That's the trick: command doesn't know, it's the shell that does the job
Consider for example
grep 'abc' *.txt. If we run trace of system calls, you will see something like this:
bash-4.3$ strace -e trace=execve grep "abc" *.txt > /dev/null
execve("/bin/grep", ["grep", "abc", "ADDA_converters.txt", "after.txt", "altera_license.txt", "altera.txt", "ANALOG_DIGITAL_NOTES.txt", "androiddev.txt", "answer2.txt", "answer.txt", "ANSWER.txt", "ascii.txt", "askubuntu-profile.txt", "AskUbuntu_Translators.txt", "a.txt", "bash_result.txt", ...], [/* 80 vars */]) = 0
+++ exited with 0 +++
The shell expanded
*.txt into all filenames in current directory that end with
.txt extension. So effectively, your shell translates the
grep 'abc' *.txt command into
grep 'abc' file1.txt file2.txt file3.txt . . .. Thus, your second assumption is correct.
First assumption is not correct - programs have no way of detecting glob. It is possible to pass
* as string argument to command, but it's the command's job to decide what to do with it then. Filename expansion, however, is property of your respective shell as I've already mentioned.
However, this only works because grep accepts multiple files at the end. It is possible that another command would only allow 1 file to be passed in.
Exactly right ! Programs don't limit the number of acceptable command-line arguments (for instance , in C that's array of strings
const char *args and in python
sys.argv ), but they can detect the length of that array or whether or not something unexpected is in wrong array position.
grep doesn't do that, and accepts multiple files, which is by design.
On side note , improper quoting coupled with globbing with grep can sometimes be a problem. Consider this:
bash-4.3$ echo "one two" | strace -e trace=execve grep *est*
execve("/bin/grep", ["grep", "self_test.sh", "test.wxg"], [/* 80 vars */]) = 0
+++ exited with 1 +++
Unprepared user would expect that grep will match any line with
est letters in it coming from pipe, but instead shell's filename expansion twisted everyting around. I've seen this happen a lot with people who do
ps aux | grep shell_script_name.sh, and they expect to find their process running, but because they ran command from same directory where script was, shell's filename expansion made
grep command to look completely different behind the scenes from what user expected.
Proper way would be to use single quotes:
bash-4.3$ echo "one two" | strace -e trace=execve grep '*est*'
execve("/bin/grep", ["grep", "*est*"], [/* 80 vars */]) = 0
+++ exited with 1 +++