The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


tail - copy the last part of a file


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

tail -[number][b|c|l][f] [file]

tail +[number][b|c|l][f] [file]


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

Copying begins at the point in the file indicated by the -c number or -n number options (or the ± number portion of the argument to the obsolescent version). The option-argument number is counted in units of lines or bytes, according to the options -n and -c (or, in the obsolescent version, the appended option suffixes l (lines), b (512-byte blocks) or c (bytes)). 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, will be no smaller than {LINE_MAX}*10 bytes.


The tail utility supports the XBD specification, Utility Syntax Guidelines  , except that the obsolescent version accepts multi-character options that can preceded by a plus sign.

The following options are supported:

-c number
The number option-argument must be 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 is 1; that is, -c +1 represents the first byte of the file, -c -1 the last.
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 will be ignored. If the input file is not a FIFO, pipe or regular file, it is unspecified whether or not the -f option will be ignored.
-n number
This option is equivalent to -c number, except the starting location in the file is measured in lines instead of bytes. The origin for counting is 1; that is, -n +1 represents the first line of the file, -n -1 the last.

In the non-obsolescent form, if neither -c nor -n is specified, -n 10 is assumed.

In the obsolescent version, an argument beginning with a "-" or "+" can be used as a single option. The argument ±number with the letter c specified as a suffix is equivalent to -c ±number; ±number with the b suffix is equivalent to -c ±number*512; ±number with the letter l specified as a suffix, or with none of b, c nor l as a suffix, is equivalent to -n ±number. If number is not specified in these forms, 10 will be used. The letter f specified as a suffix is equivalent to specifying the -f option. If the -[number]c[f] form is used and neither number nor the f suffix is specified, it will be interpreted as the -c 10 option.


The following operand is supported:
A pathname of an input file. If no file operands are specified, the standard input will be used.


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


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


The following environment variables affect the execution of tail:
Provide a default value for the internationalisation variables that are unset or null. If LANG is unset or null, the corresponding value from the implementation-dependent default locale will be used. If any of the internationalisation variables contains an invalid setting, the utility will behave as if none of the variables had been defined.
If set to a non-empty string value, override the values of all the other internationalisation variables.
Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single- as opposed to multi-byte characters in arguments and input files).
Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
Determine the location of message catalogues for the processing of LC_MESSAGES .




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


Used only for diagnostic messages.






The following exit values are returned:
Successful completion.
An error occurred.




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 XSH specification. Since this specification 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.

The b suffix in the obsolescent version is not portable outside of XSI-conformant systems.


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.





UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
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