Shell scripting provides a certain set of features as compared to what we get in other scripted programming languages such as Python, Ruby, Perl, and AWK. These languages provide additional features as compared to what we get in a shell script language. On Linux and UNIX-based system, to use these languages, we have to install them separately if they are not preinstalled.
Consider a simple example: there is a json or XML file and we want to parse it and retrieve the data stored in it. It's very hard and error-prone to do this using shell and its commands, but if we are aware of the Python or Ruby languages, we can easily do it and then embed it into a shell script. Embedding another language in a shell script should be done to reduce the effort and also to achieve better performance.
The syntax for embedding other languages in a shell script is as follows:
To embed Python language inside a shell script, we will use python -c " Python Code". To learn about Python, refer to the official website at https://www.python.org/.
A simple Python example would be printing Hello World in Python, which is done as follows:
print "Hello World"
To embed this in a shell script, we can write the code as follows
#!/bin/bash # Filename: python_print.sh # Description: Embeding python in shell script # Printing using Python python -c 'print "Hello World"'
We will now execute the python_print.sh script as follows:
$ sh python_print.sh Hello World
To embed multiple lines of Python code in a shell script, use the following code:
python - <<EOF # Python code EOF
Here, python - instructs the python command to take the input from stdin and EOF is a label that instructs to take the stdin input until it encounters the EOF text.
The following example embeds Python language in a shell script and fetches unread e-mails from the user's Gmail account:
#!/bin/bash
# Filename: mail_fetch.sh
# Description: Fetching unread email from gmail by embedding python in shell script
# Enter username and password of your gmail account
echo Enter your gmail username:
read USER
echo Enter password:
read -s PASSWD
echo Running python code
python - <<CODE
# Importing required Python module
import urllib2
import getpass
import xml.etree.ElementTree as ET
# Function to get unread messages in XML format
def get_unread_msgs(user, passwd):
auth_handler = urllib2.HTTPBasicAuthHandler()
auth_handler.add_password(
realm='mail.google.com',
uri='https://mail.google.com',
user=user,
passwd=passwd
)
opener = urllib2.build_opener(auth_handler)
urllib2.install_opener(opener)
feed = urllib2.urlopen('https://mail.google.com/mail/feed/atom')
return feed.read()
xml_data = get_unread_msgs("$USER", "$PASSWD")
root = ET.fromstring(xml_data)
# Getting Title of unread emails
print "Title of unread messages:"
print "........................"
count=0
for e in root.iter('{http://purl.org/atom/ns#}title'):
print e.text
CODE
echo "Done!"After executing this script, the sample output looks as follows:
$ sh mail_fetch.sh Enter your gmail username: [email protected] Enter password: Running python code Title of unread messages: .....................…………….. Gmail - Inbox for [email protected] Unread message1 unread message2 Unread message3 Done!
Awk is a programming language designed for text processing and is mainly used for fetching relevant data and for reporting tools. To learn more about AWK programming language, refer to its man page or visit the website at http://www.gnu.org/software/gawk/manual/gawk.html.
The Awk language can be very easily used in a shell script. For example, consider the output of the df command on a running system:
$ df -h
To fetch the fourth column—that is, the Avail field using awk—we can write a shell script using awk as follows:
#!/bin/bash
# Filename: awk_embed.sh
# Description: Demonstrating using awk in shell script
# Fetching 4th column of command df output
df -h |awk '{ print $4 }'
Consider another example in which we will use an input file that will be the /etc/passwd file of a system. This file contains the basic information about each user or account on a Linux or UNIX-based system.
Each line of a /etc/passwd file looks as follows:
root:x:0:0:root:/root:/bin/bash
There are seven fields and each field is separated by a colon (:). To learn the detailed meaning of each field, refer to the Wikipedia link at https://en.wikipedia.org/wiki/Passwd.
The following shell script makes use of awk features and displays some useful information from the /etc/passwd file. For example, we will consider the following as the content of the passwd file:
$ cat passwd root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin adm:x:3:4:adm:/var/adm:/sbin/nologin lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin sync:x:5:0:sync:/sbin:/bin/sync shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown halt:x:7:0:halt:/sbin:/sbin/halt $ cat passwd_file_info.sh # Shell script content #!/bin/bash # Filename: passwd_file_info.sh # Desciption: Fetches useful information from /etc/passwd file using awk # Fetching 1st and 3rd field i.e. Username and UID and separate them with blank space awk -F":" '{ print "Username: " $1 " UID:" $3 }' passwd # Searching line whose user is root echo "User root information" awk '$1 ~ /^root/' passwd
Running this script gives following result:
$ sh passwd_file_info.sh Username: root UID:0 Username: bin UID:1 Username: daemon UID:2 Username: adm UID:3 Username: lp UID:4 Username: sync UID:5 Username: shutdown UID:6 Username: halt UID:7 User root information root:x:0:0:root:/root:/bin/bash