Shell Programming Notes 1

写在前面

之前上课学过现在全忘光了。所以对着书再学一遍。

I have bring all I learnt to my tutor. I need to learn it again.

Using multiple commands

Using “;”(or “&&”) we could seperate multiple commands:

date ; who

The output:

Wed Mar 15 07:57:57 PM CST 2023
shenyao  tty1         2023-03-15 15:00 (:0)
shenyao  pts/0        2023-03-15 15:00 (:0)
shenyao  pts/1        2023-03-15 15:01 (:0)
shenyao  pts/2        2023-03-15 19:50 (:0)
shenyao  pts/3        2023-03-15 15:42 (:0)

Using echo

format: echo + string

we do need to use ` “” outside of the string, however, while the string itself has ’ we need to use ”`

. The same as " situation.

echo using echo

Output:

using echo

And with ':

echo "Let's do it"

Output:

Let's do it

Output in the same line:

use the -n parameter. Here is an example:

cat test1.sh

Output:

#!/bin/bash
#

echo -n "Today's time is:"
date
echo "The users's are:"
who

Run the script, we have:

Today's time is:Wed Mar 15 08:07:03 PM CST 2023
The users's are:
shenyao  tty1         2023-03-15 15:00 (:0)
shenyao  pts/0        2023-03-15 15:00 (:0)
shenyao  pts/1        2023-03-15 15:01 (:0)
shenyao  pts/2        2023-03-15 19:50 (:0)
shenyao  pts/3        2023-03-15 15:42 (:0)

Using Variables

Environment Variables

shell has a set of environment varsiables. It records the name of system, or who usr ID, home folder path, etc. Use Set to get the list of environment variables:

set

Since I used p9k the output is really at a mess…

User Variables

We could also use user variables to store the data. Use “$” to quote the variable:

a=1
$a

Output:

1

Replace the command

We could use “`” or “”$()” to pass value from command output to variables:

testing=$(date)

Or:

testing=`date`

Then we have:

echo "the time and date are" $testing

Output:

the time and date are Wed Mar 15 08:41:10 PM CST 2023

Let’s see another example:

today=$(date +%y%m%d)
ls /usr/bin -al > $today.log

It will generate another file and we’ll get a new file, recording the output of ls /usr/bin -al.

Redirection

Output redirection

Input redirection