CODEWITHSUNDEEP
bashparameters
petersohn
petersohn

Reputation: 11730

What is the difference between ${var:-word} and ${var-word}?

I found the following command in a bash script:

git blame $NOT_WHITESPACE --line-porcelain "${2-@}" -- "$file"

What does this ${2-@} mean? Trying out, it returns the 2nd argument, and "@" if it doesn't exist. According to the documentation, ${2:-@} should do the same. I tried it, and it indeed does the same. What's the difference? Where is it documented? The man page does not seem to say anything about this notation.

Upvotes: 1

Views: 460

Answers (1)

fedorqui
fedorqui

Reputation: 290415

From Bash hackers wiki - parameter expansion:

${PARAMETER:-WORD}

${PARAMETER-WORD}

If the parameter PARAMETER is unset (never was defined) or null (empty), this one expands to WORD, otherwise it expands to the value of PARAMETER, as if it just was ${PARAMETER}. If you omit the : (colon), like shown in the second form, the default value is only used when the parameter was unset, not when it was empty.

echo "Your home directory is: ${HOME:-/home/$USER}."
echo "${HOME:-/home/$USER} will be used to store your personal data."

If HOME is unset or empty, everytime you want to print something useful, you need to put that parameter syntax in.

#!/bin/bash

read -p "Enter your gender (just press ENTER to not tell us): " GENDER
echo "Your gender is ${GENDER:-a secret}."

It will print "Your gender is a secret." when you don't enter the gender. Note that the default value is used on expansion time, it is not assigned to the parameter.

Upvotes: 8

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