nginx -t: test the configuration before reloading

nginx -t parses the configuration, opens the files it references (certificates, included files) and resolves upstream hostnames, then exits: 0 if the configuration is valid, 1 if not. It does not touch the running server, so the safe way to apply changes is nginx -t && nginx -s reload (or sudo nginx -t && sudo systemctl reload nginx).

A valid and a broken configuration

mkdir -p logs temp
cat > nginx.conf <<'EOF'
pid logs/nginx.pid;
events {}
http {
  server {
    listen 127.0.0.1:8080;
    location / {
      return 200 "ok\n";
    }
  }
}
EOF
log() { sed -E 's/^[0-9/]+ [0-9:]+ \[([a-z]+)\] [0-9#]+: /[\1] /'; }
nginx -p . -c nginx.conf -e stderr -t 2>&1 | log; echo "exit status: ${PIPESTATUS[0]}"
sed -i 's/return 200/retrun 200/' nginx.conf
nginx -p . -c nginx.conf -e stderr -t 2>&1 | log; echo "exit status: ${PIPESTATUS[0]}"

nginx: the configuration file ./nginx.conf syntax is ok
nginx: configuration file ./nginx.conf test is successful
exit status: 0
[emerg] unknown directive "retrun" in ./nginx.conf:7
nginx: configuration file ./nginx.conf test failed
exit status: 1

On a packaged install you just run sudo nginx -t; -p (prefix), -c (config file) and -e (error log) are only needed here because the example runs from a scratch directory.

Show the effective configuration

nginx -T tests the configuration and then prints it, with every include expanded and each file preceded by # configuration file …:. Use it to find which file sets a directive:

sudo nginx -T 2>/dev/null | grep -n "client_max_body_size"

Command-line switches