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205 lines
8.2 KiB
Plaintext
# @(#)POSIX 8.1 (Berkeley) 6/6/93
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# $FreeBSD$
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Comments on the IEEE P1003.2 Draft 12
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Part 2: Shell and Utilities
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Section 4.55: sed - Stream editor
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Diomidis Spinellis <dds@doc.ic.ac.uk>
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Keith Bostic <bostic@cs.berkeley.edu>
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In the following paragraphs, "wrong" usually means "inconsistent with
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historic practice", as most of the following comments refer to
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undocumented inconsistencies between the historical versions of sed and
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the POSIX 1003.2 standard. All the comments are notes taken while
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implementing a POSIX-compatible version of sed, and should not be
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interpreted as official opinions or criticism towards the POSIX committee.
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All uses of "POSIX" refer to section 4.55, Draft 12 of POSIX 1003.2.
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1. 32V and BSD derived implementations of sed strip the text
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arguments of the a, c and i commands of their initial blanks,
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i.e.
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#!/bin/sed -f
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a\
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foo\
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\ indent\
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bar
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produces:
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foo
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indent
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bar
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POSIX does not specify this behavior as the System V versions of
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sed do not do this stripping. The argument against stripping is
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that it is difficult to write sed scripts that have leading blanks
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if they are stripped. The argument for stripping is that it is
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difficult to write readable sed scripts unless indentation is allowed
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and ignored, and leading whitespace is obtainable by entering a
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backslash in front of it. This implementation follows the BSD
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historic practice.
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2. Historical versions of sed required that the w flag be the last
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flag to an s command as it takes an additional argument. This
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is obvious, but not specified in POSIX.
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3. Historical versions of sed required that whitespace follow a w
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flag to an s command. This is not specified in POSIX. This
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implementation permits whitespace but does not require it.
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4. Historical versions of sed permitted any number of whitespace
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characters to follow the w command. This is not specified in
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POSIX. This implementation permits whitespace but does not
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require it.
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5. The rule for the l command differs from historic practice. Table
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2-15 includes the various ANSI C escape sequences, including \\
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for backslash. Some historical versions of sed displayed two
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digit octal numbers, too, not three as specified by POSIX. POSIX
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is a cleanup, and is followed by this implementation.
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6. The POSIX specification for ! does not specify that for a single
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command the command must not contain an address specification
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whereas the command list can contain address specifications. The
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specification for ! implies that "3!/hello/p" works, and it never
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has, historically. Note,
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3!{
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/hello/p
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}
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does work.
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7. POSIX does not specify what happens with consecutive ! commands
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(e.g. /foo/!!!p). Historic implementations allow any number of
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!'s without changing the behaviour. (It seems logical that each
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one might reverse the behaviour.) This implementation follows
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historic practice.
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8. Historic versions of sed permitted commands to be separated
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by semi-colons, e.g. 'sed -ne '1p;2p;3q' printed the first
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three lines of a file. This is not specified by POSIX.
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Note, the ; command separator is not allowed for the commands
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a, c, i, w, r, :, b, t, # and at the end of a w flag in the s
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command. This implementation follows historic practice and
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implements the ; separator.
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9. Historic versions of sed terminated the script if EOF was reached
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during the execution of the 'n' command, i.e.:
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sed -e '
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n
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i\
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hello
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' </dev/null
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did not produce any output. POSIX does not specify this behavior.
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This implementation follows historic practice.
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10. Deleted.
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11. Historical implementations do not output the change text of a c
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command in the case of an address range whose first line number
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is greater than the second (e.g. 3,1). POSIX requires that the
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text be output. Since the historic behavior doesn't seem to have
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any particular purpose, this implementation follows the POSIX
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behavior.
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12. POSIX does not specify whether address ranges are checked and
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reset if a command is not executed due to a jump. The following
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program will behave in different ways depending on whether the
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'c' command is triggered at the third line, i.e. will the text
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be output even though line 3 of the input will never logically
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encounter that command.
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2,4b
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1,3c\
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text
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Historic implementations did not output the text in the above
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example. Therefore it was believed that a range whose second
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address was never matched extended to the end of the input.
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However, the current practice adopted by this implementation,
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as well as by those from GNU and SUN, is as follows: The text
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from the 'c' command still isn't output because the second address
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isn't actually matched; but the range is reset after all if its
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second address is a line number. In the above example, only the
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first line of the input will be deleted.
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13. Historical implementations allow an output suppressing #n at the
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beginning of -e arguments as well as in a script file. POSIX
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does not specify this. This implementation follows historical
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practice.
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14. POSIX does not explicitly specify how sed behaves if no script is
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specified. Since the sed Synopsis permits this form of the command,
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and the language in the Description section states that the input
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is output, it seems reasonable that it behave like the cat(1)
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command. Historic sed implementations behave differently for "ls |
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sed", where they produce no output, and "ls | sed -e#", where they
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behave like cat. This implementation behaves like cat in both cases.
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15. The POSIX requirement to open all w files at the beginning makes
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sed behave nonintuitively when the w commands are preceded by
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addresses or are within conditional blocks. This implementation
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follows historic practice and POSIX, by default, and provides the
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-a option which opens the files only when they are needed.
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16. POSIX does not specify how escape sequences other than \n and \D
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(where D is the delimiter character) are to be treated. This is
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reasonable, however, it also doesn't state that the backslash is
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to be discarded from the output regardless. A strict reading of
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POSIX would be that "echo xyz | sed s/./\a" would display "\ayz".
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As historic sed implementations always discarded the backslash,
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this implementation does as well.
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17. POSIX specifies that an address can be "empty". This implies
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that constructs like ",d" or "1,d" and ",5d" are allowed. This
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is not true for historic implementations or this implementation
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of sed.
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18. The b t and : commands are documented in POSIX to ignore leading
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white space, but no mention is made of trailing white space.
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Historic implementations of sed assigned different locations to
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the labels "x" and "x ". This is not useful, and leads to subtle
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programming errors, but it is historic practice and changing it
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could theoretically break working scripts. This implementation
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follows historic practice.
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19. Although POSIX specifies that reading from files that do not exist
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from within the script must not terminate the script, it does not
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specify what happens if a write command fails. Historic practice
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is to fail immediately if the file cannot be opened or written.
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This implementation follows historic practice.
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20. Historic practice is that the \n construct can be used for either
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string1 or string2 of the y command. This is not specified by
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POSIX. This implementation follows historic practice.
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21. Deleted.
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22. Historic implementations of sed ignore the RE delimiter characters
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within character classes. This is not specified in POSIX. This
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implementation follows historic practice.
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23. Historic implementations handle empty RE's in a special way: the
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empty RE is interpreted as if it were the last RE encountered,
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whether in an address or elsewhere. POSIX does not document this
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behavior. For example the command:
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sed -e /abc/s//XXX/
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substitutes XXX for the pattern abc. The semantics of "the last
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RE" can be defined in two different ways:
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1. The last RE encountered when compiling (lexical/static scope).
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2. The last RE encountered while running (dynamic scope).
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While many historical implementations fail on programs depending
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on scope differences, the SunOS version exhibited dynamic scope
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behaviour. This implementation does dynamic scoping, as this seems
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the most useful and in order to remain consistent with historical
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practice.
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