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Top Forums Shell Programming and Scripting delete last line from text file Post 23558 by Perderabo on Tuesday 25th of June 2002 09:52:51 AM
Old 06-25-2002
$ means the last line. But sed doesn't know that it has the last line until it arrives. You can't do a "sed '$-1d'" or anything. Deleting the next to the last line via sed is very hard. So hard that I would normally use other techniques. But just for the heck of it, I finally got a sed script running that deletes the next to the last line:
Code:
#! /usr/bin/sed -nf
#
${P
q
}
1{h
d
}
x
p

And this solution will not easily scale up. A sed script to delete $-2 will take about 10 times the code. Deleting $-3 would be a nightmare. The problem is that you don't know in advance how many lines there are. To really have a general solution, you need to make two passes through the file. On the first pass, you count the lines. On the second pass, you delete the appropriate line. sed cannot do this alone since it always will only make a single pass through the file.
Code:
#! /usr/bin/ksh
lines=$(wc -l $1)
((target=lines-1))
sed "${target}d" < $1
exit 0

Now we have an extensible solution.
 

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SUBST(1)						      General Commands Manual							  SUBST(1)

NAME
subst - substitute definitions into file(s) SYNOPSIS
subst [ -e editor ] -f substitutions victim ... DESCRIPTION
Subst makes substitutions into files, in a way that is suitable for customizing software to local conditions. Each victim file is altered according to the contents of the substitutions file. The substitutions file contains one line per substitution. A line consists of two fields separated by one or more tabs. The first field is the name of the substitution, the second is the value. Neither should contain the character `#', and use of text-editor metacharacters like `&' and `' is also unwise; the name in particular is best restricted to be alphanumeric. A line starting with `#' is a comment and is ignored. In the victims, each line on which a substitution is to be made (a target line) must be preceded by a prototype line. The prototype line should be delimited in such a way that it will be taken as a comment by whatever program processes the file later. The prototype line must contain a ``prototype'' of the target line bracketed by `=()<' and `>()='; everything else on the prototype line is ignored. Subst extracts the prototype, changes all instances of substitution names bracketed by `@<' and `>@' to their values, and then replaces the tar- get line with the result. OPTIONS
-e Substitutions are done using the sed(1) editor, which must be found in either the /bin or /usr/bin directories. To specify a dif- ferent executable, use the ``-e'' flag. EXAMPLE
If the substitutions file is FIRST 111 SECOND 222 and the victim file is x = 2; /* =()<y = @<FIRST>@ + @<SECOND>@;>()= */ y = 88 + 99; z = 5; then ``subst -f substitutions victim'' changes victim to: x = 2; /* =()<y = @<FIRST>@ + @<SECOND>@;>()= */ y = 111 + 222; z = 5; FILES
victimdir/substtmp.new new version being built victimdir/substtmp.old old version during renaming SEE ALSO
sed(1) DIAGNOSTICS
Complains and halts if it is unable to create its temporary files or if they already exist. HISTORY
Written at U of Toronto by Henry Spencer. Rich $alz added the ``-e'' flag July, 1991. BUGS
When creating a file to be substed, it's easy to forget to insert a dummy target line after a prototype line; if you forget, subst ends up deleting whichever line did in fact follow the prototype line. 25 Feb 1990 SUBST(1)

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