shellharden/how_to_do_things_safely_in_bash.md

Safe ways to do things in bash

Why bash?

The attraction of shellscripting is that it offers the concisest way to programmatically run other programs – that is after all the purpose of any shell language.

If that sums up the duties of your program, then a shellscript may be the right tool for the job.

Unfortunately, Bash is not a language where the correct way to do something is also the easiest. Because it isn't (which is to say that Bash is an unsafe language), it takes discipline to avoid creating those systematic bugs encouraged by the language.

What Bash offers the disciplined, however, is the necessary obscure non-POSIX features to do things correctly, and strict modes to make error handling practical.

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

Preface

If there is anything like a driver's license for safe bash coding, it must be rule zero of BashPitfalls: Always use quotes.

This guide accompanies Shellharden, but your author also recommends ShellCheck: Shellharden's rules shall not disagree with ShellCheck.

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 and wildcards. 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 string is split on any of the characters in the special $IFS variable, which is whitespace by default. Furthermore, any wildcard characters (*?) in the resulting words are used to expand those words to match files on your filesystem (indirect pathname expansion). This is mostly invisible, because most of the time, the result is a 1-element array, which is indistinguishable from the original string value.

Quoting inhibits word splitting and indirect pathname expansion, both 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 is harder: Backticks require escaping when nested, and examples in the wild are improperly quoted more often than not.

Shellharden rewrites these into the dollar-parenthesis form.

Should I use curly braces?

Braces are for string interpolation, i.e. usually unnecessary:

• Bad: some_command $arg1 $arg2 $arg3
• Bad and verbose: some_command ${arg1} ${arg2} ${arg3}
• Good but verbose: some_command "${arg1}" "${arg2}" "${arg3}"
• Good: some_command "$arg1" "$arg2" "$arg3"

It does not hurt to always use braces, in theory, but in your author's experience, there is a strong negative correlation between unnecessary use of braces and proper use of quotes – nearly everyone chooses the "bad and verbose" instead of "good but verbose" form!

Your author's theories:

• Fear of the wrong thing: Instead of worrying about the real danger (missing quotes), a beginner might worry that a variable named $prefix would influence the expansion of "$prefix_postfix" – this is simply not how it works.
• Cargo cult – writing code in testament to the wrong fear perpetuates it.
• Braces compete with quotes under the limits of tolerable verbosity.

The decision was made to ban unnecessary use of braces: Shellharden will rewrite all these variants into the simplest good form.

Now onto string interpolation, where braces are actually useful:

• Bad (concatenation): $var1"more string content"$var2
• Good (concatentation): "$var1""more string content""$var2"
• Good (interpolation): "${var1}more string content${var2}"

Concatenation and interpolation are equivalent in bash (even for arrays, which is ridiculous).

Because Shellharden is not a style formatter, it is not supposed to change correct code. This is true of the "good (concatenation)" example: As far as shellharden is concerned, this is the holy (canonically correct) form.

Shellharden currently adds and removes braces on an as-needed basis: In the bad example, var1 becomes interpolated with braces, but braces are not accepted on var2 even in the good (interpolation) case, since they are never needed at the end of a string. The latter requirement may well be lifted.

Gotcha: Numbered arguments

Unlike normal identifier variable names (in regex: [_a-zA-Z][_a-zA-Z0-9]*), numbered arguments require braces (string interpolation or not). 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

Here is why arrays are such a basic feature for a shell: Command arguments are fundamentally arrays (and shell scripting is all about commands and arguments). You could say that a shell that makes it artificially impossible to pass multiple arguments around cleanly is comically unfit for purpose.

For our purposes, lack of arrays is the most pressing feature omission of the POSIX shell standard. In other words, POSIX is not the standard. The Bash array syntax is, and Zsh supports a superset of it. Awareness needs to be raised that minimal POSIX compatible shells like Dash and Busybox Ash are only doing us a disfavor.

Those exceptional cases where you actually intend to split the string

Splitting $string on the separator $sep into $array:

Bad (indirect pathname expansion):

IFS="$sep"
array=($string)

Good:

array=()
while read -rd "$sep" i; do
    array+=("$i")
done < <(printf '%s%s' "$string" "$sep")

This works for any separator byte (no UTF-8 or multi-character separator string) except NUL. To make it work for NUL, hardcode the literal $'\0' in place of $sep.

