Operate on variables in Bash directly in a loop

Question

My googlefu is failing me. I have a bunch of variables I read it from a CSV where I want to strip whitespace. I could

variable1="$(echo -e "${variable1}" | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//')"
variable2="$(echo -e "${variable2}" | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//')"
variable3="$(echo -e "${variable3}" | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//')"

but then I'm copying in and editing this for every variable change. It seems to me, from other languages, there should be a way to do a simple for loop that applies to the variables as named and strips the whitespace, but I can't figure out exactly how to do that.

for $i in variable1 variable2 variable3
do
$i=$(echo -e "${$i}" | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//')
done

but this doesn't work as far as I can tell. What I want to do is loop over all the variables, strip whitespace, but not have to manually type out each variable name in the script.

Please note that in shell code variable names are only $-prefixed when you reference a variable, not when you define_/_assign to it; thus, it should be for i in ... and i=$(...). — mklement0
– mklement0, Commented May 13, 2015 at 18:59

mklement0 · Accepted Answer · 2015-05-13 19:05:18Z

Try the following:

# Sample values that need trimming.
variable1='  a  '
variable2='      b  c  '
variable3='        sadf sdf sdf     '

for name in ${!variable*}; do
  # Trim leading and trailing whitespace.
  read -r $name <<<"${!name}"
  # Print result
  echo "Variable $name now contains: [${!name}]"
done

The solution relies on variable indirection, the ability to refer to a variable indirectly, via another variable containing the target variable's name.

^{Note that variable indirection has its pitfalls (see link) and that there often better solutions, e.g., through use of arrays.}

${!variable*} expands to the names of all defined variables whose name starts with variable.
${!name} expands to the value of the variable whose name is stored in variable $name.
read -r, by virtue of reading into a single variable, implicitly trims leading and trailing whitespace.
- -r ensures that \ chars. in the input are left alone, which is typically what you want.

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