IP Subnetting questions for IPv4
http://subnettingquestions.com/
IT Consultant
Network Consultant
www.inCOREporation.com
Wednesday, March 20, 2019
Subnet Mask Cheat Sheet
Subnet Mask Cheat Sheet
according to :
https://www.aelius.com/njh/subnet_sheet.html
Subnet Mask Cheat Sheet
See also RFC 1878.| Addresses | Hosts | Netmask | Amount of a Class C | |
|---|---|---|---|---|
| /30 | 4 | 2 | 255.255.255.252 | 1/64 |
| /29 | 8 | 6 | 255.255.255.248 | 1/32 |
| /28 | 16 | 14 | 255.255.255.240 | 1/16 |
| /27 | 32 | 30 | 255.255.255.224 | 1/8 |
| /26 | 64 | 62 | 255.255.255.192 | 1/4 |
| /25 | 128 | 126 | 255.255.255.128 | 1/2 |
| /24 | 256 | 254 | 255.255.255.0 | 1 |
| /23 | 512 | 510 | 255.255.254.0 | 2 |
| /22 | 1024 | 1022 | 255.255.252.0 | 4 |
| /21 | 2048 | 2046 | 255.255.248.0 | 8 |
| /20 | 4096 | 4094 | 255.255.240.0 | 16 |
| /19 | 8192 | 8190 | 255.255.224.0 | 32 |
| /18 | 16384 | 16382 | 255.255.192.0 | 64 |
| /17 | 32768 | 32766 | 255.255.128.0 | 128 |
| /16 | 65536 | 65534 | 255.255.0.0 | 256 |
Guide to sub-class C blocks
/25 -- 2 Subnets -- 126 Hosts/Subnet
|
/30 -- 64 Subnets -- 2 Hosts/Subnet
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/26 -- 4 Subnets -- 62 Hosts/Subnet
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/27 -- 8 Subnets -- 30 Hosts/Subnet
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
/28 -- 16 Subnets -- 14 Hosts/Subnet
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
/29 -- 32 Subnets -- 6 Hosts/Subnet
|
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Saturday, March 16, 2019
Linux cron
Linux cron jobs
as per
https://www.cyberciti.biz/faq/how-do-i-add-jobs-to-cron-under-linux-or-unix-oses/
How do I add cron job under Linux or UNIX like operating system?
Cron allows Linux and Unix users to run commands or scripts at a given date and time. You can schedule scripts to be executed periodically. Cron is one of the most useful tool in a Linux or UNIX like operating systems. It is usually used for sysadmin jobs such as backups or cleaning /tmp/ directories and more. The cron service (daemon) runs in the background and constantly checks the /etc/crontab file, and /etc/cron.*/ directories. It also checks the /var/spool/cron/ directory.
www.inCOREporation.com
as per
https://www.cyberciti.biz/faq/how-do-i-add-jobs-to-cron-under-linux-or-unix-oses/
How To Add Jobs To cron Under Linux or UNIX
last updated in Commands, Linux, UNIXCron allows Linux and Unix users to run commands or scripts at a given date and time. You can schedule scripts to be executed periodically. Cron is one of the most useful tool in a Linux or UNIX like operating systems. It is usually used for sysadmin jobs such as backups or cleaning /tmp/ directories and more. The cron service (daemon) runs in the background and constantly checks the /etc/crontab file, and /etc/cron.*/ directories. It also checks the /var/spool/cron/ directory.
crontab command
You need to use the crontab command to edit/create, install, deinstall or list the cron jobs in Vixie Cron. Each user can have their own crontab file, and though these are files in /var/spool/cron/crontabs, they are not intended to be edited directly. You need to use crontab command for editing or setting up your own cron jobs.
Types of cron configuration files
There are different types of configuration files:
- The UNIX / Linux system crontab : Usually, used by system services and critical jobs that requires root like privileges. The sixth field (see below for field description) is the name of a user for the command to run as. This gives the system crontab the ability to run commands as any user.
- The user crontabs: User can install their own cron jobs using the crontab command. The sixth field is the command to run, and all commands run as the user who created the crontab
Note: This faq features cron implementations written by Paul Vixie and included in many Linux distributions and Unix like systems such as in the popular 4th BSD edition. The syntax is compatible with various implementations of crond.
How Do I install or create or edit my own cron jobs?
To edit or create your own crontab file, type the following command at the UNIX / Linux shell prompt:
$ crontab -eDo I have to restart cron after changing the crontable file?
No. Cron will examine the modification time on all crontabs and reload those which have changed. Thus cron need not be restarted whenever a crontab file is modified.
