В bash
и других оболочках символ обратной косой черты обрабатывается по-разному в одинарных или двойных кавычках.
Когда вы sed 's#\\there#where#' test.file
, sed
видит в строке запуска s#\\there#where# test.file
, потому что одинарные кавычки предотвращают интерпретацию всех специальных символов и escape-последовательностей : даже \'
Когда вы sed "s#\\there#where#" test.file
, sed
видит в своей строке запуска s#\there#where# test.file
, потому что двойные кавычки допускают некоторые escape-последовательности, а оболочка имеет интерпретировал первый обратный слеш как выход из второго.
Еще одним осложнением является то, что sed
также позволяет интерпретацию escape-последовательности, аналогично интерпретации в двойных кавычках, поэтому в первом случае (в одинарных кавычках) строка поиска становится \there
, как вы хотите; тогда как во втором случае (в двойных кавычках) первый символ строки поиска становится символом табуляции , а затем here
.
Следующая выдержка из руководства bash
определяет эти действия:
There are three quoting mechanisms: the escape character, single quotes, and double quotes.
A non-quoted backslash (\) is the escape character. It preserves the literal value of the next character that
follows, with the exception of <newline>. If a \<newline> pair appears, and the backslash is not itself
quoted, the \<newline> is treated as a line continuation (that is, it is removed from the input stream and
effectively ignored).
Enclosing characters in single quotes preserves the literal value of each character within the quotes. A sin‐
gle quote may not occur between single quotes, even when preceded by a backslash.
Enclosing characters in double quotes preserves the literal value of all characters within the quotes, with
the exception of $, `, \, and, when history expansion is enabled, !. The characters $ and ` retain their spe‐
cial meaning within double quotes. The backslash retains its special meaning only when followed by one of the
following characters: $, `, ", \, or <newline>. A double quote may be quoted within double quotes by preced‐
ing it with a backslash. If enabled, history expansion will be performed unless an ! appearing in double
quotes is escaped using a backslash. The backslash preceding the ! is not removed.
The special parameters * and @ have special meaning when in double quotes (see PARAMETERS below).
Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped charac‐
ters replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as
follows:
\a alert (bell)
\b backspace
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\" double quote
\nnn the eight-bit character whose value is the octal value nnn (one to three digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex
digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to
eight hex digits)
\cx a control-x character
The expanded result is single-quoted, as if the dollar sign had not been present.
A double-quoted string preceded by a dollar sign ($"string") will cause the string to be translated according
to the current locale. If the current locale is C or POSIX, the dollar sign is ignored. If the string is
translated and replaced, the replacement is double-quoted.