Add flags to skip symlink creation and user interaction

[sindrel]

Adds two new flags:

1. no-symlink (n)
   Makes tfswitch detect and install a version only, without creating a permanent symlink to it. This allows tfswitch to be used as a helper when dynamically switching Terraform versions in environments where multiple tasks run with different versions in parallel, such as CI/CD agents.

2. quiet (q)
   Makes tfswitch switch to a version if it's detected or specified as an argument, but prevents it from prompting the user for input if none is found or provided. This makes tfswitch more automation friendly.

Example use case on a CI/CD agent
An agent host is scheduled to run multiple Terraform Plan/Apply tasks simultaneously from different repositories, which each contain different Terraform configurations, using different versions of Terraform.

With a regular symlink approach, they would have to be queued and run synchronously, as a version switch would have to be made between each run.

However, if the terraform binary (or symlink) is replaced with a script (example below) that catches the arguments sent to terraform, performs a tfswitch using both the no-symlink and quiet flags, then catches the output from tfswitch and uses the desired binary to run the specific task, this can be done dynamically with each run, in parallel.

This utilizes tfswitch's brilliant ability to detect and install the required version, while allowing concurrency.

Example PoC script

#!/bin/bash

TF_DEFAULT_VERSION=0.13.2
TF_BIN_LOCATION=~/.terraform.versions

TF_SWITCH=`tfswitch-test -nq`
TF_DETECTED_VERSION=`echo "$TF_SWITCH" | awk -F '\"|\"' '{print $2}' | grep "\S"`

echo "$TF_SWITCH"

if [ "$TF_DETECTED_VERSION" == "" ]; then
    TF_DETECTED_VERSION=$TF_DEFAULT_VERSION
fi

TF_BIN="$TF_BIN_LOCATION/terraform_$TF_DETECTED_VERSION"

echo "Using version: $TF_DETECTED_VERSION"
echo ""

$TF_BIN "$@"

Suggestions and improvements are welcome.

-Sindre

[mikesir87]

Any progress on this PR? I'd love to see the --quiet flag, as we're looking to use this in a CI template file that'll be shared with teams and we don't want pipelines to get hung up. If it's easier to separate this PR into two to let the quiet flag get through, I'd be happy to do that (if the original author is unavailable).