Syntax of crontab (field description)
The syntax is:
OR
Where,
- 1: Minute (0-59)
- 2: Hours (0-23)
- 3: Day (0-31)
- 4: Month (0-12 [12 == December])
- 5: Day of the week(0-7 [7 or 0 == sunday])
- /path/to/command – Script or command name to schedule
Easy to remember format:
* * * * * command to be executed - - - - - | | | | | | | | | ----- Day of week (0 - 7) (Sunday=0 or 7) | | | ------- Month (1 - 12) | | --------- Day of month (1 - 31) | ----------- Hour (0 - 23) ------------- Minute (0 - 59)
Your cron job looks as follows for system jobs:
1 2 3 4 5 USERNAME /path/to/command arg1 arg2
OR
1 2 3 4 5 USERNAME /path/to/script.sh
Example: Run backup cron job script
If you wished to have a script named /root/backup.sh run every day at 3am, your crontab entry would look like as follows. First, install your cronjob by running the following command:
Append the following entry:
Save and close the file.
# crontab -eAppend the following entry:
0 3 * * * /root/backup.shSave and close the file.
MORE EXAMPLES
To run /path/to/command five minutes after midnight, every day, enter:
Run /path/to/script.sh at 2:15pm on the first of every month, enter:
Run /scripts/phpscript.php at 10 pm on weekdays, enter:
Run /root/scripts/perl/perlscript.pl at 23 minutes after midnight, 2am, 4am …, everyday, enter:
Run /path/to/unixcommand at 5 after 4 every Sunday, enter:
5 0 * * * /path/to/commandRun /path/to/script.sh at 2:15pm on the first of every month, enter:
15 14 1 * * /path/to/script.shRun /scripts/phpscript.php at 10 pm on weekdays, enter:
0 22 * * 1-5 /scripts/phpscript.phpRun /root/scripts/perl/perlscript.pl at 23 minutes after midnight, 2am, 4am …, everyday, enter:
23 0-23/2 * * * /root/scripts/perl/perlscript.plRun /path/to/unixcommand at 5 after 4 every Sunday, enter:
5 4 * * sun /path/to/unixcommandHow do I use operators?
An operator allows you to specifying multiple values in a field. There are three operators:
- The asterisk (*) : This operator specifies all possible values for a field. For example, an asterisk in the hour time field would be equivalent to every hour or an asterisk in the month field would be equivalent to every month.
- The comma (,) : This operator specifies a list of values, for example: “1,5,10,15,20, 25”.
- The dash (-) : This operator specifies a range of values, for example: “5-15” days , which is equivalent to typing “5,6,7,8,9,….,13,14,15” using the comma operator.
- The separator (/) : This operator specifies a step value, for example: “0-23/” can be used in the hours field to specify command execution every other hour. Steps are also permitted after an asterisk, so if you want to say every two hours, just use */2.
How do I disable email output?
By default the output of a command or a script (if any produced), will be email to your local email account. To stop receiving email output from crontab you need to append >/dev/null 2>&1. For example:
To mail output to particular email account let us say vivek@nixcraft.in you need to define MAILTO variable as follows:
See “Disable The Mail Alert By Crontab Command” for more information.
0 3 * * * /root/backup.sh >/dev/null 2>&1To mail output to particular email account let us say vivek@nixcraft.in you need to define MAILTO variable as follows:
MAILTO="vivek@nixcraft.in"
0 3 * * * /root/backup.sh >/dev/null 2>&1See “Disable The Mail Alert By Crontab Command” for more information.
Task: List all your cron jobs
Type the following command:
To remove or erase all crontab jobs use the following command:
# crontab -l
# crontab -u username -lTo remove or erase all crontab jobs use the following command:
# Delete the current cron jobs #
crontab -r## Delete job for specific user. Must be run as root user ##
crontab -r -u usernameUse special string to save time
Instead of the first five fields, you can use any one of eight special strings. It will not just save your time but it will improve readability.
| Special string | Meaning |
| @reboot | Run once, at startup. |
| @yearly | Run once a year, “0 0 1 1 *”. |
| @annually | (same as @yearly) |
| @monthly | Run once a month, “0 0 1 * *”. |
| @weekly | Run once a week, “0 0 * * 0”. |
| @daily | Run once a day, “0 0 * * *”. |
| @midnight | (same as @daily) |
| @hourly | Run once an hour, “0 * * * *”. |
Examples
Run ntpdate command every hour:
Make a backup everyday:
@hourly /path/to/ntpdateMake a backup everyday:
@daily /path/to/backup/script.shMore about /etc/crontab file and /etc/cron.d/* directories
/etc/crontab is system crontabs file. Usually only used by root user or daemons to configure system wide jobs. All individual user must must use crontab command to install and edit their jobs as described above. /var/spool/cron/ or /var/cron/tabs/ is directory for personal user crontab files. It must be backup with users home directory.
Understanding Default /etc/crontab
Typical /etc/crontab file entries:
SHELL=/bin/bash PATH=/sbin:/bin:/usr/sbin:/usr/bin MAILTO=root HOME=/ # run-parts 01 * * * * root run-parts /etc/cron.hourly 02 4 * * * root run-parts /etc/cron.daily 22 4 * * 0 root run-parts /etc/cron.weekly 42 4 1 * * root run-parts /etc/cron.monthly
First, the environment must be defined. If the shell line is omitted, cron will use the default, which is sh. If the PATH variable is omitted, no default will be used and file locations will need to be absolute. If HOME is omitted, cron will use the invoking users home directory.
