/usr.bin/tail/tail.1

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  31. .\" @(#)tail.1 8.1 (Berkeley) 6/6/93
  32. .\" $FreeBSD$
  33. .\"
  34. .Dd June 5, 2009
  35. .Dt TAIL 1
  36. .Os
  37. .Sh NAME
  38. .Nm tail
  39. .Nd display the last part of a file
  40. .Sh SYNOPSIS
  41. .Nm
  42. .Op Fl F | f | r
  43. .Op Fl q
  44. .Oo
  45. .Fl b Ar number | Fl c Ar number | Fl n Ar number
  46. .Oc
  47. .Op Ar
  48. .Sh DESCRIPTION
  49. The
  50. .Nm
  51. utility displays the contents of
  52. .Ar file
  53. or, by default, its standard input, to the standard output.
  54. .Pp
  55. The display begins at a byte, line or 512-byte block location in the
  56. input.
  57. Numbers having a leading plus
  58. .Pq Ql +
  59. sign are relative to the beginning
  60. of the input, for example,
  61. .Dq Li "-c +2"
  62. starts the display at the second
  63. byte of the input.
  64. Numbers having a leading minus
  65. .Pq Ql -
  66. sign or no explicit sign are
  67. relative to the end of the input, for example,
  68. .Dq Li "-n 2"
  69. displays the last two lines of the input.
  70. The default starting location is
  71. .Dq Li "-n 10" ,
  72. or the last 10 lines of the input.
  73. .Pp
  74. The options are as follows:
  75. .Bl -tag -width indent
  76. .It Fl b Ar number
  77. The location is
  78. .Ar number
  79. 512-byte blocks.
  80. .It Fl c Ar number
  81. The location is
  82. .Ar number
  83. bytes.
  84. .It Fl f
  85. The
  86. .Fl f
  87. option causes
  88. .Nm
  89. to not stop when end of file is reached, but rather to wait for additional
  90. data to be appended to the input.
  91. The
  92. .Fl f
  93. option is ignored if the standard input is a pipe, but not if it is a FIFO.
  94. .It Fl F
  95. The
  96. .Fl F
  97. option implies the
  98. .Fl f
  99. option, but
  100. .Nm
  101. will also check to see if the file being followed has been renamed or rotated.
  102. The file is closed and reopened when
  103. .Nm
  104. detects that the filename being read from has a new inode number.
  105. .Pp
  106. If the file being followed does not (yet) exist or if it is removed, tail
  107. will keep looking and will display the file from the beginning if and when
  108. it is created.
  109. .Pp
  110. The
  111. .Fl F
  112. option is the same as the
  113. .Fl f
  114. option if reading from standard input rather than a file.
  115. .It Fl n Ar number
  116. The location is
  117. .Ar number
  118. lines.
  119. .It Fl q
  120. Suppresses printing of headers when multiple files are being examined.
  121. .It Fl r
  122. The
  123. .Fl r
  124. option causes the input to be displayed in reverse order, by line.
  125. Additionally, this option changes the meaning of the
  126. .Fl b , c
  127. and
  128. .Fl n
  129. options.
  130. When the
  131. .Fl r
  132. option is specified, these options specify the number of bytes, lines
  133. or 512-byte blocks to display, instead of the bytes, lines or blocks
  134. from the beginning or end of the input from which to begin the display.
  135. The default for the
  136. .Fl r
  137. option is to display all of the input.
  138. .El
  139. .Pp
  140. If more than a single file is specified, each file is preceded by a
  141. header consisting of the string
  142. .Dq Li "==> " Ns Ar XXX Ns Li " <=="
  143. where
  144. .Ar XXX
  145. is the name of the file unless
  146. .Fl q
  147. flag is specified.
  148. .Sh EXIT STATUS
  149. .Ex -std
  150. .Sh SEE ALSO
  151. .Xr cat 1 ,
  152. .Xr head 1 ,
  153. .Xr sed 1
  154. .Sh STANDARDS
  155. The
  156. .Nm
  157. utility is expected to be a superset of the
  158. .St -p1003.2-92
  159. specification.
  160. In particular, the
  161. .Fl F ,
  162. .Fl b
  163. and
  164. .Fl r
  165. options are extensions to that standard.
  166. .Pp
  167. The historic command line syntax of
  168. .Nm
  169. is supported by this implementation.
  170. The only difference between this implementation and historic versions
  171. of
  172. .Nm ,
  173. once the command line syntax translation has been done, is that the
  174. .Fl b ,
  175. .Fl c
  176. and
  177. .Fl n
  178. options modify the
  179. .Fl r
  180. option, i.e.,
  181. .Dq Li "-r -c 4"
  182. displays the last 4 characters of the last line
  183. of the input, while the historic tail (using the historic syntax
  184. .Dq Li -4cr )
  185. would ignore the
  186. .Fl c
  187. option and display the last 4 lines of the input.
  188. .Sh HISTORY
  189. A
  190. .Nm
  191. command appeared in PWB UNIX.