Reversing a string using only commands from the SUS (Single UNIX Specification)

Willard · 11-17-2009, 08:06 AM

Greetings.

I would like to reverse a string using only the tools which are part of the Single UNIX Specification (POSIX). The list of UNIX utilities in that specification can be seen on Wikipedia:

http://en.wikipedia.org/wiki/List_of_Unix_utilities

In Linux and BSD I can just use the command

Code:

rev

and feed input to it through standard input. However, 'rev' is not part of SOS.

So far, my best proposal is the following.

Code:

awk '{for(i=length($0);i>0;i--){printf("%c",substr($0,i,1))}}'

This program reverses the text provided to it on standard input, although it removes linebreaks if present.

In my specific scenario, I have a single word consisting of English characters only, with no spaces, punctuations, or hyphens.

I think the code looks hideous, and I have not found a more elegant way in my brief search. Any ideas?

Thanks in advance

Willard.

ghostdog74 · 11-17-2009, 08:15 AM

gawk

Code:

$ echo "abcdefghij" | awk -vFS= '{for(i=NF;i>0;i--)printf $i}'
jihgfedcba

Willard · 11-17-2009, 09:05 AM

Quote:

Originally Posted by ghostdog74

gawk

Code:

$ echo "abcdefghij" | awk -vFS= '{for(i=NF;i>0;i--)printf $i}'
jihgfedcba

Ah, much better. Neat trick

Both this script, and my script, will reverse lines at a time, and remove line breaks. For completeness and elegance, it would be nice if the script would just consider the entire input stream as a single token, and reverse it, instead of treating linebreaks in a special manner.

If I remember correctly, it should be possible to re-define how a stream is tokenized. I cannot remember how, though.

A challenge: Write the shortest SUS-compliant program which reverses a string.

ghostdog74 · 11-17-2009, 09:21 AM

Code:

$ more file
foo
bar
foo1
bar1

$ awk -vFS= -vRS= '{for(i=NF;i>0;i--)printf $i}' file
1rab
1oof
rab
oof

bigearsbilly · 11-17-2009, 11:03 AM

Quote:

Originally Posted by Willard

If I remember correctly, it should be possible to re-define how a stream is tokenized. I cannot remember how, though.

it's IFS but it won't split words into characters.

here's a neat trick:

Code:

(IFS=:;ls -1 $PATH)

bigearsbilly · 11-17-2009, 11:26 AM

Well, C99 is part of the SUS!

I think using awk is cheating

Can't cope with newlines but...

Code:

$ uname  -inps                                           
FreeBSD foghorn amd64 GENERIC

$ uname  -inps | fold -1 | nl | sort -rn | cut -f2 | tr -d  '\n'
CIRENEG 46dma nrohgof DSBeerF$

wow 10+ years and I never heard of fold or nl before.

cheers OP.

gnashley · 11-17-2009, 12:30 PM

This is longer, but uses pure shell:

Code:

FILE_LINE_COUNT=0
LINE=
IFS=
while read LINE ; do
 # add the curent line to the line counter
 (( FILE_LINE_COUNT++ ))
 OUTPUT_STRING=
 STRING=$LINE
 while [[ "$STRING" != "" ]] ; do
  OUTPUT_STRING=${STRING:0:1}$OUTPUT_STRING
  STRING=${STRING:1}
 done
echo $OUTPUT_STRING
done <"$1"

Actually this uses bash, but if that particular construct doesn't work i think it can still be done with just the shell using another construct.
That code is part of a fuller file which constitutes a pretty complete implementaion of rev using pure bash.