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TAIL(1P) POSIX Programmer's Manual TAIL(1P)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (con-
sult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on
Linux.

NAME
tail - copy the last part of a file

SYNOPSIS
tail [-f][ -c number| -n number][file]

DESCRIPTION
The tail utility shall copy its input file to the standard output beginning at a designated place.

Copying shall begin at the point in the file indicated by the -c number or -n number options. The option-argument number
shall be counted in units of lines or bytes, according to the options -n and -c. Both line and byte counts start from 1.

Tails relative to the end of the file may be saved in an internal buffer, and thus may be limited in length. Such a buf-
fer, if any, shall be no smaller than {LINE_MAX}*10 bytes.

OPTIONS
The tail utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001, Section 12.2, Utility Syntax
Guidelines.

The following options shall be supported:

-c number
The application shall ensure that the number option-argument is a decimal integer whose sign affects the location
in the file, measured in bytes, to begin the copying:

Sign Copying Starts
+ Relative to the beginning of the file.
- Relative to the end of the file.
none Relative to the end of the file.

The origin for counting shall be 1; that is, -c +1 represents the first byte of the file, -c -1 the last.

-f If the input file is a regular file or if the file operand specifies a FIFO, do not terminate after the last line
of the input file has been copied, but read and copy further bytes from the input file when they become available.
If no file operand is specified and standard input is a pipe, the -f option shall be ignored. If the input file is
not a FIFO, pipe, or regular file, it is unspecified whether or not the -f option shall be ignored.

-n number
This option shall be equivalent to -c number, except the starting location in the file shall be measured in lines
instead of bytes. The origin for counting shall be 1; that is, -n +1 represents the first line of the file, -n -1
the last.

If neither -c nor -n is specified, -n 10 shall be assumed.

OPERANDS
The following operand shall be supported:

file A pathname of an input file. If no file operands are specified, the standard input shall be used.

STDIN
The standard input shall be used only if no file operands are specified. See the INPUT FILES section.

INPUT FILES
If the -c option is specified, the input file can contain arbitrary data; otherwise, the input file shall be a text file.

ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of tail:

LANG Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions
volume of IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables for the precedence of international-
ization variables used to determine the values of locale categories.)

LC_ALL If set to a non-empty string value, override the values of all the other internationalization variables.

LC_CTYPE
Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-
byte as opposed to multi-byte characters in arguments and input files).

LC_MESSAGES
Determine the locale that should be used to affect the format and contents of diagnostic messages written to stan-
dard error.

NLSPATH
Determine the location of message catalogs for the processing of LC_MESSAGES .

ASYNCHRONOUS EVENTS
Default.

STDOUT
The designated portion of the input file shall be written to standard output.

STDERR
The standard error shall be used only for diagnostic messages.

OUTPUT FILES
None.

EXTENDED DESCRIPTION
None.

EXIT STATUS
The following exit values shall be returned:

0 Successful completion.

>0 An error occurred.

CONSEQUENCES OF ERRORS
Default.

The following sections are informative.

APPLICATION USAGE
The -c option should be used with caution when the input is a text file containing multi-byte characters; it may produce
output that does not start on a character boundary.

Although the input file to tail can be any type, the results might not be what would be expected on some character spe-
cial device files or on file types not described by the System Interfaces volume of IEEE Std 1003.1-2001. Since this vol-
ume of IEEE Std 1003.1-2001 does not specify the block size used when doing input, tail need not read all of the data
from devices that only perform block transfers.

EXAMPLES
The -f option can be used to monitor the growth of a file that is being written by some other process. For example, the
command:

tail -f fred

prints the last ten lines of the file fred, followed by any lines that are appended to fred between the time tail is ini-
tiated and killed. As another example, the command:

tail -f -c 15 fred

prints the last 15 bytes of the file fred, followed by any bytes that are appended to fred between the time tail is ini-
tiated and killed.

RATIONALE
This version of tail was created to allow conformance to the Utility Syntax Guidelines. The historical -b option was
omitted because of the general non-portability of block-sized units of text. The -c option historically meant "charac-
ters", but this volume of IEEE Std 1003.1-2001 indicates that it means "bytes". This was selected to allow reasonable
implementations when multi-byte characters are possible; it was not named -b to avoid confusion with the historical -b.

The origin of counting both lines and bytes is 1, matching all widespread historical implementations.

The restriction on the internal buffer is a compromise between the historical System V implementation of 4096 bytes and
the BSD 32768 bytes.

The -f option has been implemented as a loop that sleeps for 1 second and copies any bytes that are available. This is
sufficient, but if more efficient methods of determining when new data are available are developed, implementations are
encouraged to use them.

Historical documentation indicates that tail ignores the -f option if the input file is a pipe (pipe and FIFO on systems
that support FIFOs). On BSD-based systems, this has been true; on System V-based systems, this was true when input was
taken from standard input, but it did not ignore the -f flag if a FIFO was named as the file operand. Since the -f option
is not useful on pipes and all historical implementations ignore -f if no file operand is specified and standard input is
a pipe, this volume of IEEE Std 1003.1-2001 requires this behavior. However, since the -f option is useful on a FIFO,
this volume of IEEE Std 1003.1-2001 also requires that if standard input is a FIFO or a FIFO is named, the -f option
shall not be ignored. Although historical behavior does not ignore the -f option for other file types, this is unspeci-
fied so that implementations are allowed to ignore the -f option if it is known that the file cannot be extended.

This was changed to the current form based on comments noting that -c was almost never used without specifying a number
and that there was no need to specify -l if -n number was given.

FUTURE DIRECTIONS
None.