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Full Discussion: Shell script [Generalizing]
Top Forums Shell Programming and Scripting Shell script [Generalizing] Post 302551524 by Corona688 on Tuesday 30th of August 2011 06:06:25 PM
Old 08-30-2011
Quote:
Originally Posted by jzhang172
Hello there, I am trying to edit this code so that it will produce a shell script called "subst" that will apply a substitution to any number of files given on the command line.
That seems rather the long way around. Why not just write a script that does that in the first place?

Think I remember giving you an example for one, too.

[edit] yes, I did.
Code:
if [ "$#" -lt 3 ]
then
        echo "usage:  a b file1 file2 ..."
        exit
fi

A="$1"
shift
B="$1"
shift
while [ "$#" -gt 0 ]
do
        echo "substitute $A for $B in $1"
        shift
done

You complained you didn't know how to do a while loop yet, but it's easier than building rube-goldbergian script-generator generator.

How a while loop works is simple:
Code:
while [ condition ]
do
        something
done

where [ condition ] is a statement exactly like you'd cram in an if. As soon as the statement becomes false, it stops repeating the code between do and done. My condition, [ "$#" -gt 0 ] , is true whenever there's one or more arguments left (a number that decreases by one every time you shift).

How shift works is also simple. Say you have $1=string, $2=replacement, $3=file1, $4=file2. After a shift, you have $1=replacement, $2=file1, $3=file2. So I just save string and replacement for later, get rid of the original $1 and $2, then loop through all the remaining filenames...

Last edited by Corona688; 08-30-2011 at 07:20 PM..
 

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COMBINE(1)																COMBINE(1)

NAME
combine - combine sets of lines from two files using boolean operations SYNOPSIS
combine file1 and file2 combine file1 not file2 combine file1 or file2 combine file1 xor file2 _ file1 and file2 _ _ file1 not file2 _ _ file1 or file2 _ _ file1 xor file2 _ DESCRIPTION
combine combines the lines in two files. Depending on the boolean operation specified, the contents will be combined in different ways: and Outputs lines that are in file1 if they are also present in file2. not Outputs lines that are in file1 but not in file2. or Outputs lines that are in file1 or file2. xor Outputs lines that are in either file1 or file2, but not in both files. "-" can be specified for either file to read stdin for that file. The input files need not be sorted, and the lines are output in the order they occur in file1 (followed by the order they occur in file2 for the two "or" operations). Bear in mind that this means that the operations are not commutative; "a and b" will not necessarily be the same as "b and a". To obtain commutative behavior sort and uniq the result. Note that this program can be installed as "_" to allow for the syntactic sugar shown in the latter half of the synopsis (similar to the test/[ command). It is not currently installed as "_" by default, but you can alias it to that if you like. SEE ALSO
join(1) AUTHOR
Copyright 2006 by Joey Hess <joey@kitenet.net> Licensed under the GNU GPL. moreutils 2012-04-09 COMBINE(1)
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