how can we use multiple variables in single for loop in shell script

nagendrar · 11-11-2011, 03:45 AM

ch1="a b c d"
ch2="w x y z"

i want to the o/p as
a:w
b:x
c:y
d:z

Here I am implementing as below using for loop but failed to run the script

for i in $ch1, j in $ch2
do
echo $i:$j
done

Please help me to use multiple variables in single for loop in shell script

ThanQ
Nagendra

fukawi1 · 11-11-2011, 04:17 AM

You could do it by cycling through a pair of arrays:

Code:

#!/bin/bash

# Define the arrays
array1=("a" "b" "c" "d")
array2=("w" "x" "y" "z")

# get the length of the arrays
length=${#array1[@]}
# do the loop
for ((i=0;i<=$length;i++)); do
        echo -e "${array1[$i]} : ${array2[$i]}"
done

Code:

~ $ ./tmp.sh 
a : w
b : x
c : y
d : z

Although a more skilled bash-ist will probably have a better way...

nagendrar · 11-11-2011, 04:44 AM

Thank You for quick reply.
But here ch1 and ch2 are not arrays. Then How can I implement?

please help me.

ThanQ
Nagendra.

fukawi1 · 11-11-2011, 05:17 AM

A conversation with an associate (LQ user fukawi2) informed me that you can convert a space delimited string with:

Code:

array1=(${ch1// / })
array2=(${ch2// / })

so the example would look like:

Code:

#!/bin/bash
ch1="a b c d"
ch2="w x y z"

# Define the arrays
array1=(${ch1// / })
array2=(${ch2// / })

# get the length of the arrays
length=${#array1[@]}

# do the loop
for ((i=0;i<=$length;i++)); do
        echo -e "${array1[$i]} : ${array2[$i]}"
done

Juako · 11-11-2011, 07:47 AM

In this case you don't need arrays, for .. in can iterate over a string with delimiters:

Code:

#!/bin/bash
ch1="a b c d"
ch2="w x y z"
i=0

IFS=" "
for ch in $ch1; do
    echo ${ch1:$i:1} ${ch2:$i:1}
    (( i+=2 ))
done

David the H. · 11-11-2011, 02:50 PM

Quote:

Originally Posted by fukawi1

A conversation with an associate (LQ user fukawi2) informed me that you can convert a space delimited string with:

Code:

array1=(${ch1// / })
array2=(${ch2// / })

Actually, the "conversion" itself does nothing. All you're doing is replacing spaces with spaces.

parameter expansion

The array is set by breaking up the input string into words. Words are defined by whitespace (space+tab+newline) by default, so you don't need to do anything special in this case.

Code:

array1=( $ch1 )

Now if the input strings were delimited by a different character, then you can use a substitution to convert them to spaces for word-breaking.

Code:

ch1='a:b:c:d'

array1=( ${ch1//:/ } )

However, if the individual entries themselves include whitespace, then this won't work. They'll be broken up as well. You need to change the default delimiter, the IFS variable, so that it ignores whitespace and only divides on the character you want.

Code:

ch1='a a:b b:c c:d d'

savedifs=$IFS
IFS=":"

array1=( $ch1 )

IFS=$savedifs

Another technique is to use the read command.

Code:

IFS=":" read -a array1 <<<"$ch1"

This is a convenient method because when (most) commands are directly prefixed by a variable setting (not separated by a command terminator), then that setting only affects the invoked command. So we don't need to back up or restore the original IFS setting.

http://mywiki.wooledge.org/BashGuide/Arrays

Edit: I just want to add, arrays are certainly the way to go. Any time you have multiple related strings (i.e. lists of entries), you should start by storing them in an array, rather than a scalar variable. This way you can avoid having to break them up later.