Additionally, cron reads the files in /etc/cron.d/ directory. Usually system daemon such as sa-update or sysstat places their cronjob here. As a root user or superuser you can use following directories to configure cron jobs. You can directly drop your scripts here. The run-parts command run scripts or programs in a directory via /etc/crontab file:
| Directory | Description |
| /etc/cron.d/ | Put all scripts here and call them from /etc/crontab file. |
| /etc/cron.daily/ | Run all scripts once a day |
| /etc/cron.hourly/ | Run all scripts once an hour |
| /etc/cron.monthly/ | Run all scripts once a month |
| /etc/cron.weekly/ | Run all scripts once a week |
How do I use above directories to put my own scripts or jobs?
Here is a sample shell script called clean.cache. This script is created to clean up cached files every 10 days. This script is directly created at /etc/cron.daliy/ directory. In other words create a text file called /etc/cron.daily/clean.cache as follows.
Save and close the file. Set the permissions:
# chmod +x /etc/cron.daily/clean.cacheHow do I backup installed cron jobs entries?
Simply type the following command to backup your cronjobs to a nas server mounted at /nas01/backup/cron/users.root.bakup directory:
# crontab -l > /nas01/backup/cron/users.root.bakup
# crontab -u userName -l > /nas01/backup/cron/users.userName.bakupSee also
- See man pages for more information run-parts(8)
This entry is 2 of 15 in the Linux Cron Jobs Howto & Tutorial series. Keep reading the rest of the series:
- What is cron on a Linux or Unix-like systems?
- HowTo: Add Jobs To cron Under Linux or UNIX?
- Linux Verify crond Daemon And Cronjobs Are Running
- Linux Start Restart and Stop The Cron or Crond Service
- Linux: List / Display All Cron Jobs
- Linux / UNIX Crontab File Location
- Linux / UNIX: Change Crontab Email Settings ( MAILTO )
- Disable The Mail Alert By Crontab Command On a Linux or Unix-like Systems
- Linux: At What Time Cron Entries In cron.daily, cron.weekly, cron.monthly Run?
- Linux Execute Cron Job After System Reboot
- Linux / UNIX Setup and Run PHP Script As A Cron Job
- How to run crontab job every minute on a Linux or Unix-like system
- Run crontab (cron jobs) Every 10 Minutes
- Cron Job Script Execution on the Last Day of a Month
- Run crontab (cron jobs) every 1 minute
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Linux shell scripting for cron jobs
Linux shell scripting for cron jobs
List available shells
cat etc/shells
Where is bash located
which bash
Create script file
touch clea-ram.sh
#! /bin/bash
su -c "echo 1 >'/proc/sys/vm/drop_caches' && swapoff -a && swapon -a && printf '\n%s\n' 'Ram-cache and Swap Cleared'" root
to save file in nano
Ctrl + x
yes
enter
cat to see content
chmod +x clear-ram.sh
We want these permissions
-rwxrwxr-x
or
Running cron jobs from crontab -e)
add this line to crontab
0 3 * * * /home/scripts/clear-ram.sh
crontab -l to view
As per video
IT Consultant
www.inCOREporation.com
List available shells
cat etc/shells
Where is bash located
which bash
Create script file
touch clea-ram.sh
#! /bin/bash
su -c "echo 1 >'/proc/sys/vm/drop_caches' && swapoff -a && swapon -a && printf '\n%s\n' 'Ram-cache and Swap Cleared'" root
to save file in nano
Ctrl + x
yes
enter
cat to see content
chmod +x clear-ram.sh
We want these permissions
-rwxrwxr-x
or
chmod 775 clear-ram.sh
Running cron jobs from crontab -e)
add this line to crontab
0 3 * * * /home/scripts/clear-ram.sh
crontab -l to view
As per video
IT Consultant
www.inCOREporation.com
Linux command find largest file size recursively
Linux command find largest file size recursively
www.inCOREporation.com
sudo du -a | sort -n -r | head -n 20
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linux command search find all same file extension recursively
linux command search find all same file extension recursively
LOCATE linux command
common video file extension
other userful tool
ffmpeg tool to convert video formats and edit video audio attributes
LOCATE linux command
locate -i '*.flv'
common video file extension
Some popular types of “containers” are AVI, FLV, WMV, MP4, and MOV.
- AVI (Audio Video Interleave) ...
- FLV (Flash Video Format) ...
- WMV (Windows Media Video) ...
- MOV (Apple QuickTime Movie) ...
- MP4 (Moving Pictures Expert Group 4)
other userful tool
ffmpeg tool to convert video formats and edit video audio attributes
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