PROLOG¶

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult 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 buffer, 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 internationalization 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 standard 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 special device files or on file types not described by the System Interfaces volume of IEEE Std 1003.1-2001. Since this volume 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 initiated 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 initiated 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 "characters", 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 unspecified 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.

SEE ALSO¶

head

COPYRIGHT¶

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .