To get the port number of a service by its name in a Unix/Linux environment, you can use various methods and commands. Here are some common approaches:
1. **Using `getent` and `/etc/services`**:
You can use the `getent` command along with the `/etc/services` file to look up the port number of a service. For example, to find the port number for the SSH service (OpenSSH), you can run:
```bash
getent services ssh
```
This will return the port number associated with the SSH service, such as `22/tcp`.
2. **Using `nc` (netcat)**:
You can use the `nc` (netcat) command to attempt a connection to the service by name, and it will display the port if successful. For example:
```bash
nc -z -v -n -w 1 localhost 22
```
This will attempt to connect to the SSH service (port 22) on the localhost, and if successful, it will display the port number.
3. **Using `ss` (Socket Statistics)**:
The `ss` command can provide information about active sockets, which can include the service name and port number. For example, to list all listening sockets and their associated services:
```bash
ss -tuln
```
This will display a list of listening TCP and UDP sockets along with their associated services and port numbers.
4. **Using `netstat`**:
The `netstat` command, although deprecated on some systems, can still be used to find the port associated with a service name. For example:
```bash
netstat -nlt | grep <service_name>
```
Replace `<service_name>` with the name of the service you want to find.
5. **Using `nmap`**:
The `nmap` tool is commonly used for network discovery and security scanning. You can use it to discover the port number associated with a service on a remote host:
```bash
nmap -p <service_name> <hostname_or_ip>
```
Replace `<service_name>` with the service name and `<hostname_or_ip>` with the target host.
Note that the availability and behavior of these commands may vary depending on your Linux distribution and version. Additionally, some services may use non-standard port numbers or be defined in custom configuration files, so it's important to verify the information in your specific environment.