Category: Linux

Install Tomcat on CentOS 7

Apache Tomcat is an open source implementation of the Java Servlet and Java Server Pages. To install Apache Tomcat on CentOS 7, run

yum install tomcat

To enable tomcat start on boot

systemctl enable tomcat

You can manage tomcat with

systemctl stop tomcat
systemctl start tomcat
systemctl status tomcat
systemctl restart tomcat

To see the ports used by tomcat

[root@tomcat ~]# netstat -lntp| grep java
tcp6       0      0 127.0.0.1:8005          :::*                    LISTEN      31423/java          
tcp6       0      0 :::8009                 :::*                    LISTEN      31423/java          
tcp6       0      0 :::8080                 :::*                    LISTEN      31423/java          
[root@tomcat ~]#

webapps are stored in the directory

/var/lib/tomcat/webapps

Create the default page with

mkdir /var/lib/tomcat/webapps/ROOT
echo "Hello Cat" > /var/lib/tomcat/webapps/ROOT/index.html

Tomcat web server can be accessed using

http://your-server-ip:8080

To open 8080 port on the firewall, use commands

firewall-cmd --permanent --zone=public --add-port=8080/tcp
firewall-cmd --reload

Tomcat configurations are available in the directory.

/etc/tomcat/

Install tomcat Manager

To install the Tomcat manager GUI application, run

yum install tomcat-admin-webapps.noarch -y

To create user, edit file

vi /etc/tomcat/tomcat-users.xml

Inside ““, add

USER_NAME_HERE and PW_HERE – replace with the username and password you need.

Restart tomcat to activate Tomcat Manager GUI.

systemctl restart tomcat

To access GUI, go to

http://your-server-ip-here:8080/manager/

November 2, 2021

How to change Node Exporter port

Node Exporter by default uses port 9100. You may need to use another port for Node Exporter if port 9100 is already used by another program on your server. On an OVH VPS, noderig was running in port 9100 and Node Exporter failed to start. To fix the issue, i had to run Node Exporter on a different port.

To change the Node Exporter port, edit the file

/etc/systemd/system/node_exporter.service

Find line

ExecStart=/usr/local/bin/node_exporter

Replace it with

ExecStart=/usr/local/bin/node_exporter --web.listen-address=:9101

In the above line, 9101 is the new Node Exporter port. You can change it to any other port as required.

Here is the complete code

root@ns568267:~# cat /etc/systemd/system/node_exporter.service
[Unit]
Description=Node Exporter
Wants=network-online.target
After=network-online.target

[Service]
User=node_exporter
Group=node_exporter
Type=simple
ExecStart=/usr/local/bin/node_exporter --web.listen-address=:9101

[Install]
WantedBy=multi-user.target
root@ns568267:~#

Reload systemd

systemctl daemon-reload

Restart node exporter

systemctl start node_exporter

You can verify node_exporter is running with the command

root@ns568267:~# netstat -lntp | grep node_
tcp6       0      0 :::9101                 :::*                    LISTEN      15630/node_exporter 
root@ns568267:~#

November 2, 2021

top
top command is used to see current system resource usage

To show the current process
```
top -n 1 -b
```
Top sorted by memory
```
top -b -n 1 -o %MEM | sort -nk 6
```
Delay 3 second
```
top -cd3
```
Back to Linux Commands
November 2, 2021
ack show the matched files name only
ack is a command-line utility written in Perl for searching strings inside files. By default when you search, it shows you the file name along with the matching content of the file. Sometimes you may only want to list file names that match your search. This can be done with -l option
```
ack -l "search string here"
```
Example

See ack
October 30, 2021
Install .NET SDK on Ubuntu 20.04
Microsoft .NET SDK and Run time now available for Linux. To install .NET SDK and runtime on Ubuntu 20.04, follow the instructions below

Run the following commands as user root. If you are logged in as a normal user, use the command “sudo su” to become user root.
```
wget https://packages.microsoft.com/config/ubuntu/20.04/packages-microsoft-prod.deb -O packages-microsoft-prod.deb
dpkg -i packages-microsoft-prod.deb
rm packages-microsoft-prod.deb
```
Update the repo and install apt-transport-https
```
apt-get update
apt-get install -y apt-transport-https
```
Search for available dotnet-sdk versions
```
apt-cache search dotnet-sdk
```
On Ubuntu 20.04, Microsoft provides .NET SDK version 2.1, 3.1, and 5. Let’s install version 5.
```
apt-get install -y dotnet-sdk-5.0
```
To find the version of .NET SDK, run
```
dotnet --version
```
You can find .NET SDK and runtime available with commands
```
dotnet --list-sdks
dotnet --list-runtimes
```
October 29, 2021
Enable/Disable Nextcloud Maintenance Mode
Maintenance mode is useful when you are making changes to your nextcloud installation like a software upgrade.