The reason for appending the separator to the end is that the field separator is really a field terminator (postfix, not infix). The distinction matters to the notion of an empty field at the end. Skip this if your input is already field terminated.

Alternatively, for Bash 4:

readarray -td "$sep" array < <(printf '%s%s' "$string" "$sep")

The same notes apply to readarray (hardcoding of NUL, already field terminated input):

readarray -td $'\0' array < <(find -print0)

Readarray gets a small minus point for only working with ASCII separators (still no UTF-8).

If the separator consists of multiple bytes, it is also possible to do this correctly by string processing (such as by parameter substitution).

An alternative with 3 corner cases

The otherwise evil IFS variable has a legitimate use in the read command, where it can be used as another way to separate fields without invoking indirect pathname expansion. IFS is brought into significance by requesting either multiple variables or using the array option to read. By disabling the delimiter -d '', we read all the way to the end. Because read returns nonzero when it encounters the end, it must be guarded against errexit (|| true) if that is enabled.

Split to separate variables:

IFS="$sep" read -rd '' a b rest < <(printf '%s%s' "$string" "$sep") || true

Split to an array:

IFS="$sep" read -rd '' -a array < <(printf '%s%s' "$string" "$sep") || true

The 3 corner cases are tab, newline and space – when IFS is set to one of these as above, read drops empty fields! Because this is often useful though, this method makes the bottom of the recommendation list instead of disqualification.

Corollary: Use while loops to iterate strings and command output

Shellharden won't let you get away with this:

for i in $(seq 1 10); do
    echo "$i"
done

The intuitive fix – piping into the loop – is not always cool, because the pipe operator's right operand becomes a subshell. Not that it matters for this silly example, but it would surprise many to find that this loop can't manipulate outside variables:

seq 1 10 | while read -r i; do
    echo "$i"
done

To avoid future surprises, the bulk of the code should typically not be the subshell. This is all right:

while read -r i; do
    echo "$i"
done < <(seq 1 10)

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 the existence of env, but bash is not a posix thing.
  • 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 $x = ""), and indirect pathname expansion is still active. Furthermore, messing with this variable also messes with commands like read that use it, breaking constructs like cat /etc/fstab | while read -r dev mnt fs opt dump pass; do echo "$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 end a bash script

Goal: The script's exit status should convey its overall success or failure.

Reality: The script's exit status is that of the last command executed.

There is a wrong way to end a bash script: Letting a command used as a condition be the last command executed, so that the script "fails" iff the last condition is false. While that might happen to be correct for a script, it is a way to encode the exit status that looks accidental and is easily broken by adding or removing code to the end.

The rightness criterion here is that the last statement follows the "Errexit basics" below. When in doubt, end the script with an explicit exit status:

exit 0

How to use errexit

Aka set -e.

Errexit basics

Background: If a command that is not used as a condition returns nonzero, the interpreter exits at that point.

Failure is trivial to suppress:

command || true

Don't skimp on if-statements. You can't use && as a shorthand if-statement without always using || as an else-branch. Otherwise, the script terminates if the condition is false.

Bad:

command && …

Good (contrived):

command && … || true

Good (contrived):

! command || …

Good (idiomatic):

if command; then
    …
fi

To capture a command's output while using it as a condition, use an assignment as the condition (but see below on not using local on assignments):

if output="$(command)"; then
    …
fi

If at all using the exit status variable $?, it must be conditioned on the command (specifically, it only makes sense to use in the conditional branch where it is nonzero). Otherwise, your script won't live to see this variable whenever it is nonzero.

Bad:

command
if test $? -ne 0; then
    echo Command returned $?
fi

Good:

if ! command; then
    echo Command returned $?
fi

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.

To use local, separate the declaration from the assignment:

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

Gotcha: Errexit is ignored depending on caller context

Sometimes, POSIX is cruel. Errexit is ignored in functions, group commands 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

Group command:

{
    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.

How to write conditions

Should I use double brackets?

What for? It does not matter.

Issue: test, [ and [[ are largely interchangeable.

If you are following this guide, the usual arguments don't apply:

• Inside double brackets [[ ]], unquoted variables and command substitutions are safe (from word splitting and indirect pathname expansion). That's a partial solution to a problem we don't have – following this guide implies not doing that anywhere to begin with. If you are, you aren't after shellhardening your scripts.
• The usual counterargument is POSIX compatibility. We sacrificed that for arrays.

