As stated, if people wanted to search for all the letters whether in lowercase or uppercase in a file name "FILE", they would type command

grep '[a-zA-Z]' FILE

so my question is, as I understand how single quotes work, that any string within a pair of single quotes would be interpreted literally by the shell; therefore, in this case, if I use the aforementioned command, I would expect that the character class [a-zA-Z] would be interpreted literally (i.e. it would not be expanded to abc...z,ABC...Z). Based on this reason, I thought that it would be more sensible if I type command

grep [a-zA-Z] FILE

which apparently did not work out, but why?
Hope you guys can answer me this question. Thanks in advance!

You want grep to interpret the character classes, not the shell. For example, if you have files in your current directory that are named a, b and C, the shell will perform file name expansion and turn
into
which is probably not your intention.

What he said ^ ..
Another example would be the use of eg '*' in a find cmd; you need the wildcard char to be parsed by the find cmd, not the shell.

Thanks for replying. I just checked, and indeed the output of was the same as the output of , but what exactly is the behavior of ? Is this command syntactically correct and is this command actually meaningful? Or does it just output some garbage value?

Thanks for replying, but I still do not get why exactly the string within the single quotes here (i.e. '[a-zA-Z]') was not interpreted literally?

It's a syntactically correct and absolutely legitimate command. Meaning: Print all the lines containing string "a" in files b, C and FILE. If FILE exists, and all files are readable, there won't be an error message.

It was interpreted literally by the shell, because of the quotes. It was not interpreted literally by grep, because grep was handed the expression [a-zA-Z] without quoting (as a side remark, grep doesn't use the single quote as quote character).

@glenjoker: try reading this answer https://unix.stackexchange.com/quest...sing-a-command and at least once have a quick read through the linked bashwiki page as well.
You don't necessarily have to memorise the whole thing, but just be aware, and maybe bookmark that answer & refer to it often for while.

PS: a good exercise would be to setup a dev dir for testing stuff and play with the answer; try writing code snippets to check if he's telling the truth

Yes, I see it now, thanks for the clarification. I really appreciate it.

Sure, on it. And thanks for your advice, it means great help to beginners like me.

No worries; you're welcome