Michael22Orr · 11-12-2011, 03:28 AM

Hello Negendar you can use this code, is this case you wont need to use arrays;

#!/bin/bash
ch1="a b c d"
ch2="w x y z"
i=0

IFS=" "
for ch in $ch1; do
echo ${ch1:$i:1} ${ch2:$i:1}
(( i+=2 ))
done
Thanks
ahh bra

suicidaleggroll · 11-12-2011, 03:47 PM

Quote:

Originally Posted by Michael22Orr

Hello Negendar you can use this code, is this case you wont need to use arrays;

#!/bin/bash
ch1="a b c d"
ch2="w x y z"
i=0

IFS=" "
for ch in $ch1; do
echo ${ch1:$i:1} ${ch2:$i:1}
(( i+=2 ))
done
Thanks
ahh bra

That will break as soon as the inputs are no longer a set of single characters with single character delimiters. In other words, it will break as soon as he tries to run it with anything other than the grossly simplified example in the OP.

Juako · 11-12-2011, 03:56 PM

Quote:

Originally Posted by suicidaleggroll

That will break as soon as the inputs are no longer a set of single characters with single character delimiters. In other words, it will break as soon as he tries to run it with anything other than the grossly simplified example in the OP.

That's pretty much evident, but I don't see OP asking a generalized solution or indicating that inputs are going to be different. Perhaps he really wanted to work with that string?

@Michael22Orr:
How is your code different than what i posted? Just curious

David the H. · 11-13-2011, 04:54 AM

By the way, in bash at least, there's a more compact way to iterate through an array.

Code:

for i in "${!array1[@]}"; do
        echo "${array1[i]} : ${array2[i]}"
done

${!array[@]} outputs a list of all existing array indexes. This is especially useful with sparse arrays (arrays with missing elements) and associative arrays (which use strings as indexes).

So the above is all you need as long as array2 has exactly the same indexes as array1.

fshields · 03-08-2017, 05:54 PM

The method that I use is a little more old fashioned, but I think it is clearer to understand and is portable across shells...

Just pass the 'for' loop a list of delimited values and then split them up inside your loop.

Code:

for KeyValPair in 'a,w' 'b,x' 'c,y' 'd,z'
do
  i=`echo "$KeyValPair" | cut -d',' -f1`
  j=`echo "$KeyValPair" | cut -d',' -f2`
  echo $i:$j
done

Here's another example:

Code:

for NamePair in 'John,Smith' 'Jane,Doe' 'Betty,Doe-Smith' 'Madonna,'
do
  FirstName=`echo "$NamePair" | cut -d',' -f1`
  LastName=`echo "$NamePair" | cut -d',' -f2`
  echo "First: $FirstName  Last: $LastName"
done

You must use a delimiter that should never appear in your data. If I had used a dash delimiter, then item #3 would have been 'Betty-Doe-Smith' resulting in a LastName of just 'Doe', which is incorrect. If you use single quotes for each data pair (or triplet, or quaqruplet, etc.) then you can use more exotic characters like a pipe symbol: 'Betty|Doe-Smith'

Note that this technique allows you to have "null" items as in the example for Madonna (who does not have a last name). Just make sure that still include the delimiters.

NewUnixBee · 05-22-2017, 02:21 PM

Quote:

Originally Posted by fshields

The method that I use is a little more old fashioned, but I think it is clearer to understand and is portable across shells...

Just pass the 'for' loop a list of delimited values and then split them up inside your loop.

Code:

for KeyValPair in 'a,w' 'b,x' 'c,y' 'd,z'
do
  i=`echo "$KeyValPair" | cut -d',' -f1`
  j=`echo "$KeyValPair" | cut -d',' -f2`
  echo $i:$j
done

Here's another example:

Code:

for NamePair in 'John,Smith' 'Jane,Doe' 'Betty,Doe-Smith' 'Madonna,'
do
  FirstName=`echo "$NamePair" | cut -d',' -f1`
  LastName=`echo "$NamePair" | cut -d',' -f2`
  echo "First: $FirstName  Last: $LastName"
done

You must use a delimiter that should never appear in your data. If I had used a dash delimiter, then item #3 would have been 'Betty-Doe-Smith' resulting in a LastName of just 'Doe', which is incorrect. If you use single quotes for each data pair (or triplet, or quaqruplet, etc.) then you can use more exotic characters like a pipe symbol: 'Betty|Doe-Smith'

Note that this technique allows you to have "null" items as in the example for Madonna (who does not have a last name). Just make sure that still include the delimiters.

Can we use the same logic for a pair of dates?