Other concerns:

—What if I have n00b contributors?

This argument goes both ways: [[ has a more forgiving syntax because it is syntax, not a command. Quoting is required everywhere else. The fewer exceptions, the lesser confusion. If you want to be pedagogical, use the test command – it is honest about being a command, not syntax.

—What if [[ has a feature I need?

Chances are that you don't know the substitute.

• Pattern matching ([[ $path == *.png || $path == *.gif ]]): This is what case is for.
• Logical operators: The usual suspects && and || work just fine – outside commands – and can be grouped with group commands: if { true || false; } && true; then echo 1; else echo 0; fi.
• Checking if a variable exists ([[ -v varname ]]): Yes, this is possibly a killer argument, but consider the programming style of always setting variables, so you don't need to check if they exist.

Shellharden will not stop you from using quotes in syntactical contexts where it does not matter, but that would deviate from common practice.

How to avoid invoking the shell with improper quoting

When invoking a command from other programming languages, the wrong thing to do is often the easiest: implicitly invoking the shell. If that shell command is static, fine – either it works, or it doesn't. But if your program is doing any kind of string processing to assemble that command, realize that you are generating a shellscript! Rarely what you want, and tedious to do correctly:

• quote each argument
• escape relevant characters in the arguments

No matter which programming language you are doing this from, there are at least 3 ways to construct the command correctly. In order of preferece:

Plan A: Avoid the shell

If it's just a command with arguments (i.e. no shell features like piping or redirection), choose the array representation.

• Bad (python3): subprocess.check_call('rm -rf ' + path)
• Good (python3): subprocess.check_call(['rm', '-rf', path])

Bad (C++):

std::string cmd = "rm -rf ";
cmd += path;
system(cmd);

Good (C/POSIX), minus error handling:

char* const args[] = {"rm", "-rf", path, NULL};
pid_t child;
posix_spawnp(&child, args[0], NULL, NULL, args, NULL);
int status;
waitpid(child, &status, 0);

Plan B: Static shellscript

If the shell is needed, let arguments be arguments. You might think this was cumbersome – writing a special-purpose shellscript to its own file and invoking that – until you have seen this trick:

• Bad (python3): subprocess.check_call('docker exec {} bash -ec "printf %s {} > {}"'.format(instance, content, path))
• Good (python3): subprocess.check_call(['docker', 'exec', instance, 'bash', '-ec', 'printf %s "$0" > "$1"', content, path])

Can you spot the shellscript?

That's right, the printf command with the redirection. Note the correctly quoted numbered arguments. Embedding a static shellscript is fine.

The examples run in Docker because they wouldn't be as useful otherwise, but Docker is also a fine example of a command that runs other commands based on arguments. This is unlike Ssh, as we will see.

Last option: String processing

If it has to be a string (e.g. because it has to run over ssh), there is no way around it. We must quote each argument and escape whatever characters are necessary to escape within those quotes. The simplest is to go for single quotes, since these have the simplest escaping rules – only one: ' → '\''.

A very typical filename, in single quotes:

echo 'Don'\''t stop (12" dub mix).mp3'

Now, how to use this trick to run commands safely over ssh? It's impossible! Well, here is an "often correct" solution:

• Often correct (python3): subprocess.check_call(['ssh', 'user@host', "sha1sum '{}'".format(path.replace("'", "'\\''"))])

The reason we have to concatenate all the args to a string in the first place, is so that Ssh won't do it the wrong way for us: If you try to give multiple arguments to ssh, it will treacherously space-concatenate the arguments without quoting.

The reason this is not generally possible is that the correct solution depends on user preference at the other end, namely the remote shell, which can be anything. It can be your mother, in principle. Assuming that the remote shell is bash or another POSIX compatible shell, the "often correct" will in fact be correct, but fish is incompatible on this point.

How to be Fish compatible

This is only necessary if you are forced to interoperate with a user's favourite shell, such as when implementing ssh-copy-id.

The issue with supporting Fish is that the subset of common syntax with POSIX/Bash is mostly useless. The general approach is therefore to duplicate the code – obviously against any safety recommendation.

But if you must, so be it:

test '\'

echo "This is POSIX!"

test '

echo "This is fish!"

test \'