In this bash scripting tutorial, we will discuss about performing file comparisons, String comparisons as well as numeric comparisons. So let’s start with file comparisons in bash scripts.
Recommended Read: Bash Scripting-6- Using Logical operators in shell scripts
Also Read: Important PostgreSQL commands that every beginner should know
File comparisons
The following are the parameters that can be used for file comparisons,
|
File Comparisons |
|
|
-e file |
check if file exists |
|
-d file |
check if it is a directory |
|
-f file |
check if it is a file |
|
-r file |
check if the file is readable |
|
-s file |
check if the file is not empty |
|
-w file |
check if the file is writable |
|
-x file |
check if the file is executable |
|
-O file |
check if the file is owned by the current user |
|
-G file |
check if the default group is the same as the current user |
|
file1 -nt file2 |
check if file1 is newer than file2 |
|
file1 -ot file 2 |
check if file1 is older than file2 |
These comparisons parameters are one of the most frequently used parameters & you will end up using most of them on a regular basis. Now let’s example with these file comparisons in actions,
NOTE:- We write the comparative statements in brackets i.e. ‘[ ]’, whether its file, string or numeric comparisons.
#!/bin/bash
dir=/home/e2-user
if [ -d $dir ]
then
echo “$dir is a directory ”
cd $dir
ls –a
else
echo “$dir is not a directory”
fi
The above script is checking whether a directory exists & if does, it will print a message & will then also output the directory contents & if the directory does not exist then it will print a message saying that the directory does not exist.
Numeric comparisons
