Use Variables with Scripts

Using an editor or the cat command, write a script that uses a variable. Note that this script uses the bash shell on LINUX, but you can also use the ksh shell with UNIX.


1) Set a value for the variable first using typeset then check it using set.

[root@vmpc_rh804 root]# typeset first=you
[root@vmpc_rh804 root]# set | more
BASH=/bin/bash
BASH_ENV=/root/.bashrc
BASH_VERSINFO=([0]="2" [1]="05b" [2]="0" [3]="1" [4]="release" [5]="i686-pc-linu
x-gnu")
BASH_VERSION='2.05b.0(1)-release'
COLORS=/etc/DIR_COLORS
.
.
.
first=you
hi=hello
langfile=/root/.i18n


2) Write a short Welcome script that uses a variable.

[root@vmpc_rh804 root]# cat > Welcome.bash
#!/bin/bash
#
first=Student
echo "Welcome to Linux $first"
Ctrl/D


3) Execute your script using the shell interpreter.

$ bash Welcome.bash
Welcome to Linux Student

Invoke a Script

Use various methods to invoke a script, and determine when it runs in your shell, and when a subshell is created.


1) Using an editor or the cat command, write a korn or bash script that:
* Executes the ps command.
* Echos the value of the shell variable hi.

[root@vmpc_rh804 root]# cat > do-ps.bash
#!/bin/bash
# This script displays active processes
#
ps
echo $hi


2) Set the value of hi to hello in your current command line shell.

[root@vmpc_rh804 root]# hi=hello


3) Execute your script using the shell interpreter.

[root@vmpc_rh804 root]# bash do-ps.bash
PID TTY TIME CMD
1311 pts/4 00:00:00 bash
1358 pts/4 00:00:00 bash
1365 pts/4 00:00:00 ps

** Note that it doesn't recognize the hi variable. This is because the variable is local to the shell in which it was defined. When this script is executed, a new shell is started that is outside your local shell. **


4) Add execute privilege to the script file and run it as a command from the current directory.

[root@vmpc_rh804 root]# chmod +x do-ps.bash
[root@vmpc_rh804 root]# ./do-ps.bash
PID TTY TIME CMD
1311 pts/4 00:00:00 bash
1367 pts/4 00:00:00 do-ps.bash
1374 pts/4 00:00:00 ps


5) Use the built-in source command to execute the commands in the script file.

[root@vmpc_rh804 root]# source do-ps.bash
PID TTY TIME CMD
1311 pts/4 00:00:00 bash
1375 pts/4 00:00:00 ps
hello
[root@vmpc_rh804 root]#


** Note that it is able to recognize the hi variable. This is because the source command runs this script in the local shell. **

How to write a simple shell script in Linux

Below are the simple steps to write a simple shell script in Linux machine. Before this I don't have any experience to write a script. Thanks to HP's Virtual Lab. Its really make my day ;-)


===========
Exercise 1
===========

1) Find what shells are available on your system.

[root@vmpc_rh804 root]# more /etc/shells
/bin/sh
/bin/bash
/sbin/nologin
/bin/bash2
/bin/ash
/bin/bsh
/bin/tcsh
/bin/csh


2) Using an editor or the cat command, write a korn or bash script that:

* Comments what the script will do.
* Shows your current directory.
* Shows who is currently logged on.
* Prints the current date and time.

[root@vmpc_rh804 root]# cat > myscript
#! /bin/bash
#
# This script shows current directory
# then shows who is on the system
# then prints the date and time
#
pwd
who
date
Ctrl/D


3) Run the script by calling up the shell interpreter.

[root@vmpc_rh804 root]# bash myscript
/root
root :0 Apr 11 06:25
root pts/0 Apr 11 06:26
root pts/1 Apr 11 06:26
root pts/2 Apr 11 06:26
root pts/3 Apr 11 06:26
root pts/5 Apr 11 06:27
root pts/4 Mar 30 00:25 (16.158.13.22)
Tue Mar 30 00:55:01 EST 2010


4) Show how the script cannot be run as a command unless the file has execute privileges.

[root@vmpc_rh804 root]# ./myscript
-bash: ./myscript: Permission denied


5) Add execute privilege to the script file and try running it again.

[root@vmpc_rh804 root]# chmod +x myscript
[root@vmpc_rh804 root]# ./myscript
/root
root :0 Apr 11 06:25
root pts/0 Apr 11 06:26
root pts/1 Apr 11 06:26
root pts/2 Apr 11 06:26
root pts/3 Apr 11 06:26
root pts/5 Apr 11 06:27
root pts/4 Mar 30 00:25 (16.158.13.22)
Tue Mar 30 01:04:14 EST 2010




============
Exercise 2
============


1) [root@vmpc_rh804 root]# more /etc/shells
/bin/sh
/bin/bash
/sbin/nologin
/bin/bash2
/bin/ash
/bin/bsh
/bin/tcsh
/bin/csh

[root@vmpc_rh804 root]# cat > myscript1
#!/bin/bash
#
#
ifconfig eth0
netstat -i


2) [root@vmpc_rh804 root]# bash myscript1
eth0 Link encap:Ethernet HWaddr 00:50:56:03:04:40
inet addr:172.16.0.40 Bcast:172.16.0.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:4 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:0 (0.0 b) TX bytes:168 (168.0 b)
Interrupt:10 Base address:0x10a0

Kernel Interface table
Iface MTU Met RX-OK RX-ERR RX-DRP RX-OVR TX-OK TX-ERR TX-DRP TX-OVR Flg
eth0 1500 0 0 0 0 0 4 0 0 0 BMRU
eth1 1500 0 1335 0 0 0 1351 0 0 0 BMRU
lo 16436 0 98 0 0 0 98 0 0 0 LRU


3) [root@vmpc_rh804 root]# ./myscript1
-bash: ./myscript1: Permission denied


4) [root@vmpc_rh804 root]# chmod +x myscript1


5) [root@vmpc_rh804 root]# ./myscript1
eth0 Link encap:Ethernet HWaddr 00:50:56:03:04:40
inet addr:172.16.0.40 Bcast:172.16.0.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:4 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:0 (0.0 b) TX bytes:168 (168.0 b)
Interrupt:10 Base address:0x10a0

Kernel Interface table
Iface MTU Met RX-OK RX-ERR RX-DRP RX-OVR TX-OK TX-ERR TX-DRP TX-OVR Flg
eth0 1500 0 0 0 0 0 4 0 0 0 BMRU
eth1 1500 0 1397 0 0 0 1395 0 0 0 BMRU
lo 16436 0 98 0 0 0 98 0 0 0 LRU