shell - Bash: How to set a variable from argument, and with a default value -


it pretty clear shell scripting sort of thing can accomplished in huge number of ways (more programming languages) because of different variable expansion methods , programs test , [ , [[, etc.

right i'm looking 

dir=$1 or .

meaning, dir variable should contain either specified in first arg or current directory.

what difference between , dir=${1-.}?

i find hyphen syntax confusing, , seek more readable syntax.

why can't this? 

dir="$1" || '.'

i'm guessing means "if $1 empty, assignment still works (dir becomes empty), invalid command '.' never gets executed."

i see several questions here.

  1. “can write reflects logic”

    yes. there few ways can it. here's one:

    if [[ "$1" != "" ]];     dir="$1" else     dir=. fi

  2. “what difference between , dir=${1-.}?”

    the syntax ${1-.} expands . if $1 unset, expands $1 if $1 set—even if $1 set empty string.

    the syntax ${1:-.} expands . if $1 unset or set empty string. expands $1 if $1 set other empty string.

  3. “why can't this? dir="$1" || '.'”

    because bash, not perl or ruby or other language. (pardon snideness.)

    in bash, || separates entire commands (technically separates pipelines). doesn't separate expressions.

    so dir="$1" || '.' means “execute dir="$1", , if exits non-zero exit code, execute '.'”.


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