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30.4.1 Comparing two strings using Debian’s algorithm

The Debian program dpkg (available on all Debian and Ubuntu installations) can compare two strings using the --compare-versions option.

To use it, create a helper shell function (simply copy & paste the following snippet to your shell command-prompt):

compver() {
  dpkg --compare-versions "$1" lt "$2" \
    && printf "%s\n" "$1" "$2" \
    || printf "%s\n" "$2" "$1" ; \
}

Then compare two strings by calling compver:

$ compver 8.49 8.5
8.5
8.49

Note that dpkg will warn if the strings have invalid syntax:

$ compver "foo07.7z" "foo7a.7z"
dpkg: warning: version 'foo07.7z' has bad syntax:
               version number does not start with digit
dpkg: warning: version 'foo7a.7z' has bad syntax:
               version number does not start with digit
foo7a.7z
foo07.7z

$ compver "3.0/" "3.0.5"
dpkg: warning: version '3.0/' has bad syntax:
               invalid character in version number
3.0.5
3.0/

To illustrate the different handling of hyphens between Debian and coreutils’ algorithms (see Minus/Hyphen and Colon characters):

$ compver abb ab-cd 2>/dev/null     $ printf "abb\nab-cd\n" | sort -V
ab-cd                               abb
abb                                 ab-cd

To illustrate the different handling of file extension: (see Special handling of file extensions):

$ compver hello-8.txt hello-8.2.txt 2>/dev/null
hello-8.2.txt
hello-8.txt

$ printf "%s\n" hello-8.txt hello-8.2.txt | sort -V
hello-8.txt
hello-8.2.txt