Example from Mendel Cooper's "Advanced Bash-Scripting Guide"...

$bash --version | head -1
GNU bash, version 5.0.17(1)-release (x86_64-pc-linux-gnu)
$mkdir Quoting_Experimenting
$cd Quoting_Experimenting
$echo This is the first line of file1.txt > file1.txt
$echo This is the First line of file2.txt > file2.txt
$grep [fF]irst *.txt
file1.txt:This is the first line of file1.txt
file2.txt:This is the First line of file2.txt
$touch first
$grep [fF]irst *.txt
file1.txt:This is the first line of file1.txt

Why does grep not find file2.txt after first has been created?

CodePudding user response：

You need to quote the search pattern in grep command otherwise shell expands unquoted pattern [fF]irst to first only (due to filename first being present in your current directory):

So use:

grep '[fF]irst' *.txt

- Official BASH documentation on filename expansion

As per this doc (emphasis is mine):

After word splitting, unless the -f option has been set (see The Set Builtin), Bash scans each word for the characters ‘*’, ‘?’, and ‘[’. If one of these characters appears, and is not quoted, then the word is regarded as a pattern, and replaced with an alphabetically sorted list of filenames matching the pattern.

CodePudding user response：

If you're simply looking for a match with any case, instead of using a regex, just use the ignore case command:

grep -i first *.txt