Bash - 使用从文件中读取的条件中断循环

Question

I am using bash to run some commands until they meet the target I set.

Here is my code with explanation:

#!/bin/bash

#keep running some commands until a target is met
while : 
do
  #some executions that generate a number and write into flag.txt 

  #read the number from flag.txt and store in variable flag
  while read flag
  do
    echo $flag
  done <<< $(cat flag.txt)

  echo "hello $flag"

  #if the number from the file is >= the target I set (i.e. 0.717), then break the loop and finish the computation, otherwize, keep running.
  if [ $(echo "$flag >= 0.717" | bc) -eq 1 ]
  then
    break
  fi

done

I got some errors:

0.7172
hello 
(standard_in) 1: syntax error
./test.sh: line 14: [: -eq: unary operator expected

I think this is because the 2nd while loop runs in a subshell, the value of the $flag is kept within the 2nd while loop.

  do
    echo $flag
  done <<< $(cat flag.txt)

  echo "hello $flag"

To prove this, if I change the value of $flag to a real number:

if [ $(echo "0.72 >= 0.717" | bc) -eq 1 ]

Then I can get the correct results: Apparently, the value of $flag outside the 2nd while loop is just "blank":

0.7172
hello 

I wonder if there is any way to work around this?

Answer 1

Based on your comments, it sounds like you just need to replace:

  while read flag
  do
    echo $flag
  done <<< $(cat flag.txt)
  echo "hello $flag"
  if [ $(echo "$flag >= 0.717" | bc) -eq 1 ]; then
    break
  fi

With:

read flag < flag.txt
echo "hello $flag"
if test "$(echo "$flag >= 0.717" | bc)" -eq 1; then
    break
fi

Using the while loop to read every line in the file ends up leaving flag set to the empty string, because the loop doesn't terminate until read gets to the end of the file and sets flag to the empty string. Note that, assuming the echo "hello $flag" is purely for debugging, you could simply do:

if awk '{ exit ! ($1 >= .717)}' flag.txt; then break; fi

Answer 2

If I understand your question correctly, you should put your if statement within the nested loop. So that it can break when "target" is met. You can set another flag when this condition is met and use that flag to break out from outer loop.

The unary operator error is due to $flag not having any value and if statement evaluates only the 2nd operand.