Troubleshoot silent SSH startup failures on Linux ECS

Symptoms

When you use the service or systemctl command to start the SSH service on a Linux ECS instance, the command completes without errors, but the service fails to start.

Check the secure log by running cat /var/log/secure. It may contain error messages similar to the following:

sshd.service: main process exited, code=exited, status=203/EXEC.
init: ssh main process (1843) terminated with status 255.

Causes

This issue typically occurs due to a misconfigured PATH environment variable or missing files from the SSH package.

Procedure

Connect to the ECS instance using a VNC remote connection.

For more information, see Connect to an instance by using VNC.
Run the following command to check the PATH environment variable:
```
echo $PATH
```
- The expected response, which is the default value for the PATH environment variable, is as follows:
```
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin
```
- If the output is different, the PATH environment variable has been modified. Run the following command to reset it:
```
export PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin
```
Run the following command to find the path of the sshd program:
```
find / -name sshd
```
- The expected response includes the default path for the sshd program, as follows:
```
/usr/sbin/sshd
```
- If the file does not exist, run the following command to reinstall the SSH package:
```
yum install -y openssh
```
Run the following command to restart the SSH service:
```
systemctl restart sshd.service
```

Run the following command to check the status of the SSH service:

systemctl status sshd.service

Output similar to the following indicates that the SSH service is running.

● sshd.service - OpenSSH server daemon
   Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: enabled)
   Active: active (running) since Fri 2023-03-17 16:27:41 CST; 5 days ago
     Docs: man:sshd(8)
           man:sshd_config(5)
 Main PID: 1327 (sshd)
    Tasks: 1 (limit: 85040)
   Memory: 127.9M
   CGroup: /system.slice/sshd.service
           └─1327 /usr/sbin/sshd -D -oCiphers=aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,aes256-ctr,aes256-cbc,aes128-gcm@openssh.com,aes128-ctr,aes128-cbc -oMACs=hmac-sha2-256-etm@openssh.com,hmac-sha1-etm@openssh.com