linux – Page 19

systemd find enabled units

July 17, 2019

systemctl list-units | grep monitoring

systemctl list-unit-files | grep enabled

References
https://askubuntu.com/questions/795226/how-to-list-all-enabled-services-from-systemctl

Print without newline in Bash script

July 17, 2019

echo -n "Some string..."

echo -e "Some string...\c"

printf "a line without trailing linefeed"

printf "a line with trailing linefeed\n"

References
https://stackoverflow.com/questions/11193466/echo-without-newline-in-a-shell-script

Best way to test if a systemd service is running in bash script

July 17, 2019

systemctl is-active application.service

systemctl is-enabled application.service

systemctl is-active --quiet service

will exit with status zero if service is active, non-zero otherwise, making it ideal for scripts:

systemctl is-active --quiet service && echo Service is running

References
https://www.digitalocean.com/community/tutorials/how-to-use-systemctl-to-manage-systemd-services-and-units
https://unix.stackexchange.com/questions/396630/the-proper-way-to-test-if-a-service-is-running-in-a-script

Check if a File or Directory Exists in Bash

July 15, 2019

Check if File Exist

FILE=/etc/resolv.conf
if test -f "$FILE"; then
    echo "$FILE exist"
fi

FILE=/etc/resolv.conf
if [ -f "$FILE" ]; then
    echo "$FILE exist"
fi

FILE=/etc/resolv.conf
if [[ -f "$FILE" ]]; then
    echo "$FILE exist"
fi

Check if Directory Exist

FILE=/etc/docker
if [ -d "$FILE" ]; then
    echo "$FILE is a directory"
fi

Check if File does Not Exist

FILE=/etc/docker
if [ ! -f "$FILE" ]; then
    echo "$FILE does not exist"
fi

Check if Multiple Files Exist

FILE=/etc/docker
if [ -f /etc/resolv.conf -a -f /etc/hosts ]; then
    echo "$FILE is a directory"
fi

FILE=/etc/docker
if [ -f /etc/resolv.conf && -f /etc/hosts ]; then
    echo "$FILE is a directory"
fi

File test operators

-f FILE – True if the FILE exists and is a regular file (not a directory or device).
-G FILE – True if the FILE exists and has the same group as the user running the command.
-h FILE – True if the FILE exists and is a symbolic link.
-g FILE – True if the FILE exists and has set-group-id (sgid) flag set.
-k FILE – True if the FILE exists and has a sticky bit flag set.
-L FILE – True if the FILE exists and is a symbolic link.
-O FILE – True if the FILE exists and is owned by the user running the command.
-p FILE – True if the FILE exists and is a pipe.
-r FILE – True if the FILE exists and is readable.
-S FILE – True if the FILE exists and is socket.
-s FILE – True if the FILE exists and has nonzero size.
-u FILE – True if the exists and set-user-id (suid) flag is set.
-w FILE – True if the FILE exists and is writable.
-x FILE – True if the FILE exists and is executable.

References
https://linuxize.com/post/bash-check-if-file-exists/

Using environment variables in systemd units

July 15, 2019

Environment directive

Environment="ONE=one" 'TWO=two two'
ExecStart=/bin/echo $ONE $TWO ${TWO}

EnvironmentFile directive

EnvironmentFile similar to Environment directive but reads the environment variables from a text file. The text file should contain new-line-separated variable assignments.This environment file can then be sourced and its variables used
Example file : /run/metadata/coreos

COREOS_DIGITALOCEAN_IPV4_ANCHOR_0=X.X.X.X
COREOS_DIGITALOCEAN_IPV4_PRIVATE_0=X.X.X.X
COREOS_DIGITALOCEAN_HOSTNAME=test.example.com
COREOS_DIGITALOCEAN_IPV4_PUBLIC_0=X.X.X.X
COREOS_DIGITALOCEAN_IPV6_PUBLIC_0=X:X:X:X:X:X:X:X

[Unit]
Requires=coreos-metadata.service
After=coreos-metadata.service

[Service]
EnvironmentFile=/run/metadata/coreos
ExecStart=
ExecStart=/usr/bin/etcd2 \
  --advertise-client-urls=http://${COREOS_DIGITALOCEAN_IPV4_PUBLIC_0}:2379 \
  --initial-advertise-peer-urls=http://${COREOS_DIGITALOCEAN_IPV4_PRIVATE_0}:2380 \
  --listen-client-urls=http://0.0.0.0:2379 \
  --listen-peer-urls=http://${COREOS_DIGITALOCEAN_IPV4_PRIVATE_0}:2380 \
  --initial-cluster=%m=http://${COREOS_DIGITALOCEAN_IPV4_PRIVATE_0}:2380

References
https://stackoverflow.com/questions/37864999/referencing-other-environment-variables-in-systemd
https://coreos.com/os/docs/latest/using-environment-variables-in-systemd-units.html

How to see full log from systemctl status service

July 15, 2019

journalctl -u service-name.service

Or, to see only log messages for the current boot:

journalctl -u service-name.service -b

References
https://unix.stackexchange.com/questions/225401/how-to-see-full-log-from-systemctl-status-service

Start service after specific service in systemd

July 15, 2019

[Unit]
Description=My Website
After=syslog.target network.target mongodb.service

The important part is the mongodb.service

References
https://stackoverflow.com/questions/21830670/systemd-start-service-after-specific-service

Set the process name for a Java program

July 15, 2019

java -Dname=monitoring_core -jar core.jar

Use the ps Command to Find Running Processes:

ps -e -o pid,comm,command | grep 'monitoring_core'

References
https://stackoverflow.com/questions/1057247/how-can-i-set-the-process-name-for-a-java-program
https://www.tecmint.com/find-process-name-pid-number-linux/
https://linuxhint.com/ps_command_linux/

Configure Multiple Users for Shadowsocks-libev

July 14, 2019

For example, Cloud and Tifa, two AVALANCHE members, are planning to deploy shadowsocks-libev services on the same VPS to bypass the firewall of Shinra Inc. In this case, they could simply create cloud.json and tifa.json configurations with different ports, passwords and encryption methods in /etc/shadowsocks-libev directory. Then enable and start the systemd services using the following commands:

sudo systemctl enable shadowsocks-libev-server@cloud.service --now
sudo systemctl enable shadowsocks-libev-server@tifa.service --now

The status of shadowsocks-libev instances could be checked with the following commands:

sudo systemctl status shadowsocks-libev-server@cloud.service
sudo systemctl status shadowsocks-libev-server@tifa.service

You can start shadowsocks-libev instances with the following commands:

sudo systemctl start shadowsocks-libev-server@cloud.service
sudo systemctl start shadowsocks-libev-server@tifa.service

References
https://blog.zzhou612.com/2019/02/15/configure-multiple-users-for-shadowsocks-libev/

Check if a command succeeded on Bash

July 14, 2019

The return value is stored in $?. 0 indicates success, others indicates error.

some_command
if [ $? -eq 0 ]; then
    echo OK
else
    echo FAIL

some_command
retval=$?
do_something $retval
if [ $retval -ne 0 ]; then
    echo "Return code was not zero but $retval"
fi

References
https://askubuntu.com/questions/29370/how-to-check-if-a-command-succeeded