UNIX for Dummies Questions & Answers

I'm in the habit of using the following type of loop structure:

Code:

for num in `seq $1 $2`
do
     command
done

While `seq $1 $2` is not exactly a huge resource hog, I would like to learn a better way. It seems that brace expansion is a good way to go:

Code:

for num in {3..10}

The problem, though, is that it so far appears that there's no way to use a variable; that is, this doesn't work:

Code:

for num in {$1..$2}

It appears that Bash's order of operations is 1) perform the expansion, then 2) substitute variables, if relevant. In fact, that appears to be a serious roadblock-- Bash expands in such a way that the variable cannot be made to represent a range parameter. So, what's one to do? Brace expansion seems like it should be the lowest-impact method, but it apparently can't be used here. Is there a better way than the seq (or jot, depending on what's available) method?

Use eval, something like:

Code:

sequence=$(eval echo {$1..$2})

for i in $sequence
do
  # your commands
done

Using seq has the same problems as anything else that preloads and jams all arguments at once into a for loop: If the list gets too long it may be truncated. It also creates an extra process and waits for it to finish, a waste of time that's not trivial if you end up doing so dozens of times a second. Brace expansion avoids the process but can still be truncated. 1:10 works, 1:10000 could easily become too large. Never use it for anything where the size isn't known in advance. i.e. exactly what you're asking for

This C-style loop doesn't need to store the entire list of numbers or create any new processes. Classic, barebones bourne shell may not have this loop, most others should. You can put variables and math expressions in it where you please.

Code:

MIN=3
MAX=10
for ((N=MIN; N<=MAX; N++))
do
        echo $N
done

Even classic bourne shell can do this:

Code:

N=3;
MAX=10
while [ "$N" -le "$MAX" ]
do
        echo $N
        N=$((N+1))
done