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check environment variable is defined

# use the string "is not null" operator
if [ -n $MY_ENV_VAR ]; then
    echo "MY_ENV_VAR is defined as $MY_ENV_VAR"
else
    echo "MY_ENV_VAR is not defined."
fi

check environment variable is not defined

# use the string "is null" operator
if [ -z $MY_ENV_VAR ]; then
    echo "MY_ENV_VAR is not defined."
else
    echo "MY_ENV_VAR is defined as $MY_ENV_VAR"
fi

check available commands/aliases/etc.

compgen is a built-in bash command that can list all the Linux commands you can run.

  • compgen -b - show all the Bash builtins
  • compgen -k - show all the Bash keywords
  • compgen -c - show all the available commands
  • compegn -a - show all the aliases
  • compgen -A function - show all the Bash functions

loops on the command line

$ for f in `ls -1 /data/playlists/data*`; do  basename $f; done