如何在POSIX（dash）环境下检测变量是否包含换行符？

Great question! Let's dive into newline detection in the POSIX-compliant dash shell, since it doesn't support some of the fancy extensions you might see in bash or zsh.

Since dash sticks strictly to POSIX standards, we can't use bash-specific tricks like [[ ... ]] or $'\n' escape sequences. Here are two reliable, dash-friendly ways to check for newlines in a variable:

1. Pure Shell case Statement (No External Tools) #

This is the most efficient approach because it uses only built-in shell features, no extra processes:

# Example variable with a newline
my_var="Hello
World"

case "$my_var" in
  *"
"*) echo "Variable contains a newline" ;;
  *) echo "No newline found in variable" ;;
esac

Important note: The pattern *" "* includes a literal newline—you need to actually press Enter between the two double quotes when writing this code. It's not a typo!

2. Using grep (Cross-POSIX Compatible) #

If you don't mind using an external utility, grep works reliably across all POSIX shells including dash:

my_var="Line 1"

if printf '%s' "$my_var" | grep -qF "$(printf '\n')"; then
  echo "Newline detected!"
else
  echo "No newline present."
fi

Here, printf '\n' generates a literal newline, and grep -qF searches for it quietly without producing extra output.

Chances are the "specific shell" methods you're referencing are bash/zsh-specific ones like:

# Bash-only, will FAIL in dash
[[ "$my_var" == *$'\n'* ]]

These methods are not reliable in dash for two critical reasons:

• dash doesn't support the [[ ... ]] conditional syntax (it's an extension, not part of POSIX).
• dash doesn't recognize ANSI-C escape sequences like $'\n' to create literal newlines.

Running these in dash will throw syntax errors or return incorrect results—stick to the POSIX-compliant methods above for dash compatibility.

内容的提问来源于stack exchange，提问作者user232326