为什么glob ='*'; 回声“ $ glob”只写一个“ *”，而不是文件列表？

Question

My question is about the shell and globbing.

When doing something like :

myglob=*
echo $myglob

I get the file list of the directory, so it's ok

But when i do a :

myglob=*
echo "$myglob"
*

I got the * instead of the files in the directory. What i don't understand, doesnt the shell pass the * to the echo command ?

Answer 1

From the bash manual page on double quotes :

Enclosing characters in double quotes ('"') preserves the literal value of all characters within the quotes, with the exception of '$', '`', '\\', and, when history expansion is enabled, '!'.

---

The second command evaluates to:

echo "*"

which, according to the link, does not expand.

Answer 2

doesnt the shell pass the * to the echo command ?

Yes, that's the problem.

echo just writes out what you pass it. If you pass it a * it will always just write out a * .

When you do

myglob=*
echo $myglob

Bash does not pass the * .

Instead, bash expands the glob first, and essentially rewrites the command into echo file1 file2 file3 file4... . Unsurprisingly, echo then writes file1 file2 file3 file4... .

Answer 3

doesnt the shell pass the * to the echo command ?

Yes, when it is enclosed inside quotes. Filename expansion, globbing , is done by the shell during the line scan (not the echo command) but not when the pattern is inside quotes.

set -x ( xtrace ) is your friend, this shows the expansions and when they happen (the leading + is the xtrace prompt, PS4 ):

set -x
myglob=*
echo $myglob

Gives:

+ myglob='*'
+ echo ... filenames in the current directory ...

You can see that the expansion of * is done before echo is invoked.

Whereas:

set -x
myglob=*
echo "$myglob"

Gives:

+ myglob='*'
+ echo '*'
*

Here you can see there is no expansion, and that is one reason (there are others) for using quotes.

set +x will switch the xtrace feature off.