Em bash e outros shells, o caractere de barra invertida é tratado de maneira diferente entre aspas simples ou duplas.
Quando você digita sed 's#\there#where#' test.file , o que sed vê em sua sequência de execução é s#\there#where# test.file , porque aspas simples impedem toda interpretação de caractere especial e seqüência de escape: mesmo \' não é permitido.
Quando você digita sed "s#\there#where#" test.file , o que sed vê em sua sequência de execução é s#\there#where# test.file , porque as aspas duplas permitem algumas seqüências de escape e o shell interpretou a primeira barra invertida como escapando da segunda.
A complicação adicional é que sed também permite a interpretação da sequência de escape, semelhante à da aspas duplas, portanto, no primeiro caso (citação simples), a cadeia de pesquisa se torna \there , como você deseja; enquanto no segundo caso (aspas duplas) o primeiro caractere da string de busca se torna uma Tab , seguida por here .
A seguinte extração do manual bash define estas ações: -
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.