Take two at a time in a bash "for file in $list" construct - bash

Question

I have a list of files where two subsequent ones always belong together. I would like a for loop extract two files out of this list per iteration, and then work on these two files at a time (for an example, let's say I want to just concatenate, i.e. cat the two files).
In a simple case, my list of files is this:
FILES="file1_mateA.txt file1_mateB.txt file2_mateA.txt file2_mateB.txt"
I could hack around it and say
FILES="file1 file2"
for file in $FILES
do
actual_mateA=${file}_mateA.txt
actual_mateB=${file}_mateB.txt
cat $actual_mateA $actual_mateB
done
But I would like to be able to handle lists where mate A and mate B have arbitrary names, e.g.:
FILES="first_file_first_mate.txt first_file_second_mate.txt file2_mate1.txt file2_mate2.txt"
Is there a way to extract two values out of $FILES per iteration?

Answer 1

Use an array for the list:
files=(fileA1 fileA2 fileB1 fileB2)
for (( i=0; i<${#files[#]} ; i+=2 )) ; do
echo "${files[i]}" "${files[i+1]}"
done

Answer 2

You could read the values from a while loop and use xargs to restrict each read operation to two tokens.
files="filaA1 fileA2 fileB1 fileB2"
while read -r a b; do
echo $a $b
done < <(echo $files | xargs -n2)

Answer 3

You could use xargs(1), e.g.
ls -1 *.txt | xargs -n2 COMMAND
The switch -n2 let xargs select 2 consecutive filenames from the pipe output which are handed down do the COMMAND
To concatenate the 10 files file01.txt ... file10.txt pairwise
one can use
ls *.txt | xargs -n2 sh -c 'cat $# > $1.$2.joined' dummy
to get the 5 result files
file01.txt.file02.txt.joined
file03.txt.file04.txt.joined
file05.txt.file06.txt.joined
file07.txt.file08.txt.joined
file09.txt.file10.txt.joined
Please see 'info xargs' for an explantion.

Answer 4

How about this:
park=''
for file in $files # wherever you get them from, maybe $(ls) or whatever
do
if [ "$park" = '' ]
then
park=$file
else
process "$park" "$file"
park=''
fi
done
In each odd iteration it just stores the value (in park) and in each even iteration it then uses the stored and the current value.

Answer 5

Seems like one of those things awk is suited for
$ awk '{for (i = 1; i <= NF; i+=2) if( i+1 <= NF ) print $i " " $(i+1) }' <<< "$FILES"
file1_mateA.txt file1_mateB.txt
file2_mateA.txt file2_mateB.txt
You could then loop over it by setting IFS=$'\n'
e.g.
#!/bin/bash
FILES="file1_mateA.txt file1_mateB.txt file2_mateA.txt file2_mateB.txt file3_mat
input=$(awk '{for (i = 1; i <= NF; i+=2) if( i+1 <= NF ) print $i " " $(i+1) }'
IFS=$'\n'
for set in $input; do
cat "$set" # or something
done
Which will try to do
$ cat file1_mateA.txt file1_mateB.txt
$ cat file2_mateA.txt file2_mateB.txt
And ignore the odd case without the match.

Answer 6

You can transform you string to array and read this new array by elements:
#!/bin/bash
string="first_file_first_mate.txt first_file_second_mate.txt file2_mate1.txt file2_mate2.txt"
array=(${string})
size=${#array[*]}
idx=0
while [ "$idx" -lt "$size" ]
do
echo ${array[$idx]}
echo ${array[$(($idx+1))]}
let "idx=$idx+2"
done
If you have delimiter in string different from space (i.e. ;) you can use the following transformation to array:
array=(${string//;/ })

Answer 7

You could try something like this:
echo file1 file2 file3 file4 | while read -d ' ' a; do read -d ' ' b; echo $a $b; done
file1 file2
file3 file4
Or this, somewhat cumbersome technique:
echo file1 file2 file3 file4 |tr " " "\n" | while :;do read a || break; read b || break; echo $a $b; done
file1 file2
file3 file4