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====== Download FASTQ from SRA ======


===== An easy intro =====


The easiest way to download FASTQ files from SRA is using the fastq-dump command from the SRA Toolkit with the command: 

''fastq-dump --gzip --split3 SRR[accession ID]''

Options:
   ''--gzip'' will compress the downloaded FASTQ files (this is strictly recommended to save space)
   ''--split3'' will split your files into _1 and _2 files if it's a paired-end sequencing and will store singleton reads (those that don't have a pair) in a separate file. If it's single-end sequencing, skip this command.

However, you would like to put this command inside a script to download multiple files via slurm. In addition, it's recommended to complicate the script a bit to account for possible connection issues during download. Below is an example script that downloads a set of files with the provided accessions with multiple attempts to download each file in case of failures.


===== Script with download retry attempts =====

==== Daughter script ====

Name: ''fq_download.sh''

<code>

#!/bin/bash

# SCRIPT FOR DOWNLOADING SRA FILES USING THE NCBI SRA TOOLKIT
# NOTE: Run this script from the directory where the "log" directory is located
#
# PURPOSE:
#   This script reads SRA accession IDs from a given file (one per line)
#   and downloads each corresponding SRA file using fastq-dump.
#
# PARAMETERS:
#   1: OUTPUT DIRECTORY   - Directory where downloaded files will be stored.
#   2: ACCESSION FILE     - Text file containing SRA accession IDs (one per line).
#
# SAMPLE USAGE:
#   sbatch src/fq_download.sh <output_directory> <accession_file>
#
# IMPORTANT:
#   - This script downloads files using fastq-dump, gzips and splits paired-end reads (it does nothing to single-end read).
#   - Ensure that the SRA Toolkit is installed and available.

# Check for required parameters
if [ "$#" -ne 2 ]; then
    echo "Usage: $0 <output_directory> <accession_file>"
    exit 1
fi

# Parameters
outdir="$1"
accession_file="$2"

# Ensure the accession file exists
if [ ! -f "$accession_file" ]; then
    echo "Error: Accession file '$accession_file' does not exist."
    exit 1
fi

# Create the output directory if it doesn't exist
mkdir -p "$outdir"

# Function to download an SRA accession using fastq-dump (without split/gzip)
download_sra() {
    local acc="$1"
    echo "Downloading accession: $acc"
    if fastq-dump --gzip --split3 "$acc" -O "$outdir"; then
        echo "Successfully downloaded: $acc"
        return 0
    else
        echo "Error: fastq-dump failed for accession: $acc"
        return 1
    fi
}

# Function to download with retries
download_with_retry() {
    local acc="$1"
    local max_retries=10
    local attempt=1
    while [ $attempt -le $max_retries ]; do
        echo "Attempt $attempt for $acc"
        if download_sra "$acc"; then
            return 0  # Success
        fi
        ((attempt++))
    done
    echo "Failed all $max_retries attempts for $acc"
    return 1
}

# Export the functions and variables for use in GNU Parallel
export -f download_sra download_with_retry
export outdir

# Process all accessions in parallel
accessions=$(grep -v '^#' "$accession_file")
if [ -z "$accessions" ]; then
    echo "Error: No valid accessions found in '$accession_file'."
    exit 1
fi

echo "Processing accessions in parallel..."
parallel -j 20 download_with_retry ::: $accessions

# Check overall exit status and log the result
if [ "$?" -eq 0 ]; then
    echo "All accessions processed successfully."
else
    echo "One or more accessions encountered errors."
fi

</code>

==== Parent script (for slurm) ====

To run the script with your list of files, create a txt file with SRA Accessions per line. 
And create a separate parent script as follows: 

<code>
#!/bin/bash
#SBATCH --mem=10gb
#SBATCH --cpus-per-task=30
#SBATCH --job-name=dwnld_fq
#SBATCH --output=log/fq_download_00.log  # Main log file name

# Parameters
outdir="fq_original"
accession_file="meta/sra_accessions.txt"
logfile="log/fq_download_00.log"

# Start logging
echo "Started at: $(date)" >> "$logfile"
echo "Running fq_download.sh with:" > "$logfile"
echo "Output dir: $outdir" >> "$logfile"
echo "Accession file: $accession_file" >> "$logfile"

# Call the daughter script and redirect both stdout and stderr to the same log
src/fq_download.sh "$outdir" "$accession_file" >> "$logfile" 2>&1

# Log end time
echo "Finished at: $(date)" >> "$logfile"

</code>

Make sure the daughter script's path is correct. 

==== Why is it advisable to use parent and daughter scripts? ====
It is a great practice to log everything you do. It's useful for troubleshooting in the future. If you just have one script and add the parameters on the go, you will not be able to trace back those parameters based on the log files. In other words, you'll have no idea what input you've used to produce your log files. 

In this way, you can have a name for a parent script for each download attempt. And you'll have a log file with the same name. And you won't have to copy paste the whole code in each of the parent scripts, just call the daughter script and that's all!