mjhjmtu

I work in a relatively heterogeneous environment where I may be running different versions of Bash on different HPC nodes, VMs, or my personal workstation.

I like the default behavior of Bash <= 4.1 that expands cd $SOMEPATH into cd /the/actual/path when pressing the Tab key. This is just one example, though; complete_fullquote (though I don't know exactly what it does) may have also changed default behavior at v4.2.

However, the shopt option, direxpand which switches on this non-default behavior in Bash 4.2 and above is not recognized by earlier versions, and results in an error message being printed to the console every time I log in to a node with an older Bash:

-bash: shopt: direxpand: invalid shell option name

What I'd like to do is wrap a conditional around shop -s direxpand to enable that option on Bash > 4.1 in a robust way (i.e., not just redirecting the error output to /dev/null).

Check if direxpand is present in the output of shopt and enable it if it is:

shopt | grep -q '^direxpandb' && shopt -s direxpand

I don't see what's wrong with redirecting errors to /dev/null. If you want your code to be robust to set -e, use the common idiom … || true:

shopt -s direxpand 2>/dev/null || true

If you want to run some fallback code if the option does not exist, use the return status of shopt:

if shopt -s direxpand 2>/dev/null; then
 … # the direxpand option exists
else
 … # the direxpand option does not exist
fi

But if you really dislike redirecting the error away, you can use the completion mechanism to perform introspection. This assumes that you don't have antiquated machines with bash ≤ 2.03 that didn't have programmable completion.

shopt_exists () 
 compgen -A shopt -X !"$1" "$1" >/dev/null

if shopt_exists direxpand; then
 shopt -s direxpand
fi

This method avoids forking, which is slow on some environments such as Cygwin. So does the straightforward 2>/dev/null, I don't think you can beat that on performance.

I found most of a solution in this SE thread.

When you know for sure that a specific shopt option is available at a certain major/minor release of Bash, here's one way to enable it conditionally:

if [[ $BASH_VERSINFO[0] -eq 4 && $BASH_VERSINFO[1] -gt 1 ]]; then
 shopt -s direxpand
fi

Obviously, the conditional expression gets a little more complicated when Bash reaches version 5. Note the braces around BASH_VERINFO[index], which are required, and the use of -eq and -gt, which do integer rather than (locale-dependent) lexical comparisons.

The $BASH_VERSINFO array contains all the information you'd see in the output of bash --version:

bash --version | head -1
# result:
# GNU bash, version 4.3.48(1)-release (x86_64-pc-linux-gnu)

declare -p BASH_VERSINFO
# result:
# declare -ar BASH_VERSINFO='([0]="4" [1]="3" [2]="48" [3]="1" [4]="release" [5]="x86_64-pc-linux-gnu")'

When it isn't clear from the documentation for shopt at which Bash version(s) became supported or changed their behavior, the method proposed by Luciano is probably fine:

# note the '-q' so that the matched pattern isn't actually printed
shopt | grep -q direxpand && shopt -s direxpand

References: 1, 2, 3

Related reading: Set and Shopt - Why Two?