Bash Versioning

PRIMARY CATEGORY → BASH

RESOURCES
BASH Versioning	See here

If a Shell Script is not POSIX Compliant and a specific Non-POSIX Shell Functionality is being used (e.g. local in Bash), It may arises some errors and unintended actions if that script is interpreted by a POSIX Compliant shell like sh or dash

The same applies if the script implement some functionality that is specific of the shell for which it is being created

Therefore, It’s necessary to perform some checks at the beginning of the script to prevent above problems

Shell Check

POSIX Compliant

foo()
{
        case $( /bin/ps -p "$PPID" -o comm= ) in
 
                *bash)  return 0
                        ;;
 
                *)      printf "Shell not allowed. Exiting...\n" 1>&2
                        return 1
                        ;;
        esac
}

INFO

Above code extracts the executed command which created the Script’s Parent Process

If It is not run by a Bash, returns false. Thus, can be chained with →

$ foo || exit 99 # Script exits with a 99 Error code if not a Bash

It’s POSIX Compliant since only a case statement is used with Command Substitution and Standard Globbing

Note that It’s not necessary to quote that expansion as case statement’s parameter is one of the fews situations which not undergoes Word Splitting and Globbing

CAUTION

Be aware that this check must be performed before any other one, as It checks the shell that should run the script

Bash Version Check

It can be done through BASH_VERSION escalar parameter and BASH_VERSINFO indexed array

• BASH_VERSION →

$ declare -p -- BASH_VERSION
declare -- BASH_VERSION="5.1.16(1)-release"

• BASH_VERSINFO →

$ printf "=%s= " "${BASH_VERSINFO[@]}"
=5= =1= =16= =1= =release= =x86_64-pc-linux-gnu=

vX.0

e.g. Bash v4.0

bar()
{
      (( BASH_VERSINFO[0] < 4 )) && {
              printf "Bash Version < v4.0. Exiting...\n" 1>&2
              return 1
      }
 
      return 0
}

INFO

It is necessary when some implemented functionality is not present in previous versions

Above command uses Arithmetic Operation (( )) to perform Bash Version Checking through BASH_VERSINFO indexed array

vX.X

e.g. Bash v4.4

bar()
{
        (( BASH_VERSINFO[0] < 4 || ( BASH_VERSINFO[0] == 4 && BASH_VERSINFO[1] < 4 ) )) && {
 
                printf "Bash Version < v4.4. Exiting...\n" 1>&2
                return 1
        }
}