install.sh script fixes (#263)

* install.sh: allow user to customize install location

Fixes #262

* install.sh: verify that the install location can be used

* install.sh: don't invoke sudo if it isn't needed

* install.sh: make sure `lab` is executable

If /tmp is mounted with the noexec option, the execution bit is dropped.
Let's make sure to add it back in the install script.

* install.sh: error out if an unset variable is used

* install.sh: error out if part of a pipe chain fails

* install.sh: allow exact location of binary to be specified

By passing it as the first argument to the script.

Not that the argument overrides the environment, in case both are set.

* install.sh: replace `cp+chmod` with `install`

install.sh

@@ -1,16 +1,35 @@
 #!/usr/bin/env bash
 
-set -e
+set -eu -o pipefail
 
-if [[ ! -z $DEBUG ]]; then
+if [[ ! -z ${DEBUG-} ]]; then
     set -x
 fi
 
-if [[ $EUID != 0 ]]; then
+: ${PREFIX:=/usr/local}
+BINDIR="$PREFIX/bin"
+
+if [[ $# -gt 0 ]]; then
+  BINDIR=$1
+fi
+
+_can_install() {
+  if [[ ! -d "$BINDIR" ]]; then
+    mkdir -p "$BINDIR" 2> /dev/null
+  fi
+  [[ -d "$BINDIR" && -w "$BINDIR" ]]
+}
+
+if ! _can_install && [[ $EUID != 0 ]]; then
     sudo "$0" "$@"
     exit "$?"
 fi
 
+if ! _can_install; then
+  echo "Can't install to $BINDIR"
+  exit 1
+fi
+
 case "$(uname -m)" in
     x86_64)
         machine="amd64"
@@ -39,5 +58,5 @@ esac
 latest="$(curl -sL 'https://api.github.com/repos/zaquestion/lab/releases/latest' | grep 'tag_name' | grep --only 'v[0-9\.]\+' | cut -c 2-)"
 
 curl -sL "https://github.com/zaquestion/lab/releases/download/v${latest}/lab_${latest}_${os}_${machine}.tar.gz" | tar -C /tmp/ -xzf -
-cp /tmp/lab /usr/local/bin/lab
-echo "Successfully installed lab into /usr/local/bin/"
+install -m755 /tmp/lab $BINDIR/lab
+echo "Successfully installed lab into $BINDIR/"