tail

Display the last part of a file. Display the contents of file or, by default, it's standard input, to the standard output.

Syntax
      tail [-F | -f | -r] [-b number | -c number | -n number] [file ...]
      tail [-F | -f | -r] [-q] [-b number | -c number | -n number] [file ...]
Key
   -b number
       The location is number 512-byte blocks.

   -c number
       The location is number bytes.

   -f
       Don't stop when end of file is reached, but rather to wait for  
       additional data to be appended to the input.
       (ignored if the standard input is a pipe, but not if it is a FIFO)

   -F
       The -F option implies the -f option, but tail will also check to
       see if the file being followed has been renamed or rotated.  The
       file is closed and reopened when tail detects that the filename
       being read from has a new inode number.  The -F option is ignored
       if reading from standard input rather than a file.

   -n number
       The location is number lines.

   -q  Suppress printing of headers when multiple files are being examined.

   -r
       Causes the input to be displayed in reverse order,by line.
       Additionally, this option changes the meaning of the
       -b, -c and -n options.  When the -r option is specified, these
       options specify the number of bytes, lines or 512-byte blocks to
       display, instead of the bytes, lines or blocks from the beginning
       or end of the input from which to begin the display.  The default
       for the -r option is to display all of the input.    

If more than a single file is specified, each file is preceded by a header consisting of the string ==> XXX <== where XXX is the name of the file.

The Process Control keyboard shortcuts can be used while tail is running.

The display begins at a byte, line or 512-byte block location in the input.

Numbers having a leading plus (+) sign are relative to the beginning of the input, for example, -c +2 starts the display at the second byte of the input. Numbers having a leading minus (-) sign or no explicit sign are relative to the end of the input, for example, -n 2 displays the last two lines of the input.

The default starting location is -n 10, or the last 10 lines of the input.

The tail utility is expected to be a superset of the IEEE Std 1003.2-1992 (POSIX.2) specification. In particular, the -F, -b and -r options are extensions to that standard.

The tail utility exits 0 on success, and >0 if an error occurs.

Common Log File locations

/private/var/log
/Library/Logs
~/Library/Logs

The /private folder is hidden for protection. To view it with Finder, select Go > Go to Folder

Examples

Extract the last 20 lines from a file:

$ tail -20 file.txt

Output the newly appended lines of a file instantly:

$ tail -f /var/log/wifi.log

Output newly appended lines, and keep trying if the file is temporarily inaccessible:

$ tail -F /var/log/wifi.log

DIsplay the last 40 lines from the system.log, continue until Ctrl-C is pressed:

$ tail -40 -f /var/log/system.log

Extract lines 40-50 from a file, first using head to get the first 50 lines then tail to get the last 10:

$ head -50 file.txt | tail -10

"Money will buy a pretty good dog, but it won't buy the wag of his tail" ~ Henry Wheeler Shaw

Related macOS commands:

cat - Concatenate and print (display) the content of files
head - Display the first lines of a file.
sed - Stream Editor
Console.app (Applications/Utilities) - Display macOS log files.
LNav.org - Log file Navigator.