When to use eval

First and foremost: know what you’re doing! Secondly, while you should avoid using eval, if its use makes for cleaner code, go ahead.

Using Eval

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
}

Using Eval with Getopt

While eval may not be needed for a pop like function, it is however required whenever you use getopt:

Consider the following function that accepts -h as an option:

f()
{
local __me__="${FUNCNAME[0]}"
local argv="$(getopt -o 'h' -n $__me__ -- "$@")"
eval set -- "$argv"
while :; do
case "$1" in
-h)
echo "LOLOLOLOL"
return 0
;;
--)
shift
break
;;
done
echo "$@"
}

Without eval set — “$argv” generates

-h —
instead of the desired (-h –) and subsequently enters an infinite loop because
-h —
doesn’t match — or -h.