add get utility for obtaining binaries for use

README.md

@@ -1,13 +1,27 @@
-# Use CLI
+# use CLI
 
 This simple program allows switching between versions of an installed binary.
 Assuming you have binaries `kubectl-1.13.4` and `kubectl-1.14.1` installed,
 `use` can create a symlink `kubectl` to one of them by invoking it as
 
-```
+```sh
 use kubectl 1.14.1
 ```
 
 The eligible binaries are taken from and installed to the directory
 `$HOME/.local/bin` by default. This behaviour can be adjusted with the
 environment variables `USECLI_BIN_DIR` and `USECLI_TARGET_DIR`, respectively.
+
+# get CLI
+
+This program invokes registered helpers to obtain a binary suitable for `use`.
+
+```sh
+get kubectl 1.14.1
+```
+
+Helpers are binaries in `GETCLI_HELPER_DIR` (default: `$HOME/.local/bin`) that
+match the pattern `get-<base name>`. They read the version number from the
+first command line argument and write the binary to stdout. Examples can be
+found in the [examples](examples) directory. Binaries will be installed to
+`USECLI_BIN_DIR`.

examples/helper-dummy.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+# This helper installs a program that prints its version.
+
+set -u
+
+cat <<EOF
+#!/bin/sh
+echo "$1"
+EOF

examples/helper-kubectl.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+set -eu
+
+if [ $# -lt 1 ]
+then
+echo "Usage: $0 { version }" >&2
+exit 1
+fi
+
+curl -sSL https://dl.k8s.io/v$1/kubernetes-client-linux-arm64.tar.gz |\
+    tar -xz kubernetes/client/bin/kubectl --to-stdout

get

@@ -0,0 +1,37 @@
+#!/bin/sh
+
+set -eu
+
+iam=`basename $0`
+
+if [ $# -lt 2 ]
+then
+echo "Usage: $iam {name} {version}" >&2
+exit 1
+fi
+
+base="$1"
+version="$2"
+
+helperdir="${GETCLI_HELPER_DIR:-$HOME/.local/bin}"
+bindirdefault="${USECLI_BIN_DIR:-$HOME/.local/bin}"
+bindir="${GETCLI_TARGET_DIR:-$bindirdefault}"
+
+#TODO: verify that directories exist
+
+target="$bindir/$base-$version"
+if [ -e "$target" ]
+then
+echo "Target binary '$target' already exists.'" >&2
+exit 2
+fi
+
+helper="$helperdir/$iam-$base"
+if [ ! -e "$helper" ]
+then
+echo "Helper '$helper' not found.'" >&2
+exit 2
+fi
+
+"$helper" "$version" >"$target"
+chmod +x "$target"

test.sh

@@ -3,3 +3,4 @@
 set -ex
 
 ./test/test-use.sh >/dev/null
+./test/test-get.sh >/dev/null

test/test-get.sh

@@ -0,0 +1,44 @@
+#!/bin/sh
+
+set -u
+
+export GETCLI_HELPER_DIR=`mktemp -d`
+export USECLI_BIN_DIR=`mktemp -d`
+export GETCLI_TESTDIR=`mktemp -d`
+
+cleanup () {
+set +u
+if [ -n "$GETCLI_DEBUG" ]
+then
+return 0
+fi
+rm -rf "$GETCLI_HELPER_DIR"
+rm -rf "$USECLI_BIN_DIR"
+rm -rf "$GETCLI_TESTDIR"
+}
+
+fail () {
+echo "FAIL:" "$@" >&2
+cleanup
+exit 1
+}
+
+get="./get"
+
+sh -n "$get" || fail "did not compile"
+
+"$get" 2>&1 && fail "did not set error code"
+
+ln -s "`pwd`/examples/helper-dummy.sh" "$GETCLI_HELPER_DIR/get-dummy"
+"$get" dummy 1 2>&1 || fail "failed to produce dummy-1"
+"$get" dummy 1 2>&1 && fail "tried to overwrite existing dummy-1"
+"$get" dummy 2 2>&1 || fail "failed to produce dummy-2"
+
+for i in 1 2
+do
+echo $i >"$GETCLI_TESTDIR/expected-$i" 
+"$USECLI_BIN_DIR/dummy-$i" >"$GETCLI_TESTDIR/actual-$i"
+diff "$GETCLI_TESTDIR/expected-$i" "$GETCLI_TESTDIR/actual-$i" || fail "wrong binary $i"
+done
+
+cleanup