SC2124 – ShellCheck Wiki

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Assigning an array to a string! Assign as array, or use * instead of @ to concatenate.

Problematic code:

# Want to store multiple elements in var
var=$@
for i in $var; do ..; done

or

set -- Hello World
# Want to concatenate multiple elements into a single string
msg=$@
echo "You said $msg"

Correct code:

# Bash: use an array variable
var=( "$@" )
for i in "${var[@]}"; do ..; done

# POSIX sh: without array support, one possible workaround
# is to store elements concatenated with a delimiter (here linefeed/newline)
var=$(printf '%s\n' "$@")
printf '%s\n' "$var" | while IFS='' read -r line; do ..; done

or

#!/bin/sh
set -- Hello World
# Explicitly concatenates all the array elements into a single string
msg=$*
echo "You said $msg"

Rationale:

Arrays and $@ can contain multiple elements. Simple variables contain only one. When assigning multiple elements to one element, the default behavior depends on the shell (bash concatenates with spaces, zsh concatenates with first char of IFS).

Since doing this usually indicates a bug, ShellCheck warns and asks you to be explicit about what you want.

If you want to assign N elements as N elements in Bash or Ksh, use an array, e.g. myArray=( "$@" ).

Dash and POSIX sh do not support arrays. In this case, either concatenate the values with some delimiter that you can split on later (the example uses linefeeds and splits them back up with a while read loop), or keep the values as positional parameters without putting them in an intermediate variable.

If you want to assign N elements as 1 element by concatenating them, use * instead of @, e.g. myVar=${myArray[*]} (this separates elements with the first character of IFS, usually space).

The same is true for ${@: -1}, which results in 0 or 1 elements: var=${*: -1} assigns the last element or an empty string.

Exceptions

None.

Adding an element to an array may trigger this warning when you forget the parenthesis.

filelist="${filelist[@]}" "$filename"

What was meant is:

filelist=("${filelist[@]}" "$filename")

Note: This syntax is compatible with older shells; in later shells (bash 3.1+ and zsh 4.2+), you can do filelist+=("$filename").

ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.