/bin/bash - Proper Whitespace Handling - Whitespace Safety - End-of-Options Parameter Security

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Supporting multiple command line parameters with spaces in wrapper scripts and End-of-Options Parameter (--) for better security.

safe echo[edit]

TODO

shellcheck bug reports:

helper-scripts:

A drop-in replacement safe_echo would be useful to have.

Based on echo:

  • implementation: abort if first character is a -
  • advantage: supports colors etc.
  • disadvantage: break on legitimate output such as "--example"

Based on printf:

  • disadvantage: no support for colors etc.
  • advantae: does not break if first character is -

Bash Proper Whitespace Handling[edit]

#!/bin/bash

## https://yakking.branchable.com/posts/whitespace-safety/

set -e

app_user=user
lib_dir="/tmp/test/lib/program with space/something spacy"
main_app_dir="/tmp/test/home/user/folder with space/abc"
mkdir -p "$lib_dir"
mkdir -p "$main_app_dir"

declare -a cmd

cmd+=("cp")
cmd+=("-r")
cmd+=("${lib_dir}")
cmd+=("${main_app_dir}/")

"${cmd[@]}"

Use of End-of-Options Parameter (--)[edit]

The end-of-options parameter "--" is crucial because otherwise inputs might be mistaken for command options. This might even be a security risk. Here are examples using the `sponge` command:

echo test

Result: OK. This works because "testfilename" doesn't look like an option.

echo test

Result: Fail. The command interprets "--testfilename" as a series of options: 

sponge: invalid option -- '-'
sponge: invalid option -- 't'
sponge: invalid option -- 'e'
...
test

echo test

Result: OK. The `--` signals that "--testfilename" is a filename, not an option.

Conclusion:

  • The "--" parameter marks the end of command options.
  • Use "--" add the end of a command prevent misinterpretation.
  • This technique is applicable to many Unix/Linux commands, not just sponge.

nounset - Check if Variable Exists[edit]

#!/bin/bash

set -x
set -e
set -o nounset

## Enable for testing.
#unset HOME

if [ -z "${HOME+x}" ]; then
    echo "Error: HOME is not set."
fi

echo "$HOME"

Safely Using Find with End-Of-Options[edit]

Example:

Note: Variable could be different. Could be for example --/usr.

folder_name="/usr"

printf '%s' "${folder_name}" | find -files0-from - -perm /u=s,g=s -print0

Of if safe_echo_nonewline is available from helper-scripts.

# shellcheck disable=SC1091 source /usr/libexec/helper-scripts/safe_echo.sh safe_echo_nonewline "${folder_name}" | find -files0-from - -perm /u=s,g=s -print0

Not using bash's built-in echo, because it does not support end-of-options ("--").

Not using /usr/bin/echo, because it does not support end-of-options ("--").

## Broken! ## The '-n' option is needed to avoid piping a newline to 'find'. echo -n -- "${folder_name}" | find -files0-from - -perm /u=s,g=s -print0

misc[edit]

base_name="${file_name##*/}"
file_extension="${base_name##*.}"

coding style[edit]

  • use:
    • shellcheck
    • safe-rm
    • scurl
    • str_replace
    • append-once
  • use ${variable} style
  • use shell options
set -o errexit
set -o nounset
set -o errtrace
set -o pipefail

See Also[edit]

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