Difference between revisions of "BASH/C2/String-and-File-attributes/English"

String and File Attributes comparison in Bash.

• String comparison and
• File attributes comparison

We will do this using a few examples.

• Ubuntu Linux 12.04 Operating System and
• GNU BASH version 4.1.10

Please note, GNU Bash version 4 or above, is recommended to practise this tutorial.

1) First: using == (equal to equal to) operator

To compare two equal strings.

2) Second: using != (not equal to) operator

To compare two not equal strings.

Let us look at an example.

Open a file in your editor and save it as strcompare dot sh

Now, type the code as shown here, in your strcompare dot sh file

#!/bin/bash

if [ "$(whoami)" != 'root' ]; then

We have used not-equal to operator here to compare strings.

echo "You have no permission to run $0 as non-root user."

“You have no permission to run strcompare dot sh as non-root user.”

Here $0 (dollar zero) is the zeroeth argument which is a filename itself.

else

echo "Welcome root!. "

“Welcome root!”

exit 0;

And here with fi, we end the if statement.

A successful command returns a 0 (zero).

An erroneous command returns a non-zero value.

It can be interpreted as an error code.

We can customize the return value of the exit statement .

on the Terminal:

whoami

Type whoami and press Enter.

This will output the name of the current user.

chmod +x strcompare.sh

Type chmod +x strcompare.sh

The output

You have no permission to run dot slash strcompare dot sh as non-root user.

Type: sudo su

Type: ./strcompare.sh

Type: sudo dot slash strcompare.sh

The output

file1=“/home/ttt/fileattrib.sh”

if [ -f $file1 ];

then

And whether it is a normal file.

echo "File exists and is a normal file"

echo "File does not exist"

fi

File exists and is a normal file

gedit empty.sh &

Type gedit empty dot sh ampersand sign

Click on Save and close the file.

if [ -f $file1 ];

then

with

if [ -s $file1 ];

then

file1= “/home/ttt/fileattrib.sh”

with

file1= “/home/ttt/empty.sh”

echo "File exists and is a normal

with

echo "File exists and is not empty"

AND

"File does not exist"

with

echo “File is empty”

echo “File exists and is not empty”

And the second echo statement with:

echo “File is empty”

./fileattrib.sh

Let's execute.

Type dot slash fileattrib dot sh

and press Enter.

Output

if [ -s $file1 ];

then

with

if [ -w $file1 ];

then

file exists and is not empty"

with

echo “User has write permission to this file”

AND

echo "file is empty"

with

echo “User don't have write permission to this file”

“User has write permission to this file”

And second echo statement with:

“User doesn't have write permission to this file”

Click on Save.

I will select a file, which is not a readable file or which does not have write permission.

“/etc/mysql/debian.cnf”

Save the file

“slash etc slash mysql slash debian dot cnf”

Click on Save.

Press the up-arrow key.

Output

User doesn't have write permission to this file.

In this example, we will check whether file1 is newer than file2.

Point cursor to the filename

fileattrib2.sh

Note that the filename is fileattrib2.sh

Let's go through the code.

file1= “/home/ttt/empty1.sh”

file2= “/home/ttt/empty2.sh”

The two files have already been created and are empty.

Else, file2 is newer than file1.

Else, we print file2 is older than file1.

#!/bin/bash

echo “Hiii”

Type: echo within doubel quotes Hiii after the double quotes greater than sign empty one dot sh

Press Enter.

Type:

chmod +x fileattrib2.sh

Type -

chmod plus x fileattrib2 dot sh

Type:

./fileattrib2.sh

Output

file1 is newer than file2

file2 is older than file1

Type:

#!/bin/bash

echo “Hello”

Here also I will add an echo statement.

Let me clear the prompt.

Press Enter

Output

file2 is newer than file1

file1 is older than file2

Summary

In this tutorial we learnt,

String comparison

file attributes

==

!=

-f

-s

-w

-nt

-ot

Assignment

Explore some more attributes.

Ex: -r

-x

-o

http://spoken-tutorial.org /What\_is\_a\_Spoken\_Tutorial

About the Spoken Tutorial Project

It summarises the Spoken Tutorial project

If you do not have good bandwidth, you can download and watch it

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Acknowledgement

It is supported by the National Mission on Education through ICT, MHRD, Government of India

More information on this Mission is available at: http://spoken-tutorial.org\NMEICT-Intro

This is Ashwini Patil from IIT Bombay

Thank You for joining.