How to conditionally add flags to shell scripts?

问题

I want to run a command inside my bash script with or without a -v flag depending on if the environment variable $VERBOSE is defined. Something like this:

#!/bin/bash

/usr/local/bin/mongo-connector \
  -m $MONGO_HOST \
  -t $NEO_URI \
  [if $VERBOSE then -v ] \
  -stdout

回答1:

#!/bin/bash

/usr/local/bin/mongo-connector \
  -m $MONGO_HOST \
  -t $NEO_URI \
  ${VERBOSE:+-v} \
  -stdout

If VERBOSE is set and non-empty, then ${VERBOSE:+-v} evaluates to -v. If VERBOSE is unset or empty, it evaluates to the empty string. Note that this is an instance where you must avoid using double quotes. If you write: cmd "${VERBOSE:+-v}" rather than cmd ${VERBOSE+:v}, the behavior is semantically different when VERBOSE is empty or unset. In the former case, cmd is called with one argument (the empty string), while in the latter case cmd is called with zero arguments.

To test if VERBOSE is set to a particular string, you can do things like:

/usr/local/bin/mongo-connector \
  -m $MONGO_HOST \
  -t $NEO_URI \
  $(case ${VERBOSE} in (v1) echo foo;; (v2) echo bar;; (*) echo default; esac )\
  -stdout

回答2:

For readability, consider using an array to store the arguments.

args=(
  -m "$MONGO_HOST"
  -t "$NEO_URI"
  -stdout
)
if [[ -v VERBOSE ]]; then
    args+=(-v)
fi
/usr/local/bin/mongo-connector "${args[@]}"

来源：https://stackoverflow.com/questions/42985611/how-to-conditionally-add-flags-to-shell-scripts