Bash When to use eval Using Eval
Example
For example, consider the following that sets the contents of $@ to the contents of a given variable:
a=(1 2 3)
eval set -- "${a[@]}"
This code is often accompanied by getopt or getopts to set $@ to the output of the aforementioned option parsers, however, you can also use it to create a simple pop function that can operate on variables silently and directly without having to store the result to the original variable:
isnum()
{
# is argument an integer?
local re='^[0-9]+$'
if [[ -n $1 ]]; then
[[ $1 =~ $re ]] && return 0
return 1
else
return 2
fi
}
isvar()
{
if isnum "$1"; then
return 1
fi
local arr="$(eval eval -- echo -n "\$$1")"
if [[ -n ${arr[@]} ]]; then
return 0
fi
return 1
}
pop()
{
if [[ -z $@ ]]; then
return 1
fi
local var=
local isvar=0
local arr=()
if isvar "$1"; then # let's check to see if this is a variable or just a bare array
var="$1"
isvar=1
arr=($(eval eval -- echo -n "\${$1[@]}")) # if it is a var, get its contents
else
arr=($@)
fi
# we need to reverse the contents of $@ so that we can shift
# the last element into nothingness
arr=($(awk <<<"${arr[@]}" '{ for (i=NF; i>1; --i) printf("%s ",$i); print $1; }'
# set $@ to ${arr[@]} so that we can run shift against it.
eval set -- "${arr[@]}"
shift # remove the last element
# put the array back to its original order
arr=($(awk <<<"$@" '{ for (i=NF; i>1; --i) printf("%s ",$i); print $1; }'
# echo the contents for the benefit of users and for bare arrays
echo "${arr[@]}"
if ((isvar)); then
# set the contents of the original var to the new modified array
eval -- "$var=(${arr[@]})"
fi
}
