added environment variable expansion in config file

[sergey-safarov]

This PR add environment variables expansion in sngrep config file.

Use case example
You have logged into remote server using ssh as normal user (not root user).
Then you will need to use sudo -i command to get root user permission.
To save pcap files in normal user folder (not root folder) required to configure in /etc/sngreprc file next string

set savepath /home/${SUDO_USER}

PR tested locally.

Related to GH #529

[Kaian]

Hi @sergey-safarov!

Apologies for the long silence. I recently became a father and have been focusing on family during this time.

I get the idea, but if we're only doing this for a single setting, I would prefer to do it where settings are initializated rather than the function used to set all settings, and always try to expand the setting (we can check if there are expansion tags inside the function )

diff --git a/src/option.c b/src/option.c
index 28f011a..aaf4cb3 100644
--- a/src/option.c
+++ b/src/option.c
@@ -98,6 +98,12 @@ init_options(int no_config)
         }
     }
 
+    // Expand environment macros in save path
+    const char *save_path = setting_get_value(SETTING_SAVEPATH);
+    char expanded_value[MAX_SETTING_LEN];
+    setting_expand_env_macros(save_path, expanded_value, sizeof(expanded_value));
+    setting_set_value(SETTING_SAVEPATH, expanded_value);
+
     return 0;
 }

What do you think?

Thanks for your time and contribution!

Best regards,