multiple positional parameter assignment for "cut" command

Question

First I would like to say thanks ahead of time to all of you, that I hope to be a part of some day. So I have a file that is a list of lines that are 5 parameters long. Such as

Bob:Wilson:Nebraska:34:yes

I need to write a program in BASH that will allow me to view the standard out put horizontally and will allow me to call the lines of text in the file like

$sh filename.sh 1

Bob
Tom
Dave
etc..

or

$sh filename.sh 1 3 4 2

Bob:Nebraska:34:Wilson
Tom:Iowa:27:Anderson
etc..

What I have written so far is

cut -d : -f$1,$2,$3,$4,$5 somefile.txt | paste -s

The problem is if the user puts in less then 5 parameters after sh filename.sh I get an error code. The user is not required to put in 5 but can call up to 5.

score 0 · Answer 1 · answered Apr 05 '17 at 20:52

0

awk to the rescue!

awk -F: -v c="$1 $2 $3 $4 $5" 'BEGIN {n=split(c,a," ")} 
                                     {for(i=1;i<n;i++) printf "%s", $a[i] FS; 
                                      printf "%s\n", $a[n]}' file

try and let me know if you need clarification.

answered Apr 05 '17 at 20:52

karakfa

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I see the use of "awk" a lot to answer these question on this board (I have been creeping for a while) but we have yet to cover it in class so I not familiar with it enough to use it but thank you very much for your input!! I am learning this on line and it leaves a lot to be desired. – Grunt Apr 05 '17 at 21:02

score 0 · Answer 2 · edited May 23 '17 at 10:30

0

Expand Only Existing Parameters

IFS=,
cut -d : -f"$*" somefile.txt

$* expands to 1st parameter $IFS 2nd parameter $IFS ... $IFS last parameter, that is $1,$2 if two parameters were given, $1,$2,$3 if three were given, and so on.

Allow Rearranging

The command from above has a problem: cut cannot rearrange columns. cut -f1,2 is equivalent to cut -f2,1. See rearrange columns using cut. Therefore we use awk instead.

If the parameters were filename.sh 1 3 4 2, then we would need the following awk command:

awk -F: -v OFS=: '{ print $1, $3, $4, $2 }' somefile.txt

-F sets the field seperator for reading the file. -v OFS=: sets the field seperator of the output. print $1, $3, ... means »print the first field, then the third, ...«. Note that '$1' is not the first argument of your script, but a literal string that is interpreted by awk.

We use the same trick as before to build an awk-command from the given parameters.

IFS=,
awk -F: -v OFS=: "{ print ${*/#/$} }" somefile.txt

${*/#/$} prepends $ to each parameter. The expansion will be $ 1st parameter , $ 2nd parameter , ... , $ last parameter.

edited May 23 '17 at 10:30

Community

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1

answered Apr 05 '17 at 21:22

Socowi

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I forgot to add the ". Thanks – Grunt Apr 05 '17 at 22:01
Your script doesn't give the requested output. When called `./script 1 3 4 2` it will output `Bob:Wilson:Nebraska:34` instead of `Bob:Nebraska:34:Wilson`. – reflective_mind Apr 06 '17 at 01:10
`cut` cannot change the order of fields. See [this question](http://stackoverflow.com/q/2129123/6770384). Since Grunt asked primarily about the parameters instead of the actual cutting I assumed that wasn't a problem for him/her. But you are right, I should fix this. – Socowi Apr 06 '17 at 08:43
Thanks to inferno for letting me know that cut will not rearrange the order. – Grunt Apr 10 '17 at 22:08

reflective_mind · Answer 3 · 2017-04-06T01:53:08.753

Personally I would always avoid text processing with shell scripts. There are better tools like sed, awk or perl to name a few. This is how I would do it in pure bash since that seems to be what you are looking for (*):

#!/bin/bash
#file: script.sh

for line in $(<somefile.txt); do
   array=( ${line//:/ } )
   counter=1
   for elem in $@; do
      echo -n "${array[$elem-1]}"
      [ $counter -ne $# ] && echo -n ":"
      ((counter++))
   done
   echo
done

(*): Don't be confused by the syntax highlighting, which recognizes everything after $# as a comment (gray text). It's all syntactically correct in the example script.

Explanation:

The script reads somefile.txt, splits each line at the : characters and stores the resulting fields/elements in the array array. Then it outputs the fields in the order specified by your script arguments ($@).

Here is the output, exactly how you want it:

$ ./script.sh 1
Bob
Tom

$ ./script.sh 1 3 4 2
Bob:Nebraska:34:Wilson
Tom:Iowa:27:Anderson

Also note the following:

In your example you possibly execute your script with the Bourne Shell (sh) and not bash. The script above however requires bash because it uses the substring replacement feature.

multiple positional parameter assignment for "cut" command

3 Answers3

Expand Only Existing Parameters

Allow Rearranging