Updating (sequentially adding a value into) a variable within a for loop, in bash script

Question

I am trying to update a variable within a for loop, sequentially adding to the variable, and then printing it to a series of file names.

Actually, I want to sequentially update a series of file, from a tmp file distTmp.RST to sequentially dist1.RST, dist2.RST, etc.. The original distTmp.RST contains a line called "WWW". I want to replace the string with values called 21.5 in dist1.RST, 22.5 in dist2.RST, 23.5 in dist3.RST, etc...

My code is as follows:

#!/bin/bash

wm=1
wM=70
wS=1

F=20.5

for W in {${wm}..${wM}..${wS}}; do

    F=$(${F} + 1 | bc)
    echo ${F}
    sed "s/WWW/"${F}"/g" distTmp.RST > dist${W}.RST 
done
echo ${F}

========

But I am getting error message as follows:

change.sh: line 13: 20.5 + 1 | bc: syntax error: invalid arithmetic operator (error token is ".5 + 1 | bc")

Kindly suggest me a solution to the same.

paste your script at shellcheck.net for validation, recomendation — Jetchisel
– Jetchisel, Commented Nov 5, 2021 at 4:20
shellcheck.net will detect lots of issues with your script, but one issue that might get 'missed' is the potentially problematic bc command; try changing F=$(${F} + 1 | bc) to F=$(echo "${F} + 1" | bc) and see if you still get the syntax error — jared_mamrot
– jared_mamrot, Commented Nov 5, 2021 at 4:35
The problem isn't the addition, it's the range expression you're trying to iterate over -- bash doesn't allow you to use variables in a brace expression. See: How do I iterate over a range of numbers defined by variables in Bash? — Gordon Davisson
– Gordon Davisson, Commented Nov 5, 2021 at 4:37

Jetchisel · Accepted Answer · 2021-11-05 04:37:00Z

4

Kindly suggest me a solution to the same.

This might do what you wanted. Using a c-style for loop.

#!/usr/bin/env bash

wm=1
wM=70
wS=1

F=20.5

for ((w=wm;w<=wM;w+=wS)); do
  f=$(bc <<< "$F + $w")
  echo "$f"
  sed "s/WWW/$f/g" distTmp.RST > "dist${w}.RST"
done

The error from your script might be because the order of expansion. brace expansion expansion happens before Variable does.
See Shell Expansion

answered Nov 5, 2021 at 4:37

Jetchisel

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Comments

Nathan Mills · Accepted Answer · 2021-11-05 04:57:19Z

1

Use

F=$(echo ${F} + 1 | bc)

instead of F=$((${F} + 1 | bc)). The doubled-up parentheses are what caused your error. Double parentheses weren't in the original code, but I get a different error saying 20.5: command not found with the original code, so I tried doubling the parentheses and get the error in the question. Apparently, floating point numbers aren't supported by $(()) arithmetic evaluation expressions in Bash.

answered Nov 5, 2021 at 4:57

Nathan Mills

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1 Comment

Jetchisel Over a year ago

I don't see a double (( from the OP's post, but on the other hand, you're right about not having an echo inside the command substitution. Which obviously I have not pointed out from my answer.

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