Shell Scripting

Question 1

Executing a script
A. Writing it directly in bash interpreter

B. First write the script then execute

Answer 1

A.
1. Go to directory where the script should live
2. Create a file with .sh extension
3. Write script in that file using an editor (e.g. vi)
Indicate the file is a bash script with #!/bin/bash
4. Make script executable >chmod 755 [script name].sh
5. Run script > ./[script name].sh

B.

1. Start script with : #!/bin/bash
2. Apply executable permission to .sh file > chmod 755 [script name].sh
3. Run script

Question 2

Shell script with variable in script

Answer 2

#! /bin/bash
MY_SHELL="eat"
echo "I am ${MY_SHELL}ing my keyboard."

Output
I am eating my keyboard

Question 3

Shell script with output of a command as variable

Answer 3

!# /bin/bash
SERVER_NAME = $(hostname) »_space;this is the command
echo “You use ${hostname}”

Output
You use Linux

Question 4

Executing Python scripts via the interpreter

Answer 4

1. Write python script
   > #! /usr/bin/python
   > print “THis is a python script”
2. Add execute permissions
   >chmod 755 hi.py
3. Run script from present directory
   ./hi.py
   This is a Python script.

Question 5

File Operators

Answer 5

• d FILE True if file is a directory.
• e FILE True if file exists.
• f FILE True if file exists and is a regular file.
• r FILE True if file is readable by you.
• s FILE True if file exists and is not empty.
• w FILE True if the file is writable by you.
• x FILE True if the file is executable by you

Question 6

String Operators

Answer 6

-z STRING True if string is empty.
-n STRING True if string is not empty.
STRING1 = STRING2
True if the strings are equal.
STRING1 != STRING2
True if the strings are not equal

Question 7

Arithmetic operators

Answer 7

arg1 –eq arg2 True if arg1 is equal to arg2.
arg1 –ne arg2 True if arg1 is not equal to arg2.
arg1 –lt arg2 True if arg1 is less than arg2.
arg1 –le arg2 True if arg1 is less than or equal to arg2.
arg1 –gt arg2 True if arg1 is greater than arg2.
arg1 –ge arg2 True if arg1 is greater than or equal to arg2

Question 8

IF Statement Syntax

Answer 8

if [ condition-is-true ]
then
     command 1
     command 2
     command N
fi

Question 9

IF Statement script - Example

Answer 9

#!/bin/bash
MY_SHELL="bash"
if [ "$MY_SHELL" = "bash" ]
then
    echo "You seem to like the bash shell."
fi

Output:
You seem to like the bash shell.

Question 10

IF/THEN/ELSE Syntax

Answer 10

if [ condition-is-true ]
then
     command N
else
      command N
fi

Question 11

IF/THEN/ELSE Script Example

Answer 11

#!/bin/bash
MY_SHELL="csh"
if [ "$MY_SHELL" = "bash" ]
then
   echo "You seem to like the bash shell."
else
    echo "You don't seem to like the bash
shell."
fi

Question 12

IF / ELIF / ELSE Syntax

Answer 12

if [ condition-is-true ]
then
  command N
elif [ condition-is-true ]
then
    command N
else
    command N
fi

Question 13

IF / ELIF / ELSE Example

Answer 13

!/bin/bash

MY_SHELL=”csh”
if [ “$MY_SHELL” = “bash” ]
then
echo “You seem to like the bash shell.”
elif [ “$MY_SHELL” = “csh” ]
then
echo “You seem to like the csh shell.”
else
echo “You don’t seem to like the bash or csh shells.”
fi

Question 14

FOR Loop Syntax

Answer 14

for VARIABLE_NAME in ITEM_1 ITEM_N
do
    command 1
    command 2
    command N
done

Question 15

FOR Loop Script Example

Answer 15

#!/bin/bash
for COLOR in red green blue
do
     echo "COLOR: $COLOR"
done

Output:
COLOR: red
COLOR: green
COLOR: blue
\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
#!/bin/bash
COLORS="red green blue"
for COLOR in $COLORS
do
     echo "COLOR: $COLOR"
done

Question 16

FOR Loop Script Example 2

Answer 16

#!/bin/bash
PICTURES=$(ls *jpg)
DATE=$(date +%F)
for PICTURE in $PICTURES
do
echo "Renaming ${PICTURE} to ${DATE}
-${PICTURE}"
mv ${PICTURE} ${DATE}-${PICTURE}
done

$ ls
bear.jpg man.jpg pig.jpg rename-pics.sh
$ ./rename-pics.sh
Renaming bear.jpg to 2015-03-06-bear.jpg
Renaming man.jpg to 2015-03-06-man.jpg
Renaming pig.jpg to 2015-03-06-pig.jpg
$ ls
2015-03-06-bear.jpg 2015-03-06-man.jpg
2015-03-06-pig.jpg rename-pics.sh

Question 17

Positional Parameters Syntax

Answer 17

$ script.sh parameter1 parameter2 parameter3

$0 : “script.sh”
$1 : “parameter1”
$2 : “parameter2”
$3 : “parameter3”

Question 18

Positional Parameters script Example

Answer 18

#!/bin/bash
echo "Executing script: $0"
echo "Archiving user: $1"
# Lock the account
passwd –l $1
# Create an archive of the home directory.
tar cf /archives/${1}.tar.gz /home/${1}

$ ./archive_user.sh elvis
Executing script: ./archive_user.sh
Archiving user: elvis
passwd: password expiry information changed.
tar: Removing leading `/’ from member names

Question 19

Accepting User Input (STDIN) Syntax

Answer 19

The read command accepts STDIN.

read -p “PROMPT” VARIABLE

Question 20

Accepting User Input (STDIN) Example

Answer 20

!/bin/bash

read –p “Enter a user name: “ USER
echo “Archiving user: $USER”

# Lock the account
passwd –l $USER

Create an archive of the home directory.
tar cf /archives/${USER}.tar.gz /home/${USER}
____________________
$ ./archive_user.sh
Enter a user name: mitch
Archiving user: mitch
passwd: password expiry information changed.
tar: Removing leading `/’ from member names