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Debian Almquist Shell (Dash)

While Bash is very popular (and so is its syntax), Dash replaced Bash as non-interactive shell in 2011 with the release of Debian 6 (Squeeze). Dash is POSIX-compliant, small and fast. For the interactive shell Bash is still used – which is absolutely fine.

However, Bash is not POSIX-compliant which can be a portability problem, especially when shared with the open-source community. Instead of memorizing different syntaxes and features between different shells – which can be dangerously confusing – I decided to use exactly one shell. Since Dash is POSIX-compliant, the choice was clear and I stopped writing shell scripts in Bash.

All of the code above is POSIX-compliant. No language based special features are used ("bashism").

History

Debian Almquist Shell (dash) ‹ Almquist Shell (ash) ‹ System V Shell Release 4 (SVR4) ‹ Bourne Shell (sh)

Manual

See Chapter 2. Shell Command Language of the The Open Group Base Specifications (Issue 7, Edition 2018).

Helpful Tips

1.0 STDERR and STDOUT

In order to redirect output to STDERR, you can use >&2 either at the end or at the beginning to improve readabilty:

>&2 printf '%s\n' "This is an error message."

Suppress standard output:

nft --version >/dev/null

Lets assume the command before (nft ...) is just used in order to receive the exit code to find out, if the package is installed or not:

RESULT=$(nft --version >/dev/null)

In that case we need to both suppress STDOUT and STDERR, because either it will return the version or an error message (command not found):

RESULT=$(nft --version >/dev/null 2>&1)

2.0 Return does not return boolean

Bear in mind that return does not return a boolean, but an exit code which is a positive integer between 0 and 255. While 0 stands for success, any other value indicates an error:

# Check if string is an unsigned integer (also +1 is not considered as be valid)
func_IS_UNSIGNED_INTEGER() {
	NUMBER="$1"
	
	case "$NUMBER" in
		*[!0-9]* | '')
			# False
			return 1
		;;
	esac
	
	# True
	return 0
}

3.0 printf vs echo

You should use printf instead of echo, because it is more portable:

echo

echo "Text with a new line after it."

printf

printf '%s\n' "Text with a new line after it."
References
- https://askubuntu.com/questions/467747/which-is-better-printf-or-echo
- https://unix.stackexchange.com/questions/65803/why-is-printf-better-than-echo

4.0 Quoted Variables

In the most cases, you want to place double-quotes ($VAR) around variables. This also applies to command substitution. Otherwise not all of the content of the variable – including whitespace – is passed, but the variable gets splitted.

Correct

RESULT="$(dpkg-query -s "$PACKAGE_NAME" >/dev/null 2>&1)"

Wrong

RESULT=$(dpkg-query -s "$PACKAGE_NAME" >/dev/null 2>&1)
RESULT=$(dpkg-query -s $PACKAGE_NAME >/dev/null 2>&1)

Discouraged

While backticks (`) – or also called backquotes – are not officially deprecated, their use is discouraged. Instead use $(...).

RESULT="`dpkg-query -s "$PACKAGE_NAME"` >/dev/null 2>&1"
References
- https://unix.stackexchange.com/questions/126927/have-backticks-i-e-cmd-in-sh-shells-been-deprecated
- https://unix.stackexchange.com/questions/118433/quoting-within-command-substitution-in-bash

5.0 Comments

# Single line comment
: '''
Comment
over
multiple
lines.
'''

6.0 Strings

Length

VAR="This string is 34 characters long."
printf '%s\n' "${#VAR}"

Remove from the beginning

VAR="/foo/bar/"
printf '%s\n' "${VAR#/}"

Remove from the end

VAR="/foo/bar/"
printf '%s\n' "${VAR%/}"
References
- https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html

7.0 Assignment of Multiple Variables

Let's say you have a configuration file with following content:

# ID     Name      Country
01       Morton    US
02       Matthew   CA

One way to assign these variables would be using awk '{print $...}':

PARAM_ID="$(printf '%s' "$LINE" | awk '{print $1}')"
PARAM_NAME="$(printf '%s' "$LINE" | awk '{print $2}')"
PARAM_COUNTRY="$(printf '%s' "$LINE" | awk '{print $3}')"

However, this will create one subshell for each parameter. What you could also do is following:

read -r \
PARAM_ID \
PARAM_NAME \
PARAM_COUNTRY \
__TRAILING \
<<-EOF
$(printf '%s' "$LINE")
EOF

The variable __TRAILING at the end covers the rest of the line in case it has more words then expected.