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Top Forums UNIX for Beginners Questions & Answers Usage of sed, position of apostrophe Post 303022811 by Aia on Friday 7th of September 2018 01:16:17 PM
Old 09-07-2018
Quote:
Originally Posted by baris35
Hello,
I have never ever seen below notation for string substitution.

Code:
sed -i -e 's/tttt/pppp'/g /var/bin/czech.sh;

The strange thing for me is the position of the apostrophe. Should it be before the / or after the g?
If I had been writing that command line, I would have chosen below way:
Code:
sed -i -s 's/tttt/pppp/g' /var/bin/czech.sh

Could you please explain what it differs...
Is that about the operating system?

Many thanks
Boris
Regular expressions uses meta-characters. These are characters that have special meaning beyond the representation of the character. The shell has also meta-characters. Therefore in order to protect the meaning of these characters it must be quoted so it gets passed to sed as the regular expression intended.
In this case the quoting could be avoided all together sed s/tttt/pppp/g /var/bin/czech.sh since there are no meta-characters.
 

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GLOB(7) 					       BSD Miscellaneous Information Manual						   GLOB(7)

NAME
glob -- shell-style pattern matching DESCRIPTION
Globbing characters (wildcards) are special characters used to perform pattern matching of pathnames and command arguments in the csh(1), ksh(1), and sh(1) shells as well as the C library functions fnmatch(3) and glob(3). A glob pattern is a word containing one or more unquoted '?' or '*' characters, or ``[..]'' sequences. Globs should not be confused with the more powerful regular expressions used by programs such as grep(1). While there is some overlap in the special characters used in regular expressions and globs, their meaning is different. The pattern elements have the following meaning: ? Matches any single character. * Matches any sequence of zero or more characters. [..] Matches any of the characters inside the brackets. Ranges of characters can be specified by separating two characters by a '-' (e.g. ``[a0-9]'' matches the letter 'a' or any digit). In order to represent itself, a '-' must either be quoted or the first or last character in the character list. Similarly, a ']' must be quoted or the first character in the list if it is to represent itself instead of the end of the list. Also, a '!' appearing at the start of the list has special meaning (see below), so to represent itself it must be quoted or appear later in the list. Within a bracket expression, the name of a character class enclosed in '[:' and ':]' stands for the list of all characters belonging to that class. Supported character classes: alnum cntrl lower space alpha digit print upper blank graph punct xdigit These match characters using the macros specified in ctype(3). A character class may not be used as an endpoint of a range. [!..] Like [..], except it matches any character not inside the brackets. Matches the character following it verbatim. This is useful to quote the special characters '?', '*', '[', and '' such that they lose their special meaning. For example, the pattern ``\*[x]?'' matches the string ``*[x]?''. Note that when matching a pathname, the path separator '/', is not matched by a '?', or '*', character or by a ``[..]'' sequence. Thus, /usr/*/*/X11 would match /usr/X11R6/lib/X11 and /usr/X11R6/include/X11 while /usr/*/X11 would not match either. Likewise, /usr/*/bin would match /usr/local/bin but not /usr/bin. SEE ALSO
fnmatch(3), glob(3), re_format(7) HISTORY
In early versions of UNIX, the shell did not do pattern expansion itself. A dedicated program, /etc/glob, was used to perform the expansion and pass the results to a command. In Version 7 AT&T UNIX, with the introduction of the Bourne shell, this functionality was incorporated into the shell itself. BSD
November 30, 2010 BSD
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