if … then … fi

if iis a basic programming checking for condition and reacts based on the condition, bash uses if to check for

1. File
2. integer
3. String

For checking the File, let say your file name is stored at $fn, and you wanna check the file is exist ? do this:

if [  -e $fn ]
then
echo "$fn exist."
else
echo "$fn does not exist."
fi

Bellow are the list of other common File checking,

To check for integer, let say we read user input and store at $num, and check whether is it greater than 10

read num;
if [ $num -gt "10" ]
then
echo "$num is greater than 10"
fi

Bellow are other common Integer checking.

You can uses <, <=, >, >= but you need to put that into if ((…)), let say

if (($num >= "10" ))
then
echo "$num greater than 10"
fi

Now check for string, uses if [[...]] instead of if[...] will reduce a lots of problem of adding escape character. Let say now i wanna compare 2 string to see whether they are similar, $str1, $str2.

if [[ $str1 = $str2 ]]
then
echo "They are equal."
fi

Bellow are other common string checking