The following is a comprehensive list of system variables 

SY-ABCDE - Constant: Alphabet (A,B,C,...)
SY-APPLI - SAP applications
SY-BATCH - Background active (X)
SY-BATZD - Background SUBMIT: Daily
SY-BATZM - Background SUBMIT: Monthly
SY-BATZO - Background SUBMIT: Once
SY-BATZS - Background SUBMIT: Immediately
SY-BATZW - Background SUBMIT: Weekly
SY-BINPT - Batch input active (X)
SY-BREP4 - Background SUBMIT: Root name of request report
SY-BSPLD - Background SUBMIT: List output to spool
SY-CALLD - CALL mode active (X)
SY-CALLR - Print: ID for print dialog function
SY-CCURS - Rate specification/result field (CURRENCY CONVERT)
SY-CCURT - Table rate from currency conversion
SY-CDATE - Date of rate from currency conversion
SY-COLNO - Current column during list creation
SY-CPAGE - Current page number
SY-CPROG - Runtime: Main program
SY-CTABL - Exchange rate table from currency conversion
SY-CTYPE - Exchange rate type 'M','B','G' from CURRENCY CONVERSION
SY-CUCOL - Cursor position (column)
SY-CUROW - Cursor position (line)
SY-DATAR - Flag: Data received
SY-DATLO - Local date for user
SY-DATUM - System: Date
SY-DATUT - Global date related to UTC (GMT)
SY-DAYST - Summertime active ? ('daylight saving time')
SY-DBCNT - Number of elements in edited dataset with DB operations
SY-DBNAM - Logical database for ABAP/4 program
SY-DBSYS - System: Database system
SY-DCSYS - System: Dialog system
SY-DSNAM - Runtime: Name of dataset for spool output
SY-DYNGR - Screen group of current screen
SY-DYNNR - Number of current screen
SY-FDAYW - Factory calendar weekday
SY-FDPOS - Location of a string
SY-FMKEY - Current function code menu
SY-HOST  - Host
SY-INDEX - Number of loop passes
SY-LANGU - SAP logon language key
SY-LDBPG - Program: ABAP/4 database program for SY-DBNAM
SY-LILLI - Number of current list line
SY-LINCT - Number of list lines
SY-LINNO - Current line for list creation
SY-LINSZ - Line size of list
SY-LISEL - Interact.: Selected line
SY-LISTI - Number of current list line
SY-LOCDB - Local database exists
SY-LOCOP - Local database operation
SY-LOOPC - Number of LOOP lines at screen step loop
SY-LSIND - Number of secondary list
SY-LSTAT - Interact.: Status information for each list level
SY-MACDB - Program: Name of file for matchcode access
SY-MACOL - Number of columns from SET MARGIN
SY-MANDT - Client number from SAP logon
SY-MARKY - Current line character for MARK
SY-MAROW - No. of lines from SET MARGIN statement
SY-MODNO - Number of alternative modi
SY-MSGID - Message ID
SY-MSGLI - Interact.: Message line (line 23)
SY-MSGNO - Message number
SY-MSGTY - Message type (E,I.W,...)
SY-MSGV1 - Message variable
SY-MSGV2 - Message variable
SY-MSGV3 - Message variable
SY-MSGV4 - Message variable
SY-OPSYS - System: Operating system
SY-PAART - Print: Format
SY-PAGCT - Page size of list from REPORT statement
SY-PAGNO - Runtime: Current page in list
SY-PDEST - Print: Output device
SY-PEXPI - Print: Spool retention period
SY-PFKEY - Runtime: Current F key status
SY-PLIST - Print: Name of spool request (list name)
SY-PRABT - Print: Department on cover sheet
SY-PRBIG - Print: Selection cover sheet
SY-PRCOP - Print: Number of copies
SY-PRDSN - Print: Name of spool dataset
SY-PREFX - ABAP/4 prefix for background jobs
SY-PRIMM - Print: Print immediately
SY-PRNEW - Print: New spool request (list)
SY-PRREC - Print: Recipient
SY-PRREL - Print: Delete after printing
SY-PRTXT - Print: Text for cover sheet
SY-REPID - Program: Name of ABAP/4 program
SY-RTITL - Print: Report title of program to be printed
SY-SAPRL - System: SAP Release
SY-SCOLS - Columns on screen
SY-SLSET - Name of selection set
SY-SPONO - Runtime: Spool number for list output
SY-SPONR - Runtime: Spool number from TRANSFER statement
SY-SROWS - Lines on screen
SY-STACO - Interact.: List displayed from column
SY-STARO - Interact.: Page displayed from line
SY-STEPL - Number of LOOP line at screen step
SY-SUBRC - Return value after specific ABAP/4 statements
SY-SUBTY - ABAP/4: Call type for SUBMIT
SY-SYSID - System: SAP System ID
SY-TABIX - Runtime: Current line of an internal table
SY-TCODE - Session: Current transaction code
SY-TFDSN - Runtime: Dataset for data extracts
SY-TFILL - Current number of entries in internal table
SY-TIMLO - Local time for user
SY-TIMUT - Global time related to UTC (GMT)
SY-TITLE - Title of ABAP/4 program
SY-TLENG - Line width of an internal table
SY-TMAXL - Maximum number of entries in internal table 
SY-TSTLO - Timestamp (date and time) for user
SY-TSTUT - Timestamp (date and time) related to UTC (GMT)
SY-TVAR0 - Runtime: Text variable for ABAP/4 text elements
SY-TVAR1 - Runtime: Text variable for ABAP/4 text elements
SY-TVAR2 - Runtime: Text variable for ABAP/4 text elements
SY-TVAR3 - Runtime: Text variable for ABAP/4 text elements
SY-TVAR4 - Runtime: Text variable for ABAP/4 text elements
SY-TVAR5 - Runtime: Text variable for ABAP/4 text elements
SY-TVAR6 - Runtime: Text variable for ABAP/4 text elements
SY-TVAR7 - Runtime: Text variable for ABAP/4 text elements
SY-TVAR8 - Runtime: Text variable for ABAP/4 text elements
SY-TVAR9 - Runtime: Text variable for ABAP/4 text elements
SY-TZONE - Time difference from 'Greenwich Mean Time' (UTC) in seconds
SY-UCOMM - Interact.: Command field function entry
SY-ULINE - Constant: Underline (---------...)
SY-UNAME - Session: SAP user from SAP logon
SY-UZEIT - System: Time
SY-VLINE - Constant: Vertical bar
SY-WAERS - T001: Company code currency after reading B segment
SY-WILLI - Number of current window line
SY-WINCO - Cursor position in window (column)
SY-WINDI - Index of current window line
SY-WINRO - Cursor position in window (line)
SY-WINSL - Interact.: Selected window line
SY-WINX1 - Window coordinate (column left)
SY-WINX2 - Window coordinate (column right)
SY-WINY1 - Window coordinate (line left)
SY-WINY2 - Window coordinate (line right)
SY-WTITL - Standard page header indicator
SY-XCODE - Extended command field
SY-ZONLO - Time zone of user

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