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sed - using variable values as patterns

This is purely a learning exercise.
I have no real-world application.

I have a file of names in the format FirstName LastName.
I want to exchange the two fields to produce a new file
containing the same information in the format LastName FirstName.

This works ...

Code:

sed 's/$[[:graph:]]*$ $[[:graph:]]*$/\2 \1/'

As an experiment I tried to specify the patterns as variables.

Code:

Word=' \([[:graph:]]*\) '

sed 's/' $Word $Word '/\2 \1/'

This fails with ...

Quote:

sed: -e expression #1, char 2: unterminated `s' command

Please advise.

Daniel B. Martin

Quote:

Originally Posted by danielbmartin (Post 4581376)

Code:

Word=' \([[:graph:]]*\) '

sed 's/' $Word $Word '/\2 \1/'

There are spaces in the argument to sed, therefore sed recieves the expression as 4 separate arguments. You should remove the spaces or quote them or use double quotes:

Code:

sed 's/'$Word$Word'/\2 \1/'

sed 's/ '$Word' '$Word' /\2 \1/'

sed 's/'" $Word $Word "'/\2 \1/'

or something along those lines

Quote:

Originally Posted by millgates (Post 4581401)

There are spaces in the argument to sed, therefore sed recieves the expression as 4 separate arguments. You should remove the spaces or quote them or use double quotes:

Code:

sed 's/'$Word$Word'/\2 \1/'

sed 's/ '$Word' '$Word' /\2 \1/'

sed 's/'" $Word $Word "'/\2 \1/'

or something along those lines

Thank you, millgates, for your timely and informative response. I removed the leading and trailing blanks from the variable Word.

Code:

Word='$[[:graph:]]*$'

Having done this, all three of your sed examples work. However, the first of the three puts a leading blank on each line and I don't understand why that happens.

Daniel B. Martin

How about just use double quotes:

Code:

sed "s/$Word$Word/\2 \1/"

Whoa! I took a closer look at the suggested solutions and
they don't work (unlikely), or
I've implemented them incorrectly.

Here's some simple code. Try it yourself and tell me how I goofed it!

Code:

echo; echo The input file is...

cat < $InFile

 

echo; echo Example 1...

sed 's/\([[:graph:]]*\) \([[:graph:]]*\)/\2 \1/' $InFile



Word='\([[:graph:]]*\)'



echo; echo Example 2...

sed 's/'$Word$Word'/\2 \1/' $InFile



echo; echo Example 3...

sed 's/ '$Word' '$Word' /\2 \1/' $InFile

 

echo; echo Example 4...

sed 's/'" $Word $Word "'/\2 \1/' $InFile



echo; echo Example 5...

sed "s/$Word$Word/\2 \1/" $InFile  

echo; echo

Here is the output from running that code.

Code:

The input file is...

Charles Curtis

John Garner

Henry Wallace

Alben Barkley



Example 1...

Curtis Charles

Garner John

Wallace Henry

Barkley Alben



Example 2...

 Charles Curtis

 John Garner

 Henry Wallace

 Alben Barkley



Example 3...

Charles Curtis

John Garner

Henry Wallace

Alben Barkley



Example 4...

Charles Curtis

John Garner

Henry Wallace

Alben Barkley



Example 5...

 Charles Curtis

 John Garner

 Henry Wallace

 Alben Barkley

Daniel B. Martin

Hi.

5th example should look like this

Code:

$ Word='\([[:graph:]]*\)'

$ echo a b | sed "s/$Word $Word/\2 \1/"

b a

Note the space between two $Word's. Without the space you would get

Code:

$ echo a b | sed "s/$Word$Word/[\2] {\1}/"

[] {a} b

Space do not belong to the [:graph:] character class, so the first group matches only `a' and the second one matches empty string. IMO using double quotes (") is a preferred way to do such task.

You might find the following program helpful:

Code:



~/sandbox/49381$ cat >showargs.c

#include <stdio.h>

void main (int c, char **v, char **e)

{

        while (*v) puts(*v++);

        while (*e) puts(*e++);

}

~/sandbox/49381$ make showargs

cc    showargs.c  -o showargs

~/sandbox/49381$ ./showargs *

./showargs

UIDvsSID

a

a.cc

b

b.cc

declorder.cc

duff.cc

duff.o

opendir

opendir.cc

opendir2

opendir2.cc

sanitize

showargs

showargs.c

size

size.c

tags

SHELL=/bin/bash

TERM=xterm

XDG_SESSION_COOKIE=03b561d92e2a1484d958eade00000009-1327249405.440338-1264992931

TMPDIR=/tmp/jthill

WINDOWID=6291469

HUSHLOGIN=FALSE

XTERM_SHELL=/bin/bash

USER=jthill

  (and the rest of the environment variables...)

and you can pipe the result through 'sed -n l' or 'cat -A' if spacing or the like is a concern.

Quote:

Originally Posted by firstfire (Post 4582248)

5th example should look like this

Code:

$ Word='\([[:graph:]]*\)'

$ echo a b | sed "s/$Word $Word/\2 \1/"

b a

Note the space between two $Word's.

You are right. Example 5 now works correctly. Thank you!

Daniel B. Martin