Strange Sed !! Post: 302691099

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Top Forums Shell Programming and Scripting Strange Sed !! Post 302691099 by vidyadhar85 on Friday 24th of August 2012 12:08:06 AM

08-24-2012

Registered User

Quote:

Originally Posted by alister

I will only breakdown the first example. From that hopefully you'll be able to understand the rest.

There are three levels of parsing in `sed -e "s/\//\\\//g"` which may consider a backslash to be special (for each step, active escape sequences are highlighted in bold red):

1) With the obsolete form of command substitution, `...`, the backslash is special when it is followed by another backslash, dollar, or backtick.
`sed -e "s/\//\\\//g"` ==> sed -e "s/\//\\//g"

2) Now that the shell has processed the text of the subshell's command, it creates the subshell to execute the command. That subshell then processes the double-quoted string. In a double-quoted string, the backslash is also special when it occurs before certain characters, but it's not the same set of characters as when parsing a backtick command substition. In this context, backslash is special when followed by another backslash, a dollar, a double-quote, a backtick, or a newline.
sed -e "s/\//\\//g" ==> sed -e s/\//\//g

3) Finally, we reach sed, which sees an expression that replaces a backslash-escaped regular expression delimiter (forward slash in this case) with the same. This does not yield any change in the text.
s/\//\//g

Regards,
Alister

Thanks Alister for the reply....

Does this mean we cant achieve the required result (1/1/1/-->1\/1\/1\/) from SED with backslash as delimeter?

Regards,
Vidya

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LEARN ABOUT LINUX

qmail-pw2u

qmail-pw2u(8)						      System Manager's Manual						     qmail-pw2u(8)

NAME

       qmail-pw2u - build address assignments from a passwd file

SYNOPSIS

       qmail-pw2u [ -/ohHuUC ] [ -cchar ]

DESCRIPTION

       qmail-pw2u reads a V7-format passwd file from standard input and prints a qmail-users-format assignment file.

       A V7-format passwd file is a series of lines.  Each line has the format

	  user:password:uid:gid:gecos:home:shell

       where  user  is an account name, uid and gid are the user id and group id of that account, and home is the account's home directory.  pass-
       word, gecos, and shell are ignored by qmail-pw2u.

       If you put the output of qmail-pw2u into /var/lib/qmail/users/assign, and then run  qmail-newu,	qmail-lspawn  will  obey  the  assignments
       printed by qmail-pw2u.  WARNING: After changing any users, uids, gids, or home directories in your passwd file, you must run qmail-pw2u and
       qmail-newu again if you want qmail-lspawn to see the changes.

RULES

       By default, qmail-pw2u follows the same rules as qmail-getpw.  It skips user if (1) uid is zero, (2) home does not exist, (3) user does not
       own  home, or (4) user contains uppercase letters.  It then gives each remaining user control over the basic user address and all addresses
       of the form user-anything.  A catch-all user, alias, controls all other addresses.

       You may change these rules by setting up files in /var/lib/qmail/users:

       include
	      Allowed users, one per line.  If include exists, and user is not listed in include, user is ignored.

       exclude
	      Ignored users, one per line.  If exclude exists, and user is listed in exclude, user is ignored.

       mailnames
	      Replacement names for users.  Each line has the form

		 user:mailname1:mailname2:...

	      The addresses mailname1 and mailname1-ext and mailname2 and so on will be delivered to user.

	      WARNING: The addresses user and user-ext will not be delivered to user unless user is listed as one of the mailnames.

	      A line in mailnames is silently ignored if the user does not exist.

       subusers
	      Extra addresses.	Each line has the form

		 sub:user:pre:

	      sub will be handled by home/.qmail-pre, where home is user's home directory; sub-ext will be handled by home/.qmail-pre-ext.

       append Extra assignments, printed at the end of qmail-pw2u's output.

OPTIONS

       -o     (Default.)  Skip user if home does not exist (or is not visible to qmail-pw2u).  Skip user if home is not owned by user.

       -h     Stop if home does not exist.  This is appropriate if every user is supposed to have a home directory.  Skip  user  if  home  is  not
	      owned by user.

       -H     Do not check the existence or ownership of home.

       -U     (Default.)  Skip user if there are any uppercase letters in user.

       -u     Allow uppercase letters in user.

       -cchar Use char as the user-extension delimiter in place of -.

       -C     Disable the user-extension mechanism.

       -/     Use home/.qmail-/...  instead of home/.qmail-...

SEE ALSO

       qmail-users(5), qmail-lspawn(8), qmail-newu(8), qmail-getpw(8)

																     qmail-pw2u(8)

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