Asterisk standard channel variables

There are a number of variables that are defined or read by Asterisk. Here is a list of them. More information is available in each application's help text. All these variables are in UPPER CASE only.

Variables marked with a * are builtin functions and can't be set, only read in the dialplan. Writes to such variables are silently ignored.

${CDR(accountcode)}    * Account code (if specified)
${BLINDTRANSFER}         The name of the channel on the other side of a blind transfer
${BRIDGEPEER}            Bridged peer
${BRIDGEPVTCALLID}       Bridged peer PVT call ID (SIP Call ID if a SIP call)
${CALLERID(ani)}       * Caller ANI (PRI channels)
${CALLERID(ani2)}      * ANI2 (Info digits) also called Originating line information or OLI
${CALLERID(all)}       * Caller ID
${CALLERID(dnid)}      * Dialed Number Identifier
${CALLERID(name)}      * Caller ID Name only
${CALLERID(num)}       * Caller ID Number only
${CALLERID(rdnis)}     * Redirected Dial Number ID Service
${CALLINGANI2}         * Caller ANI2 (PRI channels)
${CALLINGPRES}         * Caller ID presentation for incoming calls (PRI channels)
${CALLINGTNS}          * Transit Network Selector (PRI channels)
${CALLINGTON}          * Caller Type of Number (PRI channels)
${CHANNEL}             * Current channel name
${CONTEXT}             * Current context
${DATETIME}            * Current date time in the format: DDMMYYYY-HH:MM:SS
                         (Deprecated; use ${STRFTIME(${EPOCH},,%d%m%Y-%H:%M:%S)})
${DB_RESULT}             Result value of DB_EXISTS() dial plan function
${EPOCH}               * Current unix style epoch
${EXTEN}               * Current extension
${ENV(VAR)}              Environmental variable VAR
${GOTO_ON_BLINDXFR}      Transfer to the specified context/extension/priority
                         after a blind transfer (use ^ characters in place of
                         | to separate context/extension/priority when setting
                         this variable from the dialplan)
${HANGUPCAUSE}         * Asterisk cause of hangup (inbound/outbound)
${HINT}                * Channel hints for this extension
${HINTNAME}            * Suggested Caller*ID name for this extension
${INVALID_EXTEN}         The invalid called extension (used in the "i" extension)
${LANGUAGE}            * Current language (Deprecated; use ${LANGUAGE()})
${LEN(VAR)}            * String length of VAR (integer)
${PRIORITY}            * Current priority in the dialplan
${PRIREDIRECTREASON}     Reason for redirect on PRI, if a call was directed
${TIMESTAMP}           * Current date time in the format: YYYYMMDD-HHMMSS
                         (Deprecated; use ${STRFTIME(${EPOCH},,%Y%m%d-%H%M%S)})
${TRANSFER_CONTEXT}      Context for transferred calls
${FORWARD_CONTEXT}       Context for forwarded calls
${UNIQUEID}            * Current call unique identifier
${SYSTEMNAME}          * value of the systemname option of asterisk.conf
${ENTITYID}            * Global Entity ID set automatically, or from asterisk.conf