How To Check If A Service Exists In Bash On CentOS And Ubuntu?

Using systemctl command

The systemctl command checks service status on Linux systems using systemd. To check if a service exists, use this syntax:

systemctl list-units --full -all | grep -Fq "servicename.service"

This command lists all units and searches for the service. If it returns a zero exit status, the service exists. You can check the exit code:

if [ $? -eq 0 ]; then
    echo "Service exists"
else
    echo "Service does not exist"
fi

systemctl list-units --full -all | grep -Fq "service name with spaces.service"

Using the service command

The service command works on both systemd and SysV init systems. To check if a service exists:

service servicename status

This command gets the status of the service. If it exists, it returns information about its status. If not, you'll see an error message. In a script:

if service servicename status >/dev/null 2>&1; then
    echo "Service exists"
else
    echo "Service does not exist"
fi

The service command works with CentOS and Ubuntu, making it useful for scripts that run on different distributions.

Checking system files and directories

You can check for a service by looking at system files and directories:

1. Examining /etc/init.d directory: For SysV init systems, check if a service script exists in the /etc/init.d directory:

   if [ -f "/etc/init.d/servicename" ]; then
      echo "Service exists"
   else
      echo "Service does not exist"
   fi

2. Looking for service unit files in systemd: For systemd, check for service unit files in the systemd directories:

   if [ -f "/etc/systemd/system/servicename.service" ] || [ -f "/usr/lib/systemd/system/servicename.service" ]; then
      echo "Service exists"
   else
      echo "Service does not exist"
   fi

These methods offer different ways to check for service existence, each with advantages depending on the Linux distribution and init system in use.

check_service() {
    if systemctl list-units --full -all | grep -Fq "$1.service"; then
        return 0
    elif service "$1" status >/dev/null 2>&1; then
        return 0
    elif [ -f "/etc/init.d/$1" ] || [ -f "/etc/systemd/system/$1.service" ] || [ -f "/usr/lib/systemd/system/$1.service" ]; then
        return 0
    else
        return 1
    fi
}

if check_service "apache2"; then
    echo "Apache2 service exists"
else
    echo "Apache2 service does not exist"
fi

Simple one-line command using systemctl

A one-line command to check if a service exists using systemctl is:

systemctl list-units --full -all | grep -Fq "$service_name.service"

This command lists all units, including inactive ones, and searches for the specified service. The -q option makes grep quiet, suppressing its output. The command returns a zero exit status if the service is found.

To use this in a bash script:

#!/bin/bash

service_name="apache2"
if systemctl list-units --full -all | grep -Fq "$service_name.service"; then
    echo "$service_name exists"
else
    echo "$service_name does not exist"
fi

This script checks for the Apache2 service and prints a message based on the result.

#!/bin/bash

service_name="apache2"
if command -v systemctl >/dev/null 2>&1; then
    if systemctl list-units --full -all | grep -Fq "$service_name.service"; then
        echo "$service_name exists"
    else
        echo "$service_name does not exist"
    fi
elif command -v service >/dev/null 2>&1; then
    if service --status-all | grep -Fq "$service_name"; then
        echo "$service_name exists"
    else
        echo "$service_name does not exist"
    fi
else
    echo "Unable to check service status"
fi

Creating a custom function for service checks

Here's a guide to write a function for service checks:

1. Define the function:

check_service() {
    local service_name="$1"

    # Check using systemctl
    if systemctl list-units --full -all | grep -Fq "$service_name.service"; then
        return 0
    fi

    # Check using service command
    if service "$service_name" status >/dev/null 2>&1; then
        return 0
    fi

    # Check in /etc/init.d and systemd directories
    if [ -f "/etc/init.d/$service_name" ] || \
       [ -f "/etc/systemd/system/$service_name.service" ] || \
       [ -f "/usr/lib/systemd/system/$service_name.service" ]; then
        return 0
    fi

    # Service not found
    return 1
}

2. Add error handling:

check_service() {
    if [ -z "$1" ]; then
        echo "Error: No service name provided" >&2
        return 2
    fi

    local service_name="$1"

    # ... (rest of the function as above)
}

3. Use the function in your script:

#!/bin/bash

service_to_check="apache2"

if check_service "$service_to_check"; then
    echo "$service_to_check exists"
else
    case $? in
        1) echo "$service_to_check does not exist" ;;
        2) echo "An error occurred while checking the service" ;;
    esac
fi

This function checks for the service using multiple methods, making it work with different Linux distributions. It returns different exit codes for different scenarios:

• 0: Service exists
• 1: Service does not exist
• 2: Error in function usage (no service name provided)

By using this function, you can perform service checks across various Linux systems.