when Windows Fast Boot is enabled. I disabled fast boot in power management, and the shared data volume is mounted rw as expected.

Iterating through files/folders with spaces in name using find in bash

Ran into a problem using Sphinx to document some Python modules and scripts that Scott put together for OpenHAB. They’re making some changes to the files to get Sphinx to process them, thus making copies of the original code. Problem is, some of the folders just weren’t showing up in the copy. Needed to change IFS in order to tokenize the find results into full paths that don’t break on spaces in file or folder names.

SAVEDIFS=$IFS
IFS=$(echo -en "\n\b")

for DIRNAME in $(find "$COMMUNITY_DIR" -maxdepth 1 -type d 2>/dev/null); do
     echo $DIRNAME
done
IFS=$SAVEDIFS

Finding Block ID

I upgraded my Fedora kernel to 5.1 and the secondary disk mounted to /var disappeared. I use the old-school device notation in fstab;  when a disk comes up with a different name, the partition fails to mount. I wanted to change fstab to use a UUID. But first I needed to find the UUID. Enter blkid

[root@linux123 ~]# blkid /dev/sdb1
/dev/sdb1: LABEL=”mnt-var” UUID=”50545e50-75c5-45q5-95b5-34f5456515d5″ TYPE=”ext4″ PARTUUID=”m50525d5-05″

The command output includes the device UUID which is used instead of the /dev/sdb# string.

#/dev/sdb1                                                                           /var ext4 nodev,nosuid 0 2
UUID=50545e50-75c5-45q5-95b5-34f5456515d5 /var ext4 nodev,nosuid 0 2

Rebooted and my partition mounted.

Using sed to insert lines into a file

I’ve used sed to replace file content — use a regex to replace the sendmail.cf line that routes mail directly with a smarthost directive

sed -i -e 's/^DS/DS\\\[mailTWB.example.com\\\]/' $strSendmailDirectory/etc/mail/sendmail.cf

But I’ve needed to prepend text to a file. Turns out sed acn do that. In fact, you can insert strings at any line number. Using “sed -i ‘5s;^;StringsToInsert\n;’ filename.xtn will insert “StringsToInsert\n” at line 5. To prepend text to a file, use “1s”

[lisa@fedora tmp]# cat test.txt;sed -i ‘5s;^;NewLine1\nNewLine2\n;’ test.txt;cat test.txt
Line 1
Line 2
Line 3
Line 4
Line 5
Line 6
**********
Line 1
Line 2
Line 3
Line 4
NewLine1
NewLine2
Line 5
Line 6
**********

 

I’ve also come across an oddity in the Win32 sed — the method I usually use to blow away everything after a newline for some reason blows away everything after the first line. Works fine on RHEL7 and Fedora29, so the quick solution is “run it from the Linux box”.

C:\temp>cat input.txt
line 1
line 2

line 3
line 4
line 5
C:\temp>sed -i ‘/^$/q’ input.txt&cat input.txt
line 1

Kernel Updates In GNOME

Since I usually do not install X11 ‘stuff’ on my Linux hosts — using the console interface — I do not have any experience installing kernel updates on “desktop” type systems. Evidently, the best practice is to drop out of the GUI into what I’d call init 3 then install the kernel updates. You can get random hangs and malfunctions when you attempt to update the kernel whilst in the graphic console.