命令检测

https://stackoverflow.com/questions/592620/how-can-i-check-if-a-program-exists-from-a-bash-script https://unix.stackexchange.com/questions/404146/what-is-the-best-method-to-test-if-a-program-exists-for-a-shell-script

POSIX compatible:

command -v <the_command>

Example use:

if ! command -v <the_command> &> /dev/null
then
    echo "<the_command> could not be found"
    exit 1
fi

For Bash specific environments:

hash <the_command> # For regular commands. Or...
type <the_command> # To check built-ins and keywords

command -v foo >/dev/null 2>&1 || { echo >&2 "I require foo but it's not installed.  Aborting."; exit 1; }
type    -v foo >/dev/null 2>&1 || { echo >&2 "I require foo but it's not installed.  Aborting."; exit 1; }
hash    -v foo >/dev/null 2>&1 || { echo >&2 "I require foo but it's not installed.  Aborting."; exit 1; }