Ubuntu: remove old kernels

remove-old-kernels-example.sh

## TL;DR

# Simulate purging all kernels apart from the two most recent ones:
dpkg-query -Wf'${binary:Package}\n' 'linux*-3.*.*-*' | sort -t. -k2 | head -n-8 | xargs apt-get -s purge

# Purge all kernels apart from the two most recent ones:
dpkg-query -Wf'${binary:Package}\n' 'linux*-3.*.*-*' | sort -t. -k2 | head -n-8 | xargs sudo apt-get -y purge


## Explanation

# dpkg -W         = short for `dpkg --show` (see below)
# dpkg -f'FORMAT' = short for `dpkg --show-format='FORMAT'` (see below)

# sort -t.  = use '.' as field separator instead of ' '
# sort -k2  = sort starting from field 2 (counted from 1) and stopping at end of line (default)
# head -n-8 = each kernel consists of 4 binary packages - show all but the last 2*4 packages
# xargs     = take standard input and pass to the end of the command that follows


## Other Related Commands

# List installed kernel images (here, of series 3.x.y):
# (show only relevant package names and nothing else)
dpkg-query --show --showformat='${binary:Package}\n' 'linux-image-3.*-generic'

# or shorter, using flags:
dpkg-query -Wf'${binary:Package}\n' 'linux-image-3.*-generic'

# or using default output format (package name + version):
dpkg-query -W 'linux-image-3.*-generic'

# or for a human-readable output (a terminal window sized table):
dpkg -l 'linux-image-3.*-generic'

# Example output from the first two commands above:
# linux-image-3.13.0-46-generic
# linux-image-3.13.0-48-generic
# linux-image-3.13.0-49-generic
# linux-image-3.13.0-51-generic
# linux-image-3.13.0-52-generic

# Simulate purging all kernels apart from the two arbitrary ones (51, 52) noted from above:
dpkg-query -Wf'${binary:Package}\n' 'linux*-3.*.*-*' | grep -Ev '51|52' | xargs apt-get -s purge

# Purge all kernels apart from the two arbitrary ones (51, 52) noted from above:
# (commented out, as potentially destructive... clearly!)
# dpkg-query -Wf'${binary:Package}\n' 'linux*-3.*.*-*' | grep -Ev '51|52' | xargs sudo apt-get -y purge

# Explanation
# grep -E REGEX_PATTERN  = same as `egrep REGEX_PATTERN`
# grep -v PATTERN        = an inversion of `grep PATTERN`