Linux:Tips and Tricks

1 Find files modified in past N days

find path -mtime -N -ls

[psmode@wort ~]$ find /kvm/ -mtime -3 -ls
102090127      4 drwxr-xr-x   7  root     root         4096 Jan  3 09:46 /kvm/
  2038539      4 drwxr-xr-x   2  root     kitsnet_adm     4096 Jan  3 09:13 /kvm/socat-kvm
  2038573      4 -rw-r--r--   1  root     root               5 Jan  3 09:13 /kvm/socat-kvm/uv040
101237691      8 -rw-r--r--   1  root     kitsnet_adm     6370 Jan  3 09:46 /kvm/uv034.xml
102163845     12 -rw-r--r--   1  root     root            9096 Jan  3 09:46 /kvm/uv039.xml
102090129     12 -rw-r--r--   1  root     root            8303 Jan  3 09:46 /kvm/uv040.xml

2 Shell script debugging modes

A good explanation of the debugging modes is given by How To Enable Shell Script Debugging Mode in Linux. Basically, there are three modes

• -v (short for verbose) – tells the shell to show all lines in a script while they are read, it activates verbose mode.
• -n (short for noexec or no execution) – instructs the shell read all the commands, however doesn’t execute them. This options activates syntax checking mode.
• -x (short for xtrace or execution trace) – tells the shell to display all commands and their arguments on the terminal while they are executed. This option enables shell tracing mode.

Which may be invoked by any of three different ways:

1. Modifying the first line of a shell script: #!/bin/sh option(s)
2. Invoking shell with debugging options: $ /bin/bash option(s) script_name argument1 ... argumentN   
3. Using set shell built-in command in the middle of the script: $ set -option to enable or $ set +option to disable

3 Network Diagnostics

See also Red Hat 7 Networking Guide 3.3. Configuring IP Networking with nmcli and RED HAT ENTERPRISE LINUX 8 Configuring and managing networking and Network Configuration - Rocky Linux 9 and How I migrated to NetworkManager keyfiles for configuration

# ip address
# ip link
# ip route
# ip -6 route
# firewall-cmd --list-all --zone=... (replace ... with whatever zones you are working with)
# nmcli connection show 
# nmcli connection show --active
# nmcli device status
# nmcli connection show <device>
# sysctl -a | grep -E 'forwarding'

4 How to use diff and patch

The diff utility can be used to generate a patch file. Basically, the differences are put togetehr with syntax from the patch utility so that the resulting file can be used to turn another copy of the original file into the desired final state file. A good reference article is available online The high level syntax is:

# diff -u file1.html file2.html > patchfile.patch
# patch file1.html patchfile.patch

To reverse the patch, use:

# patch -p0 -R -i patchfile.patch

5 Explore filesystem utilization

I have installed NCurses Disk Usage (ncdu ) on all KitsNet Linux systems.

6 Time coordination with chrony

yum install -y chrony                    # to install  
systemctl enable chronyd                 # to enable  
systemctl start chronyd                  # to start  
chronyc tracking                         # To get information about the main time reference
chronyc sources -v                       # equivalent information to the ntpq
ntpdate pool.ntp.org                     # To quickly synchronize a server
timedatectl                              # display time and synchronization status
chronyc -m 'burst 3/3' 'makestep 0.1 3'  # sync time using timesources NOW

If for some reason the clock is thrown off (e.g. after VM migration), you can force an immediate resync, but makestep just allows the clock to be corrected by step instead of slow slew. You should add the burst command to make a new set of measurements.

7 How to mount a multi-partition disk image in Linux

You can use kpartx or partx to create loop devices for the partitions on the image, and then mount them. So either:

$ sudo kpartx -v -a file.iso
 add map loop0p1 (253:17): 0 8382464 linear 7:1 2048
 $ mount /dev/mapper/loop0p1 ./mnt_point
 ...  do something with the partition  ...
 $ umount ./mnt_point
 $ kpartx -d -v file.iso
 del devmap : loop0p1
 loop deleted : /dev/loop0

or:

$ sudo partx -a -v file.iso
partition: none, disk: file.iso, lower: 0, upper: 0
Trying to use '/dev/loop0' for the loop device
/dev/loop0: partition table type 'dos' detected
range recount: max partno=1, lower=0, upper=0
/dev/loop0: partition #1 added
$ mount /dev/loop0p1 ./mnt_point
...  do something with the partition  ...
$ umount /dev/loop0p1 ./mnt_point
$ sudo partx -d -v /dev/loop0
partition: none, disk: /dev/loop0, lower: 0, upper: 0
/dev/loop0: partition #1 removed

8 DHCP Lease Renewal

Linux Force DHCP Client (dhclient) to Renew IP Address covers this in greater detail for a number of platforms and environments. For KitsNet, the two methods that work are based on dhclient or nmcli

8.1 dhclient

The -r flag explicitly releases the current lease, and once the lease has been released, the client exits. For example, open terminal application and type the command:

$ sudo dhclient -r

Now obtain fresh IP address using DHCP on Linux:

$ sudo dhclient

8.2 nmcli

The NetworkManager daemon attempts to make networking configuration and operation as painless and automatic as possible by managing the primary network connection and other network interfaces, like Ethernet, WiFi, and Mobile Broadband devices command-line tool for controlling NetworkManager. The nmcli is a command-line tool for controlling NetworkManager and getting its status. To renew IP address using nmcli for connection named ‘nixcraft_5G’ (use ‘nmcli con‘ command to get list of all connections):

nmcli con
nmcli con down id 'nixcraft_5G'
nmcli con up id 'nixcraft_5G'
# Attempt to update device with changes to the currently active 
# connection made since it was last applied and then try it again
nmcli device reapply 'nixcraft_5G'