To enable maintenance mode, run the command
```
php7.3 occ maintenance:mode --on
```
To disable maintenance mode, run
```
php7.3 occ maintenance:mode --off
```
Example
```
www-data@mail:/www/nextcloud$ php occ maintenance:mode --off
Maintenance mode disabled
www-data@mail:/www/nextcloud$ 
```
See Nextcloud
October 28, 2021
Module php-imagick in this instance has no SVG support
On Ubuntu server running Nextcloud, I got the following warning in Nextcloud
```
Module php-imagick in this instance has no SVG support. For better compatibility it is recommended to install it.
```
To fix this warning, install libmagickcore-6.q16-6-extra package with the command
```
apt-get install libmagickcore-6.q16-6-extra
```
See Nextcloud
October 28, 2021

chattr: Operation not supported while reading flags

When making a file immutable with chattr command, I get the following error

[root@vps784847 vhosts]# chattr +i serverok.in.conf 
chattr: Operation not supported while reading flags on serverok.in.conf
[root@vps784847 vhosts]#

The error was due to the file I was trying to chattr being a symlink, not a real file.

[root@vps784847 vhosts]# ls -la
total 0
drwxr-xr-x. 2 root root  31 Oct 27 08:11 .
drwxr-xr-x. 7 root root 131 Jun  1 02:03 ..
lrwxrwxrwx. 1 root root  51 Jul 24 12:39 serverok.in.conf -> /var/www/vhosts/system/serverok.in/conf/nginx.conf
[root@vps784847 vhosts]#

To fix the error, chattr the actual file.

chattr +i /var/www/vhosts/system/serverok.in/conf/nginx.conf

Back to Linux Commands

October 27, 2021

Checking NVMe Disk

You can use nvme-cli to check your NVMe disks. To install nvme-cli, run

On Ubuntu/Debian

sudo apt-get install -y nvme-cli

On RHEL

sudo yum install nvme-cli

To list of NVMe drives, use

nvme list

Example

[root@server52 ~]# nvme list
Node             SN                   Model                                    Namespace Usage                      Format           FW Rev  
---------------- -------------------- ---------------------------------------- --------- -------------------------- ---------------- --------
/dev/nvme0n1     S439NA0N900171       SAMSUNG MZQLB1T9HAJR-00007               1         531.97  GB /   1.92  TB    512   B +  0 B   EDA5402Q
/dev/nvme1n1     S439NA0N900057       SAMSUNG MZQLB1T9HAJR-00007               1         532.00  GB /   1.92  TB    512   B +  0 B   EDA5402Q
[root@server52 ~]#

To find smart data, run

nvme smart-log DEVICE_NAME_HERE

Example

[root@server52 ~]# nvme smart-log /dev/nvme0n1
Smart Log for NVME device:nvme0n1 namespace-id:ffffffff
critical_warning                    : 0
temperature                         : 39 C
available_spare                     : 100%
available_spare_threshold           : 10%
percentage_used                     : 0%
data_units_read                     : 130,334,241
data_units_written                  : 69,361,921
host_read_commands                  : 866,547,209
host_write_commands                 : 710,448,251
controller_busy_time                : 998
power_cycles                        : 88
power_on_hours                      : 5,464
unsafe_shutdowns                    : 83
media_errors                        : 0
num_err_log_entries                 : 0
Warning Temperature Time            : 0
Critical Composite Temperature Time : 0
Temperature Sensor 1                : 39 C
Temperature Sensor 2                : 43 C
Temperature Sensor 3                : 47 C
Thermal Management T1 Trans Count   : 0
Thermal Management T2 Trans Count   : 0
Thermal Management T1 Total Time    : 0
Thermal Management T2 Total Time    : 0
[root@server52 ~]#

To see the NVMe error log

nvme error-log	DEVICE_NAME_HERE

October 27, 2021

mount: unknown filesystem type ‘LVM2_member’.

