Safe ways to do things in bash

safe_base.md

Safe ways to do things in bash

Original https://github.com/anordal/shellharden/blob/master/how_to_do_things_safely_in_bash.md

Why bash?

Bash has arrays and a safe mode, which may make it just about acceptable under safe coding practices, when used correctly.

Fish is easier to use correctly, but lacks a safe mode. Prototyping in fish is therefore a good idea, provided that you know how to translate correctly from fish to bash.

Preface

This guide accompanies ShellHarden, but I also recommend ShellCheck: ShellHarden's rules shall not disagree with ShellCheck.

Bash is not a language where the correct way to do something is also the easiest. If there is anything like a driver's license for writing bash, it must be rule zero of BashPitfalls: Always use quotes.

The first thing to know about bash coding

Quote like a maniac! An unquoted variable is to be treated as an armed bomb: It explodes upon contact with whitespace. Yes, "explode" as in splitting a string into an array. Specifically, variable expansions, like $var, and also command substitutions, like $(cmd), undergo word splitting, whereby the contained string expands to an array by splitting it on any of the characters in the special $IFS variable, which is whitespace by default. This is mostly invisible, because most of the time, the result is a 1-element array, which is indistinguishable from the string you expected.

Not only that, but wildcard characters (*?) are also expanded. This process happens after word splitting, so that when a resulting word contains any wildcard characters, that word is now a wildcard pattern, expanding to any matching file paths you may happen to have. So this feature actually looks at your filesystem!

Quoting inhibits both word splitting and wildcard expansion, for variables and command substitutions.

Variable expansion:

• Good: "$my_var"
• Bad: $my_var

Command substitution:

• Good: "$(cmd)"
• Bad: $(cmd)

There are exceptions where quoting is not necessary, but because it never hurts to quote, and the general rule is to be scared when you see an unquoted variable, pursuing the non-obvious exceptions is, for the sake of your readers, questionable. It looks wrong, and the wrong practice is common enough to raise suspicion: Enough scripts are being written with broken handling of filenames that whitespace in filenames is often avoided…

The only exceptions honored by Shellharden are variables of numeric content, such as $?, $# and ${#array[@]}.

Should I use backticks?

Command substitutions also come in this form:

• Correct: "`cmd`"
• Bad: `cmd`

While it is possible to use this style correctly, it looks even more awkward in quotes and is less readable when nested. The consensus around this one is pretty clear: Avoid.

Shellharden rewrites these into the dollar-parenthesis form.

Should I use curly braces?

• Bad: some_command $arg1 $arg2 $arg3
• Extra bad (cargo culting unnecessary braces): some_command ${arg1} ${arg2} ${arg3}
• Correct: some_command "${arg1}" "${arg2}" "${arg3}"
• Better: some_command "$arg1" "$arg2" "$arg3"

In the "extra bad" and "correct" examples, braces compete with quotes under the limits of tolerable verbosity.

Shellharden will rewrite all these variants into the "better" form.

Braces on variable expansions are sometimes necessary (to limit the boundary of the variable name) if you absolutely want to include more string content within the same pair of quotes. This is always avoidable:

• Good: "${var1}more string content$var2"
• Good: "$var1""more string content""$var2"

Shellharden is neutral among these interpolation styles, but will pick the first one if asked to put down quotes anywhere.

Gotcha: Numbered arguments

Unlike normal identifier variable names (in regex: [_a-zA-Z][_a-zA-Z0-9]*), numbered arguments require braces, this time to extend the boundary of the variable name. ShellCheck says:

echo "$10"
      ^-- SC1037: Braces are required for positionals over 9, e.g. ${10}.

Shellharden will refuse to fix this (deemed too subtle).

Since braces are required above 9, Shellharden permits them on all numbered arguments.

Use arrays FTW

In order to be able to quote all variables, you must use real arrays when that's what you need, not whitespace separated pseudo-array strings.

The syntax is verbose, but get over it. This bashism is reason alone to drop posix compatibility for most shellscripts.

Good:

array=(
    a
    b
)
array+=(c)
if [ ${#array[@]} -gt 0 ]; then
    rm -- "${array[@]}"
fi

Bad:

pseudoarray=" \
    a \
    b \
"
pseudoarray="$pseudoarray c"
if ! [ "$pseudoarray" = '' ]; then
    rm -- $pseudoarray
fi

Those exceptional cases where you actually intend to split the string

Example with \v as delimiter (note the second occurence):

IFS=$'\v' read -d '' -ra a < <(printf '%s\v' "$s")

This avoids wildcard expansion, and it works no matter if the delimiter is \n. The second occurence of the delimiter preserves the last element if it's empty. For some reason, the -d option must come first, so putting the options together as -rad '', which is tempting, doesn't work. Tested with bash 4.2, 4.3 and 4.4.

