Why do I use double quotes in shell scripts [duplicate]

问题

I understand the usage single quote and double quote.

but I don't know situation need to double quotes in the script.

there is no diff that statements

$ echo hello world! $1
$ echo "hello world! $1"

please show me diff between normal and double quotes.

回答1:

Let's consider a directory with these files:

$ ls foo*
foo111.txt  foo11.txt  foo1.txt

Let's consider a minor variation on your script:

$ cat script
#!/bin/sh
echo No quotes $1
echo "Double quotes $1"

Now, let's run it:

$ bash script "foo*"
No quotes foo111.txt foo11.txt foo1.txt
Double quotes foo*

As you can see, the results are completely different. Without the double quotes, pathname expansion is performed.

To illustrate another difference:

$ bash script "long              space"
No quotes long space
Double quotes long              space

With double quotes, the long space between words is preserved. Without it, all contiguous whitespace is replaced with a single blank. This is an example of word splitting.

回答2:

An example might demonstrate the use

To accommodate string with spaces
```
var=file name # Not the intended effect.
```
file is stored in a var and name is taken by shell as a separate cmd which gives you an error.
To prevent word splitting
```
var="file name"
cp $var newfile
```
Here $var expands to file name and in effect, the command would become
```
cp file name newfile
```
and cp would take file and name as 2 source files and newfile as the destination directory which gives you the error:
```
cp: target 'newfile' is not a directory
```
If there really exists a directory named 'newfile', it will give error:
```
cp: cannot stat 'file': No such file or directory
cp: cannot stat 'name': No such file or directory
```
The correct method is
```
cp "$var" newfile
```
In this case, the fully expanded $var is considered a single string.

来源：https://stackoverflow.com/questions/38757649/why-do-i-use-double-quotes-in-shell-scripts

标签

Linux

bash

shell

scripting