How to make while loop with grep and variable equality work?

Question

I am trying to execute the following loop, however I'm failing. I have read about double parentheses and other things, however, I cannot manage to make it work (especially the second part in combination with grep). I can make each part work separately, but not both together.

First try:

a=1;
b=1;

while { ( n1=$(grep 'too many requests' *.htm* )) && (( $a -eq $b  )) }; do
    echo "aa"
done
    

Second try:

while [[  n=$(grep 'too many requests' *.htm* ) && $a==$b ]]; do
    echo "error"
done

Third try

while [[  n=$(grep 'too many requests' *.htm* ) &&  "$a" -eq "$b"  ]]; do
    echo "aa"
done

What I expect: if the HTML files do not contain the phrase "too many requests", and a is equal b, enter the loop.

Answer 1

a=1
b=1

if ! grep -q -F -- 'too many requests' *.htm* && [ "$a" = "$b" ]; then
    echo error >&2
fi

This uses the negated exit status of grep and the result of comparing $a and $b for string equality (use ['s -eq for integer evaluation of equality). If both are true (the string is not found in any file matching *.htm* and the two variables are identical), then the echo is executed, outputting the string error to the standard error stream.

Note the use of single = with spaces around it. The = (or -eq) operator is an argument to the [ (test) utility and not intrinsically tied to the if keyword.

The options -q and -F to the grep utility cause the utility to be quiet and to exit as soon as a match is found (-q) and to treat the given pattern as a text string rather than as a regular expression (-F).

---

Your examples use a while loop, which should be an if statement. If you want to handle each file separately, then use a for loop:

for name in *.htm*; do
    if ! grep -q -F -- 'too many requests' "$name" && [ "$a" = "$b" ]
    then
        printf 'error for "%s"\n' "$name" >&2
    fi
done

You don't have to save the output of grep if you only want to test whether a pattern matches in a file or set of files. You may instead use grep -q and react to the exit status of the utility, as shown above.

In your first code snippet, you use an arithmetic evaluation, (( $a -eq $b )). This could be better written as (( a == b )), which evaluates to a result that an if statement could use as a boolean. The -eq operator is specific to the [ utility and not valid in (( ... )).

In the second code snippet, you use $a==$b. This would evaluate into a string, 1==1, which the shell would try to use as a command.

In the third code snippet, you use "$a" -eq "$b" within [[ ... ]], which correctly performs an integer equality test between $a and $b. This does not work as expected due to the grep command and the way you don't negate its exit status (and the fact that you use a while loop rather than an if statement).

Within [ ... ], = tests for string equality and -eq tests for integer equality. Technically, == is not a valid operator for [ ... ] tests, but bash supports it. The bash shell does shell pattern matching within its special [[ ... ]] syntax when using ==, as in

[[ $name == *.txt ]] && echo 'The name matches *.txt'