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Schiff Heimlich
Schiff Heimlich

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Bash Parameter Expansion Forms You Will Actually Use

#bash #cli #devops #linux

You know the basics: ${var:-default} gives you a fallback. ${var:=default} assigns it. Fine. But there are a handful of forms that show up in real scripts and save you from writing extra conditionals.

The Ones Worth Remembering

${var@Q} — shell-quote the value

This is the one I reach for when passing arguments through SSH or building command strings that need to survive parsing.

cmd="docker run --rm alpine echo ${msg@Q}"
ssh host "$cmd"
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Without @Q, a message like hello world becomes two arguments. With it, you get 'hello world' — single argument, preserved.

${arr[@]@Q} — quote every array element

Same idea, but for arrays. Invaluable when you're forwarding a command with its arguments:

run_remote() {
    local cmd=("$@")
    ssh host "${cmd[@]@Q}"
}

run_remote docker run --rm alpine echo "hello world"
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Each element gets quoted individually. The remote shell sees exactly what you passed.

${var@P} — prompt expansion

Expands prompt escape sequences. Useful if you're generating PS1-style strings programmatically:

prompt="\u@\h:\w\$ "
echo "${prompt@P}"
# saiki@server:/home/saiki$
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Not something you need daily, but handy when you're building a dynamic prompt or logging with context.

The Glob Replacement Form

${var//pattern/repl} — global replace (there is also ${var/pattern/repl} for first match only)

path="/usr/local/bin:/usr/bin:/bin"
echo "${path//:/ }"
# /usr/local/bin /usr/bin /bin
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Cleaner than tr or sed for simple delimiters. Works with extended globs too if you shopt -s extglob.

The Error Form

${var:?error message} — fail with message if unset or empty

: "${DEPLOY_TARGET:?DEPLOY_TARGET not set}"
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Fails fast with your message. Better than a cryptic "unbound variable" later.

Why This Matters

These aren't clever tricks. They're standard Bash (since 4.4 for @Q/@P, earlier for the rest). Using them means fewer if blocks, fewer printf '%q' calls, fewer bugs from word-splitting surprises.

Next time you reach for a conditional to handle a default or quote a string, check if there's a parameter expansion form that does it inline. Often there is.

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