Add private key stored in S3 to ssh-agent

add_ssh_key_from_s3.sh

#!/bin/bash
#
# Download the specified ssh private key from s3 and add it to the ssh-agent so
# we can make requests to a git remote using it.
#
# Works by piping the ascii key through a named pipe to get it from standard
# out to a file descriptor that the ssh-add utility can read. The benefit of
# this over just saving it to a tmp file is the key never touches disk.

# Parse args
if [[ "$#" -lt 1 ]]; then
  echo "Usage: add_ssh_key_from_s3.sh [s3 file path]"
  exit 1
fi
_private_key_s3_path="$1"

# If no agent running just exit. Error code 2 used for not running
ssh-add -l &> /dev/null

if [[ "$?" -eq 2 ]]; then
  echo "No ssh-agent running, exiting"
  exit 1
fi

# Make sure file downloaded successfully
_private_key_contents=$(aws s3 cp "$_private_key_s3_path" -)
if [[ "$?" -ne 0 ]];then
  echo "Error downloading s3 file ${_private_key_s3_path}, exiting"
  exit 1
fi

# Name the temp file based on the s3 file path
_tmp_file="$(echo -n $_private_key_s3_path | perl -C -pe 's/\W+/-/g')"
mkfifo -m 600 "$_tmp_file"

ssh-add "$_tmp_file" &

echo -e "$_private_key_contents" > "$_tmp_file"

rm "$_tmp_file"