I checked it with bash-4.4.0
:
#!/usr/bin/env bash
set -eu
check() {
if [[ ${array[@]} ]]; then
echo not empty
else
echo empty
fi
}
check # empty
array=(a b c d)
check # not empty
array=()
check # empty
and bash-4.1.5
:
#!/usr/bin/env bash
set -eu
check() {
if [[ ${array[@]:+${array[@]}} ]]; then
echo non-empty
else
echo empty
fi
}
check # empty
array=(a b c d)
check # not empty
array=()
check # empty
In the latter case you need the following construct:
${array[@]:+${array[@]}}
for it to not fail on empty or unset array. That's if you do set -eu
like I usually do. This provides for more strict error checking. From the docs:
-e
Exit immediately if a pipeline (see Pipelines), which may consist of a single simple command (see Simple Commands), a list (see Lists), or a compound command (see Compound Commands) returns a non-zero status. The shell does not exit if the command that fails is part of the command list immediately following a while or until keyword, part of the test in an if statement, part of any command executed in a && or || list except the command following the final && or ||, any command in a pipeline but the last, or if the command’s return status is being inverted with !. If a compound command other than a subshell returns a non-zero status because a command failed while -e was being ignored, the shell does not exit. A trap on ERR, if set, is executed before the shell exits.
This option applies to the shell environment and each subshell environment separately (see Command Execution Environment), and may cause subshells to exit before executing all the commands in the subshell.
If a compound command or shell function executes in a context where -e is being ignored, none of the commands executed within the compound command or function body will be affected by the -e setting, even if -e is set and a command returns a failure status. If a compound command or shell function sets -e while executing in a context where -e is ignored, that setting will not have any effect until the compound command or the command containing the function call completes.
-u
Treat unset variables and parameters other than the special parameters ‘@’ or ‘*’ as an error when performing parameter expansion. An error message will be written to the standard error, and a non-interactive shell will exit.
If you don't need that, feel free to omit :+${array[@]}
part.
Also do note, that it's essential to use [[
operator here, with [
you get:
$ cat 1.sh
#!/usr/bin/env bash
set -eu
array=(a b c d)
if [ "${array[@]}" ]; then
echo non-empty
else
echo empty
fi
$ ./1.sh
_/1.sh: line 4: [: too many arguments
empty