docs: add a manpage for ndn-log
This commit also removes reference to non-existing ndnsec-validator.conf
refs: #3918, #4184
Change-Id: I0413a2418db18db07ed9803ed6c9789374a56ded
diff --git a/docs/manpages/ndn-log.rst b/docs/manpages/ndn-log.rst
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+ndn-log
+=======
+
+The ndn-cxx logging facility exposes the internal state of NDN libraries and
+applications so the user can investigate internal interactions, such as interest
+dispatch inside ndn::Face, Data and Interest validation in the security framework,
+sync and recovery Interest processing inside ChronoSync, etc. During runtime, the
+user is able to specify the types of messages he or she would like to receive from
+a set of logging modules pre-configured by library and application developers.
+
+Environment Variable
+--------------------
+
+One way to control ndn-cxx logging facility is through the environment variable
+``NDN_LOG``. Using this variable, one can set the log levels for the available logging
+modules. Logging modules within different libraries and applications usually have
+distinguishing prefixes (``ndn.``, ``sync.``, etc.), which can be specified when
+setting the environment variable. Wildcards can be used to enable logging for all
+modules (just ``*``) or all modules under a selected prefix (e.g., ``ndn.*``).
+
+ndn-cxx logging facility provides a mechanism to manage the type of log messages
+that are written by classifying log messages by severity levels. Listed below
+are the available log levels.
+
+**Log Levels:**
+
+::
+
+ TRACE
+ DEBUG
+ INFO
+ WARN
+ ERROR
+ FATAL
+
+A message's severity level will determine whether the log is written. For instance,
+if an application sets its log severity level to DEBUG, all messages marked with
+DEBUG, or any of those below that level, are written. FATAL level logs are always
+written.
+
+Setting NDN_LOG requires the following syntax with as many prefixes and
+corresponding loglevels as the user desires:
+
+ export NDN_LOG="<prefix1>=<loglevel1>:<prefix2>=<loglevel2>"
+
+**Examples:**
+
+::
+
+ export NDN_LOG="ndn.*=DEBUG"
+ export NDN_LOG="ndn.UnixTransport=INFO"
+ export NDN_LOG="sync.Logic=ERROR"
+ export NDN_LOG="*=DEBUG:ndn.UnixTransport=INFO:sync.Logic=ERROR"
+
+**Note:**
+
+Shorter (general) prefixes should be placed before longer (specific) prefixes.
+Otherwise, the specific prefix's loglevel will be overwritten. For example,
+`export NDN_LOG="ndn.UnixTransport=TRACE:ndn.*=ERROR:*=INFO"` sets all modules
+to INFO; it should be written as
+`export NDN_LOG="*=INFO:ndn.*=ERROR:ndn.UnixTransport=TRACE"` for the desired effect.
+
+**Note:**
+
+Setting the environment variable with sudo requires the application to be run
+in the same command.
+
+::
+
+ Correct:
+
+ sudo env NDN_LOG=logLevel ./ndn-application
+
+ sudo -s
+ export NDN_LOG=logLevel
+ ./ndn-application
+
+ Incorrect:
+
+ sudo export NDN_LOG=logLevel
+ sudo ./ndn-application
+
+ NDN_LOG=logLevel sudo ./ndn-application