Check Linux configuration files for syntax errors

This article describes how to check the Linux® configuration for Secure Shell (SSH), Apache®, and Nginx®, which helps you to troubleshoot errors. For example, you might make a change to a configuration file and introduce a syntax error. As a result, when you restart or reload the service, it shows an error and fails to restart, which results in downtime.


You can find the SSH configuration file in /etc/ssh/sshd_config. After you change the configuration file, run the command sshd -t to check the configuration. If the configuration has errors, the following error message displays:

# sshd -t

/etc/ssh/sshd_config: line 17: Bad configuration option: ZPort
/etc/ssh/sshd_config: terminating, 1 bad configuration options

If there are no errors, nothing displays when you run the command.


The syntax check command checks the default Apache configuration file and any virtual hosts (vhosts) that you have configured on the server. To check the syntax, run httpd -t or apache2ctl -t for the Ubuntu® operating system and Debian®. If there is an error with the syntax check, the error message displays output similar to the following example:

# httpd -t

AH00112: Warning: DocumentRoot [/var/www/] does not exist
AH00526: Syntax error on line 3 of /etc/httpd/conf.d/test.conf:
ServerName takes one argument, The hostname and port of the server

If there are no errors, the message Syntax OK displays. Even if there are no vhosts on the server, the syntax check still shows Syntax OK when the syntax for the default configuration file for Apache is correct.


Nginx syntax checks behave differently than Apache. To check the syntax of the default Nginx configuration, run the command nginx -t. However, if you want to check the syntax of a specific server block, you need to add the -c flag and specify the path to the server block as shown in the following example:

[root@localhost ~]# nginx -t
nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
nginx: configuration file /etc/nginx/nginx.conf test is successful

[root@localhost ~]# nginx -t -c /etc/nginx/conf.d/
nginx: [emerg] "server" directive is not allowed here in /etc/nginx/conf.d/
nginx: configuration file /etc/nginx/conf.d/ test failed