从内核提取次要版本到bash变量

Question

I am new to bash and writing a script that needs to compare the minor version of the kernel to see if it is greater than or equal to 10, and exit if it is not. Currently I have something like this:

KERNEL=$(uname -r)
declare -i MINOR_VERSION=$(echo $KERNEL | cut -c 3-4)
if [ "10" -gt "$MINOR_VERSION"]; then exit 0; fi

This is bad code, and doesn't work if the minor version is < 10 since I am using cut and depending on it being two digits. What I probably need is something that parses the minor version by using the dots. Example:

$ uname -r
3.4.0-60-generic
$ MNR_VAR=<awesome bash code, with cut or sed or something>
$ echo $MNR_VAR
4

I have been reading cut and sed documentation but have just been slow picking it up. I would appreciate the help!

TL;DR - looking for a bash command that will extract an int surrounded by the first two dots in a variable. "3.13.0.x" returns '13', "3.2.0.x" returns '2', etc.

EDIT: Some answers as one liners below for those curious.

uname -r | cut -d '.' -f2

uname -r | awk -F . '{print $2}'

kernel="$(uname -r)" | tmp="${kernel#*.}" | minor="${tmp%%.*}" | echo "$minor"

Answer 1

In pure bash:

#!/bin/bash
ker="$(uname -r)"
minker="${ker#*.}"
minker="${minker%%.*}"
echo "$minker"

"${ker#*.}" is the string after the first match of a . in $ker . Thus $minker becomes 13.0-generic... from 3.13.0-generic...

"${minker%%.*}" is the string left by cutting all matches (from right) of . and whatever after it, in $minker . Thus $minker becomes 13 from 13.0-generic...

See the Bash Parameter Expansion Manual for more info

Using Bash Regex:

#!/bin/bash
regex='([0-9]+)\.([0-9]+)'
[[ $(uname -r) =~ $regex ]]
echo ${BASH_REMATCH[2]}

Answer 2

The problem is you are using -c to cut . Don't do that.

Use the -f and -d flags instead to control the delimiter and fields to output.

Or use awk -F . '{print $2}' <<< "$(uname -r)" awk -F . '{print $2}' <<< "$(uname -r)" .

Or use IFS=. read -r _ minor _rest <<< "$(uname -r)"; echo "$minor" IFS=. read -r _ minor _rest <<< "$(uname -r)"; echo "$minor" IFS=. read -r _ minor _rest <<< "$(uname -r)"; echo "$minor" (which has the benefit of not using any external utilities).

The usage of <<< "$(uname -r)" is bash-specific (I believe) but avoids the need for a pipe ( | ) and the sub-shell that it involves.

Answer 3

Extracting just minor version & comparing it with something is risky, because major number can change too...

I normally prefer padding the numbers with zeros, so that they can be easily compared using simple string compare.

kernel_version=$(uname -r | sed -r 's/([0-9]+)/0000\1/g; s/0*([0-9]{4})/\1/g') # gives 0003.0004.0000-0060-generic
if [[ "$kernel_version" < "0003.0010" ]]; then exit 0; fi