bashbrace-expansion

bash brace expansion based on variable content not working

how can i make bash expand whatever it finds in a variable i pass to mkdir? so far i've tried using eval and bash -c, but nothing seems to work

LEVEL_1=1,2,3
LEVEL_2=a,b,c

DATA_L1="/tmp/{$LEVEL_1}"
DATA_L2="$DATA_L1/{$LEVEL_2}"

for LINE in $(cat file.txt) ; do
  #"cat" here returns values like 
  #$DATA_L2/yy/data

  mkdir -pv $LINE #it actually contains e.g. this $DATA_L2/yy/data
done

i would expect that this will expand to 

mkdir -p /tmp/1/a/yy/data
mkdir -p /tmp/2/a/yy/data
mkdir -p /tmp/3/a/yy/data
mkdir -p /tmp/1/b/yy/data
mkdir -p /tmp/2/b/yy/data
mkdir -p /tmp/3/b/yy/data
mkdir -p /tmp/1/c/yy/data
mkdir -p /tmp/2/c/yy/data
mkdir -p /tmp/3/c/yy/data
Solution

  • I suspect your problem is that you have two layers of expansion, even though you don't show that in your question. That is, you show:

    mkdir -pv $DATA_L2/yy/data

    And this would work if you simply added an eval to it:

    eval mkdir -pv $DATA_L2/yy/data

    But inside your loop, you're not actually running the above command. I think you're running something like:

    mkdir -pv $LINE

    If we stick an echo in front of that mkdir, we can see that:

    echo mkdir -pv $LINE

    Results in:

    mkdir -pv $DATA_L2/yy/data

    And:

    eval echo mkdir -pv $LINE

    Results in:

    mkdir -pv /tmp/{1,2,3}/{a,b,c}/yy/data

    If you double the eval:

    eval eval echo mkdir -pv $LINE

    You get what you are looking for:

    mkdir -pv /tmp/1/a/yy/data /tmp/1/b/yy/data /tmp/1/c/yy/data /tmp/2/a/yy/data /tmp/2/b/yy/data /tmp/2/c/yy/data /tmp/3/a/yy/data /tmp/3/b/yy/data /tmp/3/c/yy/data

    But a double eval is a really good sign that need to rethink your approach to the problem.