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Bash Cheatsheet

Multi-line Command

To execute multi-line command in bash script simple put a \ after breaking up your commands. There should be no white spaces after the backslash, else it will fail!

Every single line should be followed by a \ until you finished typing the command.

curl https://cat.png \#no spaces after!
	-o here.txt \

If-Statements

In Bash the if-statements follows the structure:

if CONDITION
then
	COMMAND
elif CONDITION
then
	COMMAND
else
	COMMAND
fi

As you can see every if chain will end with fi keyword. Every if/elif condition statement will be followed by the then keyword.

If you want to put the then on the same line with the if keyword, and because if, then, else, elif, fi are all shell keyword they cannot be used on the same-line. To fix this you have to put a ; to end the previous statement and the keyword before you can use another keyword. As an example:

if CONDITION; then
	COMMAND
fi

Conditions

Okay, now we know the basic of if-statements how do I put it into use by filling in the conditions? There are couple different ways of writing conditions, here I will only go over the most commonly used ones.

1.

# The first way is using test
if test <expressions>; then
	COMMAND
fi

Using this method you can test whether the files exists, and compare values. It has it's own set of syntax for example to check if a certain file named "foo.txt" exists then you would type test -f "foo.txt" and it will evaluate to true if only the file "foo.txt" exists.

For comparing values we cannot use the symbols directly, ==, <, >, <=, or >=. Instead we have to use their flag equivalent below:

Compartor

Flag Equivalent
==

-eq
>
 -gt
<
 -lt
>=
 -ge
<=
 -le
or
 ||
and
 &&
not
 !

You can also append ! to negate the expression to check the opposite. Use = to do string equality comparison.

2.

# The second way is using [] square brackets
if [ some test ]; then
	COMMAND
fi

The square bracket is like test and essentially all the operators that you can use with test you can also use in the square brackets.

There must be a space between the test and the left bracket, [ and the right bracket, ], otherwise Bash cannot understand it!

3.

# The third way is using [[]] double square brackets
if [[ some test ]]; then
	COMMAND
fi

The double square brackets is like an upgrade of the normal square bracket. It comes from ksh.

With the double square brackets you can use some of the comparison operators without using the flags. So we are allowed to use >, =, and < without using the flag equivalent. However, <= and >= still requires the flag equivalent.

4.

if command;then
	COMMAND
fi

Bash runs the command you have provided and then will run the if-statement according to the exit code. It will run it if the exit code is 0, and will not run it if it is not 0.

Remember in programming, 0 represent the command carried out successfully, and anything not 0 represent some sort of errors occurred.

For Loops

 

https://opensource.com/article/18/5/you-dont-know-bash-intro-bash-arrays



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