Line by line parsing with bash -c-CodePudding

In bash, I parse a file line by line and extract the first and second field, like this:

$ cat myfile
a1 1
a2 2
a3 3
a4

$ while read -r first second; do echo first is $first second is $second; done < myfile
first is a1 second is 1
first is a2 second is 2
first is a3 second is 3
first is a4 second is

Now, I need to enclose the above command in bash -c, since it will be executed through kubectl exec. It doesn't work as expected and only evaluates what it has parsed in the last line:

$ bash -c "while read -r first second; do echo first is $first second is $second; done < myfile"
first is a4 second is
first is a4 second is
first is a4 second is
first is a4 second is

What's missing here?

Thanks!

CodePudding user response：

Your parameters expands in the parent shell, change your quotes or escape the parameter expansions:

$ bash -c 'while read -r first second; do echo first is $first second is $second; done < myfile'

$ bash -c "while read -r first second; do echo first is \$first second is \$second; done < myfile"

Note that you should almost always wrap your parameter expansions in double quotes:

echo "$a"

instead of

echo $a

To avoid word splitting and pathname expansion.

Conside the following example, in a POSIX shell:

a="hello *"; echo $a;

a="hello *"; echo "$a";

So that would make your script end up looking like:

$ bash -c 'while read -r first second; do echo first is "$first" second is "$second"; done < myfile'

CodePudding user response：

And if you do not want to deal with intricacies of shell double-qouting, first define a function where you type your code manually where also shellcheck can help you typing proper code:

f() { 
   while IFS=' ' read -r first second; do
        echo "first is $first second is $second";
   done
}

then do:

printf "%q\n" "$(declare -f f); f"
$'f () \n{ \n    while IFS=\' \' read -r first second; do\n        echo "first is $first second is $second";\n    done\n}; f'

declare prints the function deifnition in a re-usable form, and after it we f call the function. Then you can copy the output and re-use it in shell:

bash -c $'f () \n{ \n    while IFS=\' \' read -r first second; do\n        echo "first is $first second is $second";\n    done\n}; f'

The $'...' is specific to Bash C-quoting style, so use a different quoting function than printf %q when needed.