How can I embed a Python program in a bash command if it needs both a loop and an import?

There are several options here:

• Pass your code as a multi-line string. Note that " is used to delimit Python strings rather than the original ' here for the sake of simplicity: A POSIX-compatible mechanism to embed a literal ' in a single-quoted string is possible, but quite ugly.

  extractImageIds() {
    python -c '
  import sys, json
  for image in json.load(sys.stdin)["Images"]:
      print image["ImageId"]
  ' "$@"
  }
  

• Use bash's C-style escaped string syntax ($'') to embed newlines, as with $'\n'. Note that the leading $ is critical, and that this doesn't work with /bin/sh. See the bash-hackers' wiki on ANSI C-like strings for details.

  extractImageIds() { python -c $'import sys, json\nfor image in json.load(sys.stdin)["Images"]:\n\tprint image["ImageId"]' "$@"; }
  

• Use __import__() to avoid the need for a separate import command.

  extractImageIds() { python -c 'for image in __import__("json").load(__import__("sys").stdin)["Images"]: print image["ImageId"]' "$@"; }
  

• Pass the code on stdin and move the input onto argv; note that this only works if the input doesn't overwhelm your operating system's allowed maximum command-line size. Consider the following example:

  extractImageIds() {
    # capture function's input to a variable
    local input=$(</dev/stdin) || return
    # ...and expand that variable on the Python interpreter's command line
    python - "$input" "$@" <<'EOF'
  import sys, json
  for image in json.loads(sys.argv[1])["Images"]:
      print image["ImageId"]
  EOF
  }
  

  Note that $(</dev/stdin) is a more efficient bash-only alternative to $(cat); due to shell builtin support, it works even on operating systems where /dev/stdin doesn't exist as a file.

All of these have been tested as follows:

extractImageIds <<<'{"Images": [{"ImageId": "one"}, {"ImageId": "two"}]}'

To efficiently provide stdin from a variable, one could run extractImageIds <<<"$variable" instead. Note that the "$@" elements in the wrapper are there to ensure that sys.argv is populated with arguments to the shell function -- where sys.argv isn't referenced by the Python code being run, this syntax is optional.