If variable1 is the name of a variable, then $variable1 is a reference to its value, the data item it contains.
variable1=23
echo variable1
echo $variable1No space permitted on either side of = sign when initializing variables. What happens if there is a space?
# bad examples
VARIABLE =value
VARIABLE= value
VARIABLE = valueBelow are a few examples of variable referencing. Try them out and make sure you understand each one of the cases.
HELLO=Hi
echo HELLO # HELLO
echo $HELLO # Hi
echo ${HELLO} # Hi
echo "$HELLO" # Hi
echo "${HELLO}" # Hi
echo "$HELLO, I like them squishy"
# Variable referencing disabled (escaped) by single quotes
echo '$HELLO'There are MUCH more functionalities.
Positional arguments are arguments passed to a command or script in a specific order, usually separated by spaces. Positional arguments can be accessed, within a bash script file, using special variables such as $1, $2, $3, and so on, where $1 refers to the first argument, $2 refers to the second argument, and so on.
Let's see them in action... create a file called BarackObama.sh as follows:
#!/bin/bash
# This script reads 3 positional parameters and prints them out.
echo "$0 invoked with the following arguments: $@"
POSPAR1="$1"
POSPAR2="$2"
POSPAR3="$3"
echo "$1 is the first positional parameter, \$1."
echo "$2 is the second positional parameter, \$2."
echo "$3 is the third positional parameter, \$3."
echo
echo "The total number of positional parameters is $#."
if [ -n "${10}" ] # Parameters > $9 must be enclosed in {brackets}.
then
echo "Parameter #10 is ${10}"
fiExecute the script by:
bash positional.sh Yes We Can
bash positional.sh Yes We Can bla bla 1 2 3Investigate the script output and make sure you understand each variable.
In Unix-like operating systems, every command that is executed returns an exit status to the shell that invoked it. The exit status is a numeric value that indicates the success or failure of the command. A value of 0 indicates success, while a non-zero value indicates failure.
The exit status of the most recently executed command can be accessed via the $? variable in Bash.
[myuser@hostname]~$ ls /non-existing-dir
ls: cannot access '/non-existing-dir': No such file or directory
[myuser@hostname]~$ echo $?
2In the above example, if you run a command like ls /non-existing-dir, you will receive an error message saying that the directory does not exist, and the exit status will be non-zero. You can access the exit status of this command by typing echo $?. The output will be the exit status of the previous command (in this case, the value is 2).
Some common non-zero exit status values include:
1: General catch-all error code2: Misuse of shell built-ins (e.g. incorrect number of arguments)126: Command found but not executable127: Command not found128+: Exit status of a program that was terminated due to a signal
Explore the man page of the grep command. List all possible exit codes, and specify the reason for every exit code.
The bash shell allows users to join multiple commands on a single command line by separating the commands with a ; (semicolon).
[myuser@hostname]~$ cd /etc/ssh; ls
moduli ssh_config.d sshd_config.d ssh_host_ecdsa_key.pub ssh_host_ed25519_key.pub ssh_host_rsa_key.pub
ssh_config sshd_config ssh_host_ecdsa_key ssh_host_ed25519_key ssh_host_rsa_key ssh_import_id
[myuser@hostname]/etc/ssh$Nothing special in the above example… just two commands that were executed one after the other.
The bash shell uses && and || to join two commands conditionally. When commands are conditionally joined, the first will always execute. The second command may execute or not, depending on the return value (exit code) of the first command. For example, a user may want to create a directory, and then move a new file into that directory. If the creation of the directory fails, then there is no reason to move the file. The two commands can be coupled as follows:
[myuser@hostname]~$ echo "one two three" > numbers.txt
[myuser@hostname]~$ mkdir /tmp/boring && mv numbers.txt /tmp/boringBy coupling two commands with &&, the second command will only run if the first command succeeded (i.e., had a return value of 0).
What if the mkdir command failed?
Similarly, multiple commands can be combined with ||. In this case, bash will execute the second command only if the first command "fails" (has a non zero exit code). This is similar to the "or" operator found in programming languages. In the following example, myuser attempts to change the permissions on a file. If the command fails, a message to that effect is echoed to the screen.
[myuser@hostname]~$ chmod 600 /tmp/boring/numbers.txt || echo "chmod failed."
[myuser@hostname]~$ chmod 600 /tmp/mostly/boring/primes.txt || echo "chmod failed"
chmod: failed to get attributes of /tmp/mostly/boring/primes.txt': No such file or directory
chmod failedIt's common in bash scripts to create a directory and immediately cd to the directory, if the creations succeeded. Use conditional the && operator to create the dir and cd into it only if the creation succeeded.