Alternatively, for bash 4.4:

readarray -td $'\v' a < <(printf '%s\v' "$s")

How to begin a bash script

Something like this:

#!/usr/bin/env bash
if test "$BASH" = "" || "$BASH" -uc 'a=();true "${a[@]}"' 2>/dev/null; then
    # Bash 4.4, Zsh
    set -euo pipefail
else
    # Bash 4.3 and older chokes on empty arrays with set -u.
    set -eo pipefail
fi
shopt -s nullglob globstar

This includes:

• The hashbang:
  • Portability consideration: The absolute path to env is likely more portable than the absolute path to bash. Case in point: NixOS. POSIX mandates /bin/sh and the existence of env, but bash is not a posix thing. If you want to be 100% covered by posix, give up – wrap it in a posix script instead.
  • Safety consideration: No language flavor options like -euo pipefail here! It is not actually possible when using the env redirection, but even if your hashbang begins with #!/bin/bash, it is not the right place for options that influence the meaning of the script, because it can be overridden, which would make it possible to run your script the wrong way. However, options that don't influence the meaning of the script, such as set -x would be a bonus to make overridable (if used).
• What we need from Bash's unofficial strict mode, with set -u behind a feature check. We don't need all of Bash's strict mode because being shellcheck/shellharden compliant means quoting everything, which is a level beyond strict mode. Furthermore, set -u must not be used in Bash 4.3 and earlier. Because that option, in those versions, treats empty arrays as unset, which makes arrays unusable for the purposes described herein. With arrays being the second most imporant advice in this guide (after quoting), and the sole reason we're sacrificing POSIX compatibility, that's of course unacceptable: If using set -u at all, use Bash 4.4 or another sane shell like Zsh. This is easier said than done if there is a possibility that someone might run your script with an obsolete version of Bash. Fortunately, what works with set -u will also work without (unlike set -e). Thus why putting it behind a feature check is sane at all. Beware of the presupposition that testing and development happens with a Bash 4.4 compatible shell (so the set -u aspect of the script gets tested). If this concerns you, your other options are to give up compatibility (by failing if the feature check fails) or to give up set -u.
• shopt -s nullglob is what makes for f in *.txt work correctly when *.txt matches zero files. The default behavior (aka. passglob) – pass the pattern as-is if it happens to match nothing – is dangerous for several reasons. As for globstar, that enables recursive globbing. Globbing is easier to use correctly than find. So use it.

But not:

IFS=''
set -f
shopt -s failglob

• Setting the internal field separator to the empty string disables word splitting. Sounds like the holy grail. Sadly, this is no complete replacement for quoting variables and command substitutions, and given that you are going to use quotes, this gives you nothing. The reason you must still use quotes is that otherwise, empty strings become empty arrays (as in test $x = ""), and indirect wildcard expansion is still active. Furthermore, messing with this variable also messes with commands like read that use it, breaking constructs like cat /etc/fstab | while read -r dev mnt fs opt dump pass; do echo "$fs"; done'.
• Disabling wildcard expansion: Not just the notorious indirect one, but also the unproblematic direct one, that I'm saying you should want to use. So this is a hard sell. And this too should be completely unnecessary for a script that is shellcheck/shellharden conformant.
• As an alternative to nullglob, failglob fails if there are zero matches. While this makes sense for most commands, for example rm -- *.txt (because most commands that take file arguments don't expect to be called with zero of them anyway), obviously, failglob can only be used when you are able to assume that zero matches won't happen. That just means you mostly won't be putting wildcards in command arguments unless you can assume the same. But what can always be done, is to use nullglob and let the pattern expand to zero arguments in a construct that can take zero arguments, such as a for loop or array assignment (txt_files=(*.txt)).

How to use errexit

Aka set -e.

Program-level deferred cleanup

In case errexit does its thing, use this to set up any necessary cleanup to happen at exit.

tmpfile="$(mktemp -t myprogram-XXXXXX)"
cleanup() {
    rm -f "$tmpfile"
}
trap cleanup EXIT

Gotcha: Errexit is ignored in command arguments

Here is a nice underhanded fork bomb that I learnt the hard way – my build script worked fine on various developer machines, but brought my company's buildserver to its knees:

set -e # Fail if nproc is not installed
make -j"$(nproc)"

Correct (command substitution in assignment):

set -e # Fail if nproc is not installed
jobs="$(nproc)"
make -j "$jobs"

Caution: Builtins like local and export are also commands, so this is still wrong:

set -e # Fail if nproc is not installed
local jobs="$(nproc)"
make -j"$jobs"

ShellCheck warns only about special commands like local in this case.

Gotcha: Errexit is ignored depending on caller context

Sometimes, POSIX is cruel. Errexit is ignored in functions, scopes and even subshells if the caller is checking its success. These examples all print Unreachable and Great success, despite all sanity.

Subshell:

(
    set -e
    false
    echo Unreachable
) && echo Great success

Scope:

{
    set -e
    false
    echo Unreachable
} && echo Great success

Function:

f() {
    set -e
    false
    echo Unreachable
}
f && echo Great success

This makes bash with errexit practically incomposable – it is possible to wrap your errexit functions so that they still work, but the effort it saves (over explicit error handling) becomes questionable. Consider splitting into completely standalone scripts instead.