The simplest way is to execute the commands in the background, by adding &
to the end of the command:
#!/bin/bash
#script to loop through directories to merge fastq files
sourcedir=/path/to/source
destdir=/path/to/dest
for f in $sourcedir/*
do
fbase=$(basename "$f")
echo "Inside $fbase"
zcat $f/*R1*.fastq.gz | gzip > $destdir/"$fbase"_R1.fastq.gz &
zcat $f/*R2*.fastq.gz | gzip > $destdir/"$fbase"_R2.fastq.gz &
done
From the bash manual:
If a command is terminated by the control operator ‘&’, the shell executes the command asynchronously in a subshell. This is known as executing the command in the background. The shell does not wait for the command to finish, and the return status is 0 (true). When job control is not active (see Job Control), the standard input for asynchronous commands, in the absence of any explicit redirections, is redirected from /dev/null.