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Computer Architecture and Operating Systems

Course taught at Faculty of Computer Science of Higher School of Economics

Workshop: Shell Scripts

Shell allows creating simple scripts that run various Linux system utilities and connect them. So, it is possible to automate solving tasks related to managing system resources.

From a practical point of view, a Shell script is an executable text file that can be run with a command like this: interpreter script_file (e.g. sh script).

Also, there is a special marker «shebang» (#!/bin/sh) that specifies the interpreter to be used to run the file. It is placed to the first line of a script. Then the script is given the “execute” permission (chmod +x script). After that the script can be run simply like this:

acos@acos-vm:~$ ./script

Also, it is possible to run a Shell script from any directory if its directory is added to the $PATH variable.

acos@acos-vm:~$ export  PATH=directory_with_script:$PATH

After this the script can be called from directory in the following way:

acos@acos-vm:~$ script

Examples and Tasks

Input and Output

This script reads text into the A variable from the console (standard input) and then prints it back to the console.

#!/bin/sh

read A
echo "$A"

The script below reads the user name from the standard input and greats the user with a message containing the user name and the information about the system (returned by the uname utility).

#!/bin/sh

read A
Sys=`uname`

echo "Hello, $A!
Welcome to $Sys!"

Task 01:

Modify the previous program to get the user name from system instead of reading the input. Use the whoami utility to get the user name.

Task 02:

Modify the previous program to store the message to be printed in a macro variable called Msg. Pass the variable to the echo utility.

Command-Line Arguments

This script demonstrates the use of special symbols to access command-line arguments passed to the interpreter.

#!/bin/sh

# $1, $2 … — command line argument 1, 2 ...
# $0 — name of the script itself
# $* — all command line arguments
# $# — number of command line arguments so

echo "$0"
echo "$1"

# ...

echo "$*"
echo "$#"

Conditions and Exit Status

When a utility is called from a Shell script, it is possible to interpret its exit status in a conditional expression. For example, the script below calls the ls utility and prints “YES” only if its status code is successful.

#!/bin/sh

if ls $*; then
    echo YES
else
    echo NO $?
fi

The script below redirects the output of the ls utility and prints it only if the status code of the utility is successful.

#!/bin/sh

if ls $* -I out > out; then
    echo YES
    cat out
    rm -f out
else
    echo NO $?
fi

Using 2> /dev/null to redirect error messages out of a terminal:

acos@acos-vm:~$ ls nofile
ls: cannot access 'nofile': No such file or directory
acos@acos-vm:~$ ls nofile 2> err
acos@acos-vm:~$ cat err
ls: cannot access 'nofile': No such file or directory
acos@acos-vm:~$

Task 03:

Rewrite the previous example so that it:

Advanced Shell Features

The Shell language has features common for many programming languages such as conditions, loops, and functions. For example the script below enumerates over a collection of values:

#!/bin/sh

for n in 1 2 3 10 20 30 qwe asd xcv; do
    echo "Next value $n"
done

The script below defines a function that enumerates over a collection of its arguments. Arguments of a function are accessed using special symbols in the same way as arguments of the Shell script.

#!/bin/sh

fun() {
    echo "$0: $# $*"
    for arg; do
        echo "> $arg"
    done
}

fun 1 QWE "3 4"
fun "$*"
fun  $*
fun "$@"

Task 04:

Create a program with the following structure:

#!/bin/sh

sum() {
    # insert your code here
}

while read a b; do
    sum $a $b
done

Then do the following:

Homework

  1. Experiments with the read builtin command.
    • Research: What exit status the command read generates? Try it. Consult help read command instead of man read, because read is a Shell builtin and you got manual page on all Shell builtins.
    • Research: How to suppress «\n» output after echo?

    • Task: Write a script that asks user for name, and prints a welcome message either if a user has entered a name or not:

      $ ./c8
Enter your name: Spot
Hello, Spot!
$ ./c8
Enter your name:
Hello, tmpuser?
    • Hint: In second example user had not pressed «Enter»

  2. Read documentation on expr again. Write Shell script sumsum.sh that:

    • has function sum() that:
      • sums all of its arguments (any number of arguments is permitted);
      • returns this sum if there are no errors;
      • returns 0 if there was an error (e.g. attempt to sum non-numbers);
    • redirects all error messages to /dev/null;
    • reads two lines of numbers;
    • prints whether their sums was equal or not (0 values as a result of errors are considered equal).
    $ ./sumsum.sh
1 3 5
2 4 6
Not equal
$ ./sumsum.sh
1 5 6
4 4 4
Equal
$ ./sumsum.sh
1 2 w
3 4 e
Equal
$ ./sumsum.sh
1 2 1
2 2 Q
Not equal
$ ./sumsum.sh
qwe 3 4
10 20 -30
Equal

    Hint: errors can be checked using the $? symbol (status of the last command, must be 0 if successful).

Commit the programs to your private GitHub account. Place them into the folder os/lab03.

References