Shell scripting basics

✔️ Running shell scripts

• Any command that can be executed from the shell may be placed into a script and behave exactly the same (and vice versa)
• A script must have the execute permission x set for the current user before it may be run.
• It is important to note a script started from the command line doesn't run in the current shell process, instead a new shell is started and runs the script.
• A notable exception to the above is what is called "sourced scripts".

✔️ Writing shell scripts

• The first line of any shell script has to be the shebang #! followed by the path to interpreter :

#!/bin/bash

echo "some script"

• That line identifies the interpreter to use and should never contain any additional character
• Shell comments are prefixed with #

Bash scripts are particularly picky when it comes to formatting. Make sure spaces are put where they are needed and not put when they are not needed.

✔️ Using vi as an editor

• There are two modes in vi : insert (or input) mode and edit mode.
• In input mode you may input or enter content into the file.
• In edit mode you can move around the file, perform actions such as deleting, copying, search and replace, saving etc...
• vi regexp uses Vim syntax

✔️ Basics

• Built in control structures in the shell are more or less the same as in any other programming language
• They differ in that the expressions used do not evaluate through operators but are instead based on test expressions
• Those expressions can perform various tests on variables, files, etc ... as seen below

✔️ Control structures list

• if conditional structure

# if a file exists in my home directory ... 
if [ -f "$HOME/myfile" ]; then
    # run commands ...
fi

• while loop structure

# as long as a variable is greater than zero ... 
while [ $counter -gt 0 ]; do
    # run commands ...
done

• until loop structure

# as long as a variable is different than zero ... 
until [ $counter -eq 0 ]; do
    # run commands ...
done

• for loop structure

# execute a sequence of instructions for each value in a list or an array ...
# reference the value inside the loop by using $value ...
for value in $list; do
    # run commands ...
done

• case conditional structure

case $var in
    # if $var equals value1 ...
    value1)
        # run commands ...
        ;;
    # if $var equals value2 ...
    value2)
        # run commands ...
        ;;
    # if $var is different ...
    *)
        # run commands ...
        ;;
esac

• Notes :
  • When using the bash shell, it's best to wrap up expressions in double brackets (although single brackets work too)
  • One of the most useful applications of for loops is the processing of a set of files (wildcards and shell globbing allow that).
  • break and continue keywords are available in bash loops, as they are in many other languages to the same effect.

✔️ Functions

• Shell functions accept a list of words as argument so as multiple parameters can be passed

function some_function {
    # variable local to a functionare not accessible from outside
    # do stuff ...
}
# call the function and pass a word list as argument
some_function some arguments and more for a total of 4

• Function parameters are accessible within the function as $1, $2, etc ... like in a normal script
• The function definition must appear in the script before any calls to the function
• Shell functions do no return values but exit codes like any other linux process
• Workaronds to retrieve values from functions exist, such as this :

function sample_function {
    # print return value to the function's stdout
    echo "function actual return value" > /dev/stdout
    # "return" a success code
    return 0
}
# use command substitution to read a value from the function
result=$(sample_function)

✔️ Operators

• Type let --help to view the list of supported arithmetic, logical and bitwise operators.
• Evaluation of expressions that use operators can be performed in 2 ways

# evaluate an expression and store the result in a variable with let
let var="4 + 2"
# prints 6
echo $var
# wrap the expression in parentheses and run command substitution on it
# prints 1 for true
echo $((5 != 3))

• ${#var} returns the length of a variable

✔️ User input

• User input can be processed by the shell using the read command
• read prompts user for input and then assigns the input values to variables

# ask for user name, first name and employment
echo "Please enter your name and first name:"
read -r lname fname
echo "Please enter your profession:"
read -r empl

• User input will be processed as a list of words, each word being assigned to the variables which were passed as arguments to read
• If there are more words than variable, the remaining words will all be added to the last variable
• If there are less words than variable, the remaining variable names will be set to blank or null
• read comes with a variety of options (for instance -p to specify a prompt and -s to make the input silent)