Add some key directory customization variables

Author: eritbh

Diff for: README.md

@@ -6,6 +6,8 @@ Utilities for storing and loading SSH keys with 1password
 
 Note that interactive execution is not supported for any of these scripts, since `op signin` is required for all scripts and itself requires interactive password input.
 
+By default, the key storage location is `$TMPDIR/op-ssh-utils`, where `TMPDIR` defaults to `/dev/shm` or `/tmp`, whichever is available. The key storage location can be overridden for all commands with the `OP_KEY_STORAGE_LOCATION` environment variable.
+
 ### Create a new SSH item in the vault
 
     $ ./op-create-identity -H <hostname>
@@ -27,7 +29,6 @@ Search for SSH key items in your vault and read them all into temporary storage.
 
 # Todos
 
-- Find a better place to store keys locally (is /dev/shm appropriate?) + allow user to customize that location
 - Use a different temporary location to allow multiple users on the same system to use the tool (random folder names in the folder symlinked to `~/.local` or something? maybe just make it a bashrc script that automates adding the `Include` rule to the user's SSH config without requiring a persistent directory name across logins?)
 - Better vault item searching/handling, customization of the item template
   - Guidance for adding items to 1Password manually so that this tool can pick them up

Diff for: common.sh

@@ -1,13 +1,18 @@
+# If the user doesn't specify anything, use /dev/shm if
+# available, otherwise use /tmp and hopefully it's tmpfs.
+if [ -d /dev/shm ]; then
+	default_tmpdir="/dev/shm"
+else
+	default_tmpdir="/tmp"
+fi
+export default_temp_storage_root="${OP_KEY_STORAGE_LOCATION:-${TMPDIR:-$default_tmpdir}/op-ssh-utils}"
+
 # Signs into `op` or exits the script
 function op_signin {
 	echo "Signing into 1Password..."
 	eval "$(op signin $@ || echo 'echo "Sign-in failed." >&2; exit 1')"
 }
 
-# Ideally something that won't ever actually be written to disk, but we'll worry
-# about that later
-export default_temp_storage_root="/tmp/op-ssh-utils"
-
 # Writes the public/private key pair for a given vault item to the temporary
 # storage directory, and adds a rule to the temporary ssh config for the host
 # and user.