bash: a for loop does not increment by 1. How can I treat it like it does in order to get the index-CodePudding

I have this loop that allows me to deal only with certain time steps from a simulation:

    let ALLSTEPS=820000
    for ((step=20000; step <= ALLSTEPS; step =20000)); do
        echo "Step: $step"
...

Within the loop I need to read in a row from each line of an external file. This is what I have:

i=$((step));
k=$(sed "${i}q;d" externalFile.txt)
echo ${k%}

This does not work because in the external file, my rows go: 1, 2, 3, 4, etc whereas "step" is "20000, 40000, 60000, ..."

I could set up another loop but that seems unwieldy and I wonder if there's a cleaner way to do it?

CodePudding user response：

You can use multiple variables per arithmetic for loop:

ALLSTEPS=820000
for ((i=1, step=20000; step <= ALLSTEPS; i  , step =20000)); do
    echo "Step: $step, i: $i"
done

CodePudding user response：

Rather than using sed (which will read through the entire file, just to pick out a single line), why not use read in the loop to read the next line each time through?

for ((step=20000; step <= ALLSTEPS; step =20000)); do
    IFS= read -r k <&3    # Read the next line from the external file
    echo "Step: $step, k: $k"
    ...
done 3<externalFile.txt

In the above, I used FD #3 for the file (the 3< and <&3 stuff), to avoid conflicts with standard input if anything else in the loop reads from that.

(And the IFS= and -r stuff in the read command are standard ways to tell read not just read the raw lines, don't "helpfully" preprocess them.)