Step 1: Follow the below commands one by one to uninstall libre office.
$sudo apt-get remove --purge libreoffice*
$sudo apt-get clean
$sudo apt-get autoremove
Step 2: To remove the menu options, these following steps will help us.
#cd /home/<username>/.local/share/applications/Menus will disappear after this.
The cron utility runs based on commands specified in a cron table (crontab)
Each user, including root, can have a cron file. These files don't exist by default, but can be created in the /var/spool/cron directory using the crontab -e command that's also used to edit a cron file (see the script below). I strongly recommend that you not use a standard editor (such as Vi, Vim, Emacs, Nano, or any of the many other editors that are available). Using the crontab command not only allows you to edit the command, it also restarts the crond daemon when you save and exit the editor.
The crontab command uses Vi as its underlying editor, because Vi is always present (on even the most basic of installations).
crontab -e
SHELL=/bin/bash
MAILTO=root@example.com
PATH=/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin
# Example of job definition:
# .---------------- minute (0 - 59)
# | .------------- hour (0 - 23)
# | | .---------- day of month (1 - 31)
# | | | .------- month (1 - 12) OR jan,feb,mar,apr ...
# | | | | .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# | | | | |
# * * * * * user-name command to be executed
# Set the hardware clock to keep it in sync with the more accurate system clock
03 05 * * * /sbin/hwclock --systohc
# Perform monthly updates on the first of the month
# 25 04 1 * * /usr/bin/dnf -y update
Task: List all your cron jobs
Type the following command:
# crontab -l
# crontab -u username -l
To 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 username
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
crond and cron jobs log file
You can use the cat command/grep command/tail command to view crond log file. For example, on a CentOS 7 Linux on can use the following commands:
cat /var/log/cron
tail -f /var/log/cron
grep "my-script.sh"
tail -f /var/log/cron
On modern Linux distro one can use the systemctl command or journalctl command:
sudo systemctl status cron
sudo journalctl -u cron
how to block not to run cron jobs for user:
Add user name into file /etc/cron. deny, each user per line (Typical method which affect only listed users in the file). The other easy workaround is to have the /etc/cron. deny file empty and add only root user name in to the file /etc/cron.
https://www.thegeekdiary.com/how-to-prevent-non-root-user-from-creating-crontab-entry-in-linux/
File permissions:
https://www.linux.com/training-tutorials/understanding-linux-file-permissions/
u – Owner
g – Group
o – Others
a – All users
r – Read
w – Write
x – Execute
chmod u+rw file1
Using Binary References to Set permissions
A sample permission string would be chmod 640 file1, which means that the owner has read and write permissions, the group has read permissions, and all other user have no rights to the file.
r = 4
w = 2
x = 1
SUID, SGID and sticky bit
https://www.thegeekdiary.com/linux-interview-questions-special-permissions-suid-sgid-and-sticky-bit/
There are two special permissions that can be set on executable files:
Set User ID (setuid) and Set Group ID (sgid). These permissions allow the file being executed to be executed with the privileges of the owner or the group.
Similarly, there are two special permissions for directories: the sticky bit and the setgid bit.
What is Set User ID (setuid)
SUID is a special permission assigned to a file. These permissions allow the file being executed to be executed with the privileges of the owner. For example, if a file was owned by the root user and has the setuid bit set, no matter who executed the file it would always run with root user privileges.
How to set SUID bit on a file:
You must be the owner of the file or the root user to set the setuid bit.
Run the following command to set the setuid bit:
# chmod u+s file1
# verify the file permission
# ls -ld fil2
#root@oc1784140854 Filepermissions]# touch file1
#root@oc1784140854 Filepermissions]# ls -lrt
-rw-r--r-- 1 root root 0 Jun 17 13:58 file1
#root@oc1784140854 Filepermissions]# chmod u+s file1
[root@oc1784140854 Filepermissions]# ls -l file1
-rwSr--r-- 1 root root 0 Jun 17 13:58 file1
Note the capital S. This means there are no execute permissions. Run the following command to add execute permissions to the file1 file, noting the lower case s.
# [root@oc1784140854 Filepermissions]# chmod u+x file1
[root@oc1784140854 Filepermissions]# ls -ld file1
-rwsr--r-- 1 root root 0 Jun 17 13:58 file1
Alternatively, you can set the setuid bit using the numeric method by prepending a 4 to the mode. For example, to set the setuid bit, read, write, and execute permissions for the owner of the file1 file, run the following command:
# chmod 4700 file1
What is Set Group ID (setgid) for files
When the Set Group ID bit is set, the executable is run with the authority of the group. For example, if a file was owned by the users’ group, no matter who executed that file it would always run with the authority of the user’s group.
How to set the SGID bit for files?
Run the following command as to set the setgid bit on the file1 file:
# chmod g+s
Note: Both the setuid and setgid bits are set using the s symbol. The setgid is represented the same as the setuid bit, except in the group section of the permissions.
[root@oc1784140854 Filepermissions]# chmod g+s file1
[root@oc1784140854 Filepermissions]# ls -l file1
-rwsr-Sr-- 1 root root 0 Jun 17 13:58 file1
Run the following command as root to set the setgid bit, and read, write, and execute permissions for the owner of the file1 file:
[root@oc1784140854 Filepermissions]# chmod 2700 file1
[root@oc1784140854 Filepermissions]# ls -l file1
-rwx--S--- 1 root root 0 Jun 17 13:58 file1
#Use the chmod g+s command to set the setgid bit.
How to set GUID on a file?
# chmod 2555 [path_to_file]
(base) [root@oc1784140854 Filepermissions]# chmod 2555 file1
(base) [root@oc1784140854 Filepermissions]# ls -l file1
-r-xr-sr-x 1 root root 0 Jun 17 13:58 file1
What is Set Group ID permissions for directories
When the setgid bit is set on a directory, all files created within said directory inherit the group ownership of that directory. For example, the folder1 folder is owned by the user user1, and the group group1
[root@oc1784140854 Filepermissions]# mkdir folder1
[root@oc1784140854 Filepermissions]# chmod 664 folder1/
[root@oc1784140854 Filepermissions]# cd folder1/
[root@oc1784140854 folder1]# touch file1
[root@oc1784140854 folder1]# pwd
/opt/Filepermissions/folder1
[root@oc1784140854 folder1]# chmod 664 file1
How to set the SGID bit for directories?
To set the setgid bit on a directory, use the chmod g+s command:
(base) [root@oc1784140854 Filepermissions]# chmod g+s folder1
(base) [root@oc1784140854 Filepermissions]# ls -ld folder1/
drw-rwSr-- 2 root root 4096 Jun 17 14:19 folder1/
(base) [root@oc1784140854 Filepermissions]#
change the S – to small s – use this command – chmod g+x folder1
(base) [root@oc1784140854 Filepermissions]# chmod g+x folder1
(base) [root@oc1784140854 Filepermissions]# ls -ld folder1
drw-rwsr-- 2 root root 4096 Jun 17 14:19 folder1
(base) [root@oc1784140854 Filepermissions]#
Alternatively, prepend a 2 to the directories mode:
# chmod 2770 folder1
What is sticky bit on a directory
When the sticky bit is set on a directory, only the root user, the owner of the directory, and the owner of a file can remove files within said directory.
Eventhouh has write permissions
An example of the sticky bit is the /tmp directory. Use the ls -ld /tmp command to view the permissions:
[root@oc1784140854 Filepermissions]# ls -ld /tmp/
drwxrwxrwt 25 root root 640 Jun 17 14:29 /tmp/
[root@oc1784140854 Filepermissions]#
The t at the end symbolizes that the sticky bit is set. A file created in the /tmp directory can only be removed by its owner, or the root user. For example, run the following command to set the sticky bit on the folder1 folder:
[root@oc1784140854 Filepermissions]# chmod a+t folder1/
[root@oc1784140854 Filepermissions]# ls -ld folder1/
drw-rwsr-T 2 root root 4096 Jun 17 14:19 folder1/
[root@oc1784140854 Filepermissions]# chmod a+x folder1
[root@oc1784140854 Filepermissions]# ls -ld folder1
drwxrwsr-t 2 root root 4096 Jun 17 14:19 folder1
[root@oc1784140854 Filepermissions]#
Alternatively, prepend a 1 to the mode of a directory to set the sticky bit:
# chmod 1777 folder1
how to find files with SUID/SGID but set
1. To find all the files with SUID but set, use the below command :
# find / -perm /4000
2. Tofind all the files with SGID bit set, use the below command :
# find / -perm /2000
how to find out sticky bit directories
find / -type d -perm -1000 -exec ls -ld {} \;
