bash shellscripting reminders

On the first line of your shellscript file, add a comment and a bang and define what shell you want your script to use

#!/bin/bash

Comments are a hash/pound sign. Everything after that symbol will be ignored (is commented out)

#echo is a command that outputs a specified string to the screen
echo "Hello world"

A variable is a specific label that you can use as a placeholder for an actual value. You can define these on the fly in your code, and you have a series of pre-defined variables, called “environment variables” that you can use directly as they are implicitly part of the script from your shell

Enter a unique label that is not one of the environment variables. It generally should not start with $ when being declared and initially set. There must be no spaces between the variable name and it's assignment operator and value. It must start with $ when being used - but not when it is created!

#Define the variable var and load it with a string. Remember no space between the variable, assignment operator and the value to assign.
var="Hello World"
#Now use that variable
echo $var

You can also assign the output of a command to a variable. There are 2 ways of doing this

#Get the list of files in the /tmp directory then display them all
var=`dir -b /tmp`
echo $var

#Get the list of files in the /tmp directory then display them all
var=$(dir -b /tmp)
echo $var

You can use this for example to load values into a for loop, using say a directory listing and parsing each returned element in a list.

You can run commands in a loop. This is useful for example when you want to do something on individual files in a directory for instance:

for i in 1 2 3 4 5 6 7 8 9 10;
do
    echo $i;
done;

You can load the in part of the loop with a regular expression or with the output of a command line tool.

The output of this snippet is identical to the snippet above.

for i in {1..10};
do
    echo $i;
done;

There are 2 ways of doing this, by encapsulating the command within slanted quotes or encapsulating the command in a sort of dynamic variable.

In this example, the dir command will list file in a given directory, and these are loaded one at a time as a list into i which can then be used to output a filename or use as a value for another command line tool, such as grep

for i in `dir -b /tmp`;
do
    echo $i;
    grep "something" $i;
done;

An if structure will allow you to check if a value matches a specific condition, then runs the code defined in that section. You can add multiple conditions to check in an if, or as an “elseif” structure, called elif, and you may define an else structure to do something if nothing else matches the conditions you set. In this case, remember to space out the operators in the condition block.

Remember:

• Always put the then condition on a new line after your comparison condition
• An if condition always ends with the opposite: fi.

val=1;
if [ $val = 1 ]
then
    echo "val is 1";
fi

Use the equivalence operator -eq for “equals”, and the logical-OR operator (the logical operators are same as in the C language), so here || , add comparison blocks separated each by whatever needed logical operators.

val=1;
if [ $val -eq 1 ] || [ $val -eq 2 ]
then
    echo "val is 1";
fi

Checking the value of a command line argument sent to the script as the scripts first argument, check if it equals 1 or if it's something else:

val=$1;
if [ $val -eq 1 ]
then
    echo "val equals 1"
else
    echo "val does not equal 1"
fi

This introduces the elif structure, which works the exact same way as if allowing you to match several different values in a condition:

val=$1;
if [ $val = 1 ]
then
    echo "Val = 1";
elif [ $val = 2 ]
then
    echo "Val = 2";
elif [ $val = "Hello" ] || [ $val = "World" ]
then
    echo "Somebody typed Hello or World;
else
    echo "No matching values were entered"
fi

• AND: &&
• OR: ||

You will use the read instruction that will pause execution until something is entered interactively on the command line and enter is pressed. The entered value will be pushed into a variable that follows the command, here input captured by read will be saved into the userInput variable:

echo "Enter something and press enter"
read userInput
echo $userInput

There are 2 types of quotes used to encapsulate strings in Shell Script: Single and double.

Single quoted strings are used where data needs to be output literally, including names of variables rather than their values

Double quoted strings are used where variables need to be displayed as their actual variable value

val="Hello World"
echo 'this will display $val'
#outputs this will display $val
echo "this will display $val"
#outputs this will display Hello World