Detailed introduction to basic learning of Linux shell scripts (4)

In the previous article on the basic study of Linux shell scripts, we talked about if, select, and case that control the process in Linux shell scripts. Here we will introduce the loop and quotation marks that control the process in Linux shell scripts. There is more content in the control process, and there are also Some of the content is about the here document.
4.loop
loop expression:

while ...; do
....
done

while-loop will run until the expression tests true. will run while the expression that we test for is true.
The keyword "break" is used to break out of the loop. The keyword "continue" is used to jump directly to the next loop without executing the remaining part.
The for-loop expression looks at a list of strings (strings separated by spaces) and assigns it to a variable:

for var in ....; do
....
done

In the following example, ABC will be printed to the screen respectively Above:

#!/bin/sh
for var in A B C ; do
echo "var is $var"
done

The following is a more useful script showrpm, its function is to print some statistical information of RPM packages:

#!/bin/sh
# list a content summary of a number of RPM packages
# USAGE: showrpm rpmfile1 rpmfile2 ...
# EXAMPLE: showrpm /cdrom/RedHat/RPMS/*.rpm
for rpmpackage in $*; do
if [ -r "$rpmpackage" ];then
echo "=============== $rpmpackage =============="
rpm -qi -p $rpmpackage
else
echo "ERROR: cannot read file $rpmpackage"
fi
done

The second special thing appears here The variable $* contains the values ​​of all entered command line parameters.
If you run showrpm openssh.rpm w3m.rpm webgrep.rpm
At this time $* contains 3 strings, namely openssh.rpm, w3m.rpm and webgrep.rpm.
5. Quotes
The program expands wildcards and variables before passing any parameters to the program. The so-called expansion here means that the program will replace wildcard characters (such as *) with appropriate file names and replace variables with variable values. To prevent the program from making this substitution, you can use quotes: Let's look at an example, assuming that there are some files in the current directory, two jpg files, mail.jpg and tux.jpg.
1.2 Compile the SHELL script
#ch#!/bin/sh mod +x filename
cho *.jpg ∪螅螞枞果暙耄?./filename to execute your script.
This will print out the results of "mail.jpg tux.jpg".
Quotation marks (single and double) will prevent this wildcard expansion:

#!/bin/sh
echo "*.jpg"
echo '*.jpg'

This will print "*.jpg" twice.
Single quotes are more strict. It prevents any variable expansion. Double quotes prevent wildcard expansion but allow variable expansion.

#!/bin/sh
echo $SHELL
echo "$SHELL"
echo '$SHELL'

The running result is:

/bin/bash
/bin/bash
$SHELL

Finally, there is another way to prevent this expansion, which is to use the escape character-the backslash:

echo *.jpg
echo $SHELL

This will output:

*.jpg
$SHELL

That’s it for the basics of Linux shell scripts. The control process still has a little here document content that will be analyzed next time.

The above is a detailed introduction to the basic learning of Linux shell scripts (4). For more related content, please pay attention to the PHP Chinese website (www.php.cn)!