Bash: Two for loops at once?

I am not sure how to do this at all.

I have two text files, FILE1 and FILE2.

FILE1

FILE2

I would like to run a for loop for each file at the same time and display the
contents next to each other.

For example,

for $i in $(cat FILE1); do echo $i; done

for $i in $(cat FILE1); do echo $i; done

for $j in $(cat FILE2); do echo $j; done

for $j in $(cat FILE2); do echo $j; done

I would like to combine these two commands, so I can run both files at the same time and have an output like $i $j

$i $j

paste

paste

5 Answers
5

Solution 1

Use the paste command

paste

paste FILE1 FILE2

paste FILE1 FILE2

Details for paste command

Another resource

Solution 2

You can do this if they have the same number of lines.

#!/bin/bash
t=$(cat FILE1 | wc -l)
for i in `seq 1 $t`;
do
cat FILE1|head -n $i|tail -n 1
cat FILE2|head -n $i|tail -n 1
done


You can extend it to what you want for unequal number of lines.

You shouldn't be using for loops at all; see Bash FAQ 001. Instead, use two read commands in a single while loop.

for

read

while

while IFS= read -r line1 && IFS= read -r line2 <&3; do
printf '%s | %sn' "$line1" "$line2"
done < FILE1 3< FILE2


Each read command reads from a separate file descriptor. In this version, the loop will exit when the shorter of the two files is exhausted.

read

There are two different questions being asked here. Other answers address the question of how to display the contents of the file in 2 columns. Running two loops simultaneously (which is the wrong way to address the first problem) can be done by running them each asynchronously: for i in $seqi?; do $cmdi?; done & for j in $seqj?; do $cmdj?; done & wait

for i in $seqi?; do $cmdi?; done & for j in $seqj?; do $cmdj?; done & wait

Although you could also implement paste -d ' ' file1 file2 with something like:

paste -d ' ' file1 file2

while read line_from_file1; p=$?; read line_from_file2 <&3 || test "$p" = 0; do
echo "$line_from_file1" "$line_from_file2"
done < file1 3< file2


Another option, in bash v4+ is to read the two files into 2 arrays, then echo the array elements side-by-side:

bash

# Load each file into its own array
readarray -t f1 < file1
readarray -t f2 < file2

# Print elements of both arrays side-by-side
for ((i=0;i<$#f1[@];i++)) ; do echo $f1[i] $f2[i]; done


Or change the echo to printf if you want the columns to line up:

echo

printf

printf "%-20s %-20sn" $f1[i] $f2[i]


I'm not suggesting you do this if your files are 100s of megabytes.

You can use another tab by pressing Command T and then run the each bash script on another tab.

Thank you.

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