Unix - tail Command

Purpose:

Writes a file to standard output, beginning at a specified point.

Standard Syntax:

tail [ -f ] [ -c Number | -n Number | -m Number | -b Number | -k Number ] [File ]

Description:

The tail command writes the file specified by the File parameter to standard
output beginning at a specified point. If no file is specified, standard input
is used. The Number variable specifies how many units to write to standard
output. The value for the Number variable can be a positive or negative
integer. If the value is preceded by + (plus sign), the file is written to
standard output starting at the specified number of units from the beginning of
the file. If the value is preceded by - (minus sign), the file is written to
standard output starting at the specified number of units from the end of the
file. If the value is not preceded by + (plus sign) or - (minus sign), the file
is read starting at the specified number of units from the end of the file.

The type of unit used by the Number variable to determine the starting point
for the count is determined by the -b, -c, -k, -m, or -n flag. If one of these
flags is not specified, the tail command reads the last ten lines of the
specified file and writes them to standard output. This is the same as entering
-n 10 at the command line.

The -m flag provides consistent results in both single- and double-byte
character environments. The -c flag should be used with caution when the input
is a text file containing multibyte characters, because output can be produced
that does not start on a character boundary.

Flags

-b Number Reads the specified file beginning at the 512-byte block location
indicated by the Number variable.

-c Number Reads the specified file beginning at the byte location indicated by
the Number variable.

-f If the input file is a regular file or if the File parameter specifies a
FIFO (first-in-first-out), the tail command does not terminate after the last
specified unit of the input file has been copied, but continues to read and
copy additional units from the input file as they become available. If no File
parameter is specified and standard input is a pipe, the -f flag is ignored.
The tail -f command can be used to monitor the growth of a file being written
by another process.

-k Number Reads the specified file beginning at the 1KB block location
indicated by the Number variable.

-m Number Reads the specified file beginning at the multibyte character
location indicated by the Number variable. Using this flag provides consistent
results in both single- and double-byte character-code-set environments.

-n Number Reads the specified file beginning at the line location indicated by
the Number variable.

-r Displays the output from the end of the file in reverse order. The default
for the -r flag prints the entire file in reverse order. If the file is larger
than 20,480 bytes, the -r flag displays only the last 20,480 bytes.

The -r flag is valid only with the -n flag. Otherwise, it is ignored.

Examples:

1. To display the last 10 lines of the notes file, enter:

tail notes

2. To specify the number of lines to start reading from the end of the notes
file, enter:

tail -n 20 notes

3. To display the notes file a page at a time, beginning with the 200th byte,
enter:

tail -c +200 notes | pg

4. To follow the growth of a file, enter:

tail -f accounts

This displays the last 10 lines of the accounts file. The tail command
continues to display lines as they are added to the accounts file. The
display continues until you press the Ctrl-C key sequence to stop it.