View Issue Details
| ID | Project | Category | View Status | Date Submitted | Last Update |
|---|---|---|---|---|---|
| 0000407 | 1003.1(2008)/Issue 7 | Shell and Utilities | public | 2011-04-14 23:36 | 2024-06-11 08:53 |
| Reporter | eblake | Assigned To | ajosey | ||
| Priority | normal | Severity | Objection | Type | Enhancement Request |
| Status | Closed | Resolution | Accepted As Marked | ||
| Name | Eric Blake | ||||
| Organization | Red Hat | ||||
| User Reference | ebb.head | ||||
| Section | head | ||||
| Page Number | 2791 | ||||
| Line Number | 91030 | ||||
| Interp Status | --- | ||||
| Final Accepted Text | 0000407:0000752 | ||||
| Summary | 0000407: add head -c | ||||
| Description | The rationale for head states (XCU line 91101): There is no −c option (as there is in tail) because it is not historical practice and because other utilities in this volume of POSIX.1-2008 provide similar functionality. but does not name what those other utilities might be. The only one I can think of is dd; however, while dd can indeed truncate input after a particular number of bytes, it requires the use of ibs=1 which results in rather inefficient operation. This is tracked separately as 0000406 Both GNU and BSD implementations of head support -c with much more efficient operation than dd, and the ability to truncate input after an arbitrary number of bytes seems useful enough to standardize for Issue 8. | ||||
| Desired Action | At line 91030 [XCU head SYNOPSIS], change: head [−n number] [file...] to: head [-c number|−n number] [file...] At line 91034 [head DESCRIPTION], change: Copying shall end at the point in each input file indicated by the −n number option. The option-argument number shall be counted in units of lines. to: Copying shall end 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. At line 91039 [head OPTIONS], change: −n number The first number lines of each input file shall be copied to standard output. The application shall ensure that the number option-argument is a positive decimal integer. When a file contains less than number lines, it shall be copied to standard output in its entirety. to: −c number The first number bytes of each input file shall be copied to standard output. The application shall ensure that the number option-argument is a positive decimal integer. −n number This option shall be equivalent to −c number, except the ending location in the file shall be measured in lines instead of bytes. When a file contains less than number bytes or lines, it shall be copied to standard output in its entirety. At line 91054 [head INPUT FILES], change: Input files shall be text files, but the line length is not restricted to {LINE_MAX} bytes. to: If the −c option is specified, the input files can contain arbitrary data; otherwise, the input files shall be text files, but the line length is not restricted to {LINE_MAX} bytes. At line 91101 [head RATIONALE], change: There is no −c option (as there is in tail) because it is not historical practice and because other utilities in this volume of POSIX.1-2008 provide similar functionality. to: Head and tail have not historically been symmetric. For example, this standard only requires tail to support at most one file operand, while head must operate on multiple files. Conversely, this standard requires tail to be able to start at a position relative to the start of a file, but head need not support stopping at a position relative to the end of the file. Implementations may make the choose to make head and tail symmetric as an extension, but applications should not rely on this. Older implementations of head did not support -c number, but emulating this via dd ibs=1 count=number is less efficient. | ||||
| Tags | issue8 | ||||
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I would suggest a slight change to the end of the proposed rationale addition, to acknowledge the alternative solution of two piped dd processes (in cases where the number of bytes is a convenient multiple): Older implementations of head did not support -c number, but emulating this via dd ibs=1 count=number is much less efficient and emulating via dd obs=pipe_buf | dd ibs=size count=number_of_blocks is cumbersome, somewhat less efficient, and can only be used if the number of bytes to be copied is a multiple of a suitable block size less than or equal to PIPE_BUF. |
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Take Desired Action as modified by 0000407:0000743 |
| Date Modified | Username | Field | Change |
|---|---|---|---|
| 2011-04-14 23:36 | eblake | New Issue | |
| 2011-04-14 23:36 | eblake | Status | New => Under Review |
| 2011-04-14 23:36 | eblake | Assigned To | => ajosey |
| 2011-04-14 23:36 | eblake | Name | => Eric Blake |
| 2011-04-14 23:36 | eblake | Organization | => Red Hat |
| 2011-04-14 23:36 | eblake | User Reference | => ebb.head |
| 2011-04-14 23:36 | eblake | Section | => head |
| 2011-04-14 23:36 | eblake | Page Number | => 2791 |
| 2011-04-14 23:36 | eblake | Line Number | => 91030 |
| 2011-04-14 23:36 | eblake | Interp Status | => --- |
| 2011-04-16 10:49 | geoffclare | Note Added: 0000743 | |
| 2011-04-21 16:13 | msbrown | Tag Attached: issue8 | |
| 2011-04-21 16:15 | msbrown | Note Added: 0000752 | |
| 2011-04-21 16:15 | msbrown | Status | Under Review => Resolved |
| 2011-04-21 16:15 | msbrown | Resolution | Open => Accepted As Marked |
| 2011-04-21 16:15 | msbrown | Final Accepted Text | => 0000407:0000752 |
| 2011-04-28 16:52 | eblake | Relationship added | related to 0000406 |
| 2020-03-03 14:43 | geoffclare | Status | Resolved => Applied |
| 2024-06-11 08:53 | agadmin | Status | Applied => Closed |