Solution
mkdir newdir && cd newdirCommand substitution allows users to run arbitrary commands in a subshell and incorporate the results into the command line. The modern syntax supported by the bash shell is:
$(subcommand)As an example of command substitution, myuser would like to create a directory that contains the date in its name. After examining the date(1) man page, he devises a format string to generate the date in a compact format.
[prince@station prince]$ date +%d%b%Y
04May2023He now runs the mkdir command, using command substitution.
[prince@station prince]$ mkdir reports.$(date +%d%b%Y)
[prince@station prince]$ ls
reports.04May2003The bash shell implements command substitution by spawning a new subshell, running the command, recording the output, and exiting the subshell. The text used to invoke the command substitution is then replaced with the recorded output from the command.
Before you start, review the man page of the test command.
The first example checks for the existence of a file:
echo "This script checks the existence of the messages file."
echo "Checking..."
if [ -f /var/log/messages ]
then
echo "/var/log/messages exists."
fi
echo
echo "...done."🧐 What is the relation between the
testcommand and[?
Recall that the $? variable holds the exit status of the previously executed command. The following example utilizes this variable to make a decision according to the success or failure of the previous command:
curl google.com &> /dev/null
if [ $? -eq 0 ]
then
echo "Request succeeded"
else
echo "Request failed, trying again..."
fiThe curl command can be used to perform a request to an external website and return the response's status code:
curl -o /dev/null -s -w "%{http_code}" www.google.comThis curl command suppresses output (-o /dev/null), runs silently without printing the traffic progress (-s), and prints the HTTP status code (-w "%{http_code}").
Create an availability_test.sh script that receives an address as the 1st (and only) argument, and perform the above curl command.
The script should be completed successfully if the returned HTTP status code is < 500,
or fail otherwise (you can exit the script with exit 1 to indicate failure).
Here is the expected behaviour:
myuser@hostname:~$ ./availability_test.sh www.google.com
www.google.com is up!
myuser@hostname:~$ ./availability_test.sh http://cnn.com
http://cnn.com is up!
myuser@hostname:~$ ./availability_test.sh abcdefg
abcdefg is not available.
myuser@hostname:~$ echo $?
1
myuser@hostname:~$ ./availability_test.sh
A valid URL is required
myuser@hostname:~$ ./availability_test.sh google.com cnn.com
The script is expected a single argument only, but got 2.
myuser@hostname:~$ ./availability_test.sh httpbin.org/status/500 # this url should return a status code of 500
httpbin.org/status/500 is not available.Write a bash script geo_by_ip.sh that, given an ip address, prints geo-location details, as follows:
- The script first checks if
jqcli is installed. If not installed, it prints a message to the user with a link to download the tool: https://stedolan.github.io/jq/download/ - The script checks that exactly one argument was sent to it, which represents the ip address to check. Otherwise, an informative message is printed to stdout.
- The script checks that the given IP argument is not equal to
127.0.0.1. - The script performs an HTTP GET request to
http://ip-api.com/json/<ip>, where<ip>is the IP argument. The results should be stored in a variable. - Using the jq tool and the variable containing the HTTP response, check that the request has succeeded by checking that the
statuskey has a value ofsuccess. The commandjq -r '.<key>'can extract a key from the json (e.g.echo $RESPONSE | jq -r '.status') - If the request succeed, print the following information to the user:
- country
- city
- regionName
In our course repo, copy the file under theatre_nighout/init.sh into an empty directory and execute it.
This script creates 5 directories, each for a famous theater show.
In each directory there are 50 files, representing 50 available seats for the show.
Create a bash script available_seat.sh that takes one argument which is the name of a show and prints the available seats for the show (by simply using ls command).
Create another bash script booking.sh that takes two arguments - the name of a show and a seat number.
The selected seat should be marked as booked by deleting the file that represents the seat number. You should print an informative message to the user upon successful or failed booking.
You can always re-run init.sh to test your script again.
For example:
$ ./init.sh && cd shows
$ ./available_seat.sh Hamilton
Available seats for Hamilton:
1 2 3 4 5 6 7 8 9 10 ... 48 49 50
$ ./booking.sh Hamilton 5
Seat 5 for Hamilton has been booked!
$ ./available_seat.sh Hamilton
Available seats for Hamilton:
1 2 3 4 6 7 8 9 10 ... 48 49 50
$ ./booking.sh Hamilton 5
Error: Seat 5 for Hamilton is already booked!