>openssl x509 -in www.sample.jp.cer -text -noout
意味：www.sample.jp.cer を取り込んで テキストで結果を出力する
出力例
ーーーーーー
Certificate:
Data:
Version: 3 (0x2)
Serial Number:
2d:10:88:83:4d:6e:a1:10:e4:5d:11:e3:0b:a7:19:8f
Signature Algorithm: sha256WithRSAEncryption
Issuer: C = JP, O = "SECOM Trust Systems CO.,LTD.", CN = NII Open Domain CA - G7 RSA
Validity
Not Before: Apr 17 07:29:12 2023 GMT <--新しい有効期限
Not After : May 17 07:29:12 2024 GMT <-- ここまで
Subject: C = JP, ST = Miyagi, L = Sendai-shi, O = Tohoku University, CN = www.icd.riec.tohoku.ac.jp
Subject Public Key Info:
Public Key Algorithm: rsaEncryption
RSA Public-Key: (2048 bit)
