Introduction to Linux in HPC/Shell scripting /
Video
Quiz
CollapseWhich bash command below assigns "value" to variable var?
Exercises in Terminal (slides 115 - 116) and solution (slide 117)
1. a. Write a script that
b. Prints an environment variable
c. Saves the output of the date command to a variable
d. Sleeps briefly
e. Prints the new and old date and time
| ExpandAnswer:
|
One possible way to write the script is
#!/bin/bash
echo "I am $USER, my home directory is $HOME"
olddate=$(date)
sleep 10s
echo "old date: ${olddate}"
echo "new date: $(date)"
Explanation:
#!/bin/bash: the shebang line for bash script
echo "I am $USER, my home directory is $HOME": the variables $USER and $HOME are your username and home directory, respectively. This line prints a sentence embedded with your username and home directory.
olddate=$(date) assigns the output of the date command to the variable olddate as a string.
sleep 10s puts the terminal into idle for 10 seconds.
echo "old date: ${olddate}" prints the previously saved date in the olddate variable. The form of ${olddate} is for using (or referencing) this variable.
echo "new date: $(date)" prints the current date, in which the output of the command date will take the place of $(date).
|
2. Find a way to execute a script without setting execute permissions.
| ExpandAnswer:
|
bash script.sh
|
Explanation: bash will be used as an interpreter to execute script.sh and the commands contained in script.sh will be executed sequentially.
|
3. Find out how to do other programming things in bash (e.g. functions, classes). How convenient do they look?
| ExpandAnswer:
|
To define a function hello in bash, which prints a message "hello, world" on the terminal. bash script.sh
function hello {
echo "hello, world"
}
To use this function in bash (after defining it):
|
4. Look at different ways you can define if conditions.
| ExpandAnswer:
|
Both single square brackets [] and double square brackets [[]] can be used to define if conditions. But the effects are different. For example, we define a variable str by using a string with spaces:
The if condition with double square brackets works fine.
if [[ $str = "a b" ]]; then
echo "it works"
fi
However, the if condition with single square brackets will emit a syntax error.
if [ $str = "a b" ]; then
echo "it works"
fi
To have the correct syntax for if condition with single square brackets, the variable str must be double quoted inside the if condition.
if [ "$str" = "a b" ]; then
echo "it works"
fi
|
5. Find out what different types of quotes (single ' vs. double ") do?
| ExpandAnswer:
|
single quote ' gives literal string, the variable will not be interpreted, e.g.
This script prints the literal string $var (instead of its value abc) to terminal.
double quote " allows the variable to be interpreted, e.g.
This script prints the value of var, which is abc (instead of the literal string $var) to terminal.
|
6. create an shell variable MYIDENTITY and export it as below:
$ export MYIDENTITY=whoami
How will you list the shell variable MYIDENTITY?
Execute the shell variable MYIDENTITY, what is the output?
| ExpandAnswer:
|
you can list the variable by using the echo command as follows
$ echo $MYIDENTITY
whoami
The arguments passed to echo are printed to the standard output.
The shell variable upon execution runs the command whoami, which is assigned to it. whoami command prints the user name of the effective user ID
|