When I try to mount a partition, I get the following error

[root@sysresccd ~]# mount /dev/nvme0n1p3 /mnt
mount: /mnt: unknown filesystem type 'LVM2_member'.
[root@sysresccd ~]#

This is because the disk is LVM. First of all identify the name of the volume group and logical volume.

vgs
lvs

Example

[root@sysresccd ~]# vgs
  VG #PV #LV #SN Attr   VSize  VFree
  vg   2   3   0 wz--n- <1.82t    0 
[root@sysresccd ~]# lvs
  LV   VG Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  root vg -wi-a-----   1.80t                                                    
  swap vg -wi-a----- <15.69g                                                    
  tmp  vg -wi-a-----   1.00g                                                    
[root@sysresccd ~]#

From the above result, we found the server has 3 logical volumes. The biggest volume is root. Other volumes swap and tmp we can ignore. So the device name is

/dev/vg/root

Next run

modprobe dm-mod
lvscan
vgchange -ay

Example

[root@sysresccd ~]# lvscan
  ACTIVE            '/dev/vg/swap' [<15.69 GiB] inherit
  ACTIVE            '/dev/vg/tmp' [1.00 GiB] inherit
  ACTIVE            '/dev/vg/root' [1.80 TiB] inherit
[root@sysresccd ~]# vgchange -ay
  3 logical volume(s) in volume group "vg" now active
[root@sysresccd ~]#

Now you should be able to mount with disk using command

mount /dev/vg/root /mnt/

October 26, 2021

Delayed block allocation failed for inode

On a CentOS server, nginx web server stopped serving files. On checking logs, I found the following error message

2021/10/25 19:08:35 [crit] 42762#0: *78977627 pread() "/var/www/html/995.mp4" failed (5: Input/output error) while sending response to client, client: 59.92.71.53, server: 158.58.173.92, request: "GET /995.mp4 HTTP/1.1", host: "domain.tld"

I tried to copy the file, copy filed with error message “Input/output error”

[root@mta1 html]# cp /var/www/html/995.mp4 ~/
cp: error reading '/var/www/html/995.mp4': Input/output error
[root@mta1 html]#

On /var/log/messages, I had the following error message

Oct 25 19:19:03 mta1 kernel: EXT4-fs error (device dm-0): ext4_wait_block_bitmap:520: comm kworker/u32:2: Cannot read block bitmap - block_group = 7705, block_bitmap = 252182537
Oct 25 19:19:03 mta1 kernel: EXT4-fs (dm-0): Delayed block allocation failed for inode 39976964 at logical offset 33810 with max blocks 14 with error 5
Oct 25 19:19:03 mta1 kernel: EXT4-fs (dm-0): This should not happen!! Data will be lost

The problem is fixed by rebooting the server into rescue mode, then running the fsck command.

[root@sysresccd ~]# fsck -y /dev/vg/root
fsck from util-linux 2.35.2
e2fsck 1.45.6 (20-Mar-2020)
/dev/mapper/vg-root contains a file system with errors, check forced.
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Unconnected directory inode 61210647 (/var/cache/dnf/epel-modular-95d9a0c53e492cbd/???)
Connect to /lost+found? yes

Unconnected directory inode 61210656 (/var/cache/dnf/extras-2770d521ba03e231/???)
Connect to /lost+found? yes

Unconnected directory inode 61210668 (/var/cache/dnf/epel-6519ee669354a484/???)
Connect to /lost+found? yes

Pass 4: Checking reference counts
Inode 39845944 ref count is 1, should be 2.  Fix? yes

...

Unattached inode 61210637
Connect to /lost+found? yes

Inode 61210637 ref count is 2, should be 1.  Fix? yes

Inode 61210641 ref count is 3, should be 2.  Fix? yes

Directories count wrong for group #7474 (5, counted=0).
Fix? yes

Free inodes count wrong (120894192, counted=120894225).
Fix? yes

Padding at end of inode bitmap is not set. Fix? yes


/dev/mapper/vg-root: ***** FILE SYSTEM WAS MODIFIED *****
/dev/mapper/vg-root: 44271/120938496 files (2.2% non-contiguous), 235802671/483741696 blocks
[root@sysresccd ~]#

See fsck

October 26, 2021

qdbus: not found

When running a program Ubuntu server with XFCE, I get the error

ubuntu@ubuntu:~$ ~/Downloads/grab-song-master/check-media-players.sh
/home/ubuntu/Downloads/grab-song-master/check-media-players.sh: 3: qdbus: not found
ubuntu@ubuntu:~$

The error is fixed by installing qbus

apt-get install qdbus

On CentOS, you need to install qdbus-qt5 package

yum install qdbus-qt5

October 26, 2021