When attempting to create compressed disk backups using dd piped to gzip, many users encounter the frustrating "Permission denied" error. The command syntax appears correct:

sudo dd if=/dev/sda2 | gzip > /media/disk/sda2-backup.gz

Yet the operation fails despite using sudo. This happens because the permission elevation only applies to the dd command, not the entire pipeline.

There are three effective solutions to maintain permission context throughout the entire operation:

1. Using sudo with the entire pipeline:

sudo sh -c 'dd if=/dev/sda2 | gzip > /media/disk/sda2-backup.gz'

2. Using tee with sudo:

dd if=/dev/sda2 | gzip | sudo tee /media/disk/sda2-backup.gz > /dev/null

3. Writing to a temporary file first:

dd if=/dev/sda2 | gzip > /tmp/tempbackup.gz
sudo mv /tmp/tempbackup.gz /media/disk/sda2-backup.gz

For better compression ratios or faster operation, consider these alternatives to basic gzip:

# Using pigz for multi-core compression (faster)
dd if=/dev/sda2 | pigz -c | sudo tee /media/disk/sda2-backup.gz > /dev/null

# Using xz for better compression (slower)
dd if=/dev/sda2 | xz -z -c | sudo tee /media/disk/sda2-backup.xz > /dev/null

# Using lz4 for fastest operation (lower compression)
dd if=/dev/sda2 | lz4 -c | sudo tee /media/disk/sda2-backup.lz4 > /dev/null

Always verify your compressed backups. For gzip compressed images:

gunzip -c /media/disk/sda2-backup.gz | md5sum
sudo dd if=/dev/sda2 bs=1M | md5sum

The checksums should match. For other compression formats, adjust the decompression command accordingly.

For frequent backups, create a script with progress monitoring:

#!/bin/bash
echo "Starting compressed backup of $1 to $2"
sudo sh -c 'dd if=$1 bs=1M status=progress | gzip -c > $2'
echo "Verifying backup..."
gunzip -c $2 | md5sum
sudo dd if=$1 bs=1M | md5sum
echo "Backup complete"

Save as compressed_backup.sh and run with:

sudo ./compressed_backup.sh /dev/sda2 /media/disk/sda2-backup.gz

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When attempting to compress a disk image during creation using:

sudo dd if=/dev/sda2 | gzip > /media/disk/sda2-backup-10august09.gz

You encounter a "permissions denied" error because the shell handles the redirection (>) with your user's permissions, not root's. The sudo only applies to the dd command.

Here are three working methods:

Method 1: Using tee with sudo

sudo dd if=/dev/sda2 bs=64k | gzip | sudo tee /media/disk/sda2-backup.gz > /dev/null

Method 2: Using sudo for the entire pipeline

sudo sh -c 'dd if=/dev/sda2 bs=64k | gzip > /media/disk/sda2-backup.gz'

Method 3: Using pv for progress monitoring

sudo dd if=/dev/sda2 bs=64k | pv | gzip | sudo tee /media/disk/sda2-backup.gz > /dev/null

For better compression ratios:

sudo dd if=/dev/sda2 bs=1M | pigz -9 | sudo tee /media/disk/sda2-backup.img.gz > /dev/null

Or using parallel bzip2:

sudo dd if=/dev/sda2 bs=1M | pbzip2 -c | sudo tee /media/disk/sda2-backup.img.bz2 > /dev/null

After creation, verify with:

gunzip -c /media/disk/sda2-backup.gz | md5sum
sudo dd if=/dev/sda2 bs=1M | md5sum

The two checksums should match.

To restore:

gunzip -c /media/disk/sda2-backup.gz | sudo dd of=/dev/sda2 bs=1M

Or using pv for progress:

pv /media/disk/sda2-backup.gz | gunzip | sudo dd of=/dev/sda2 bs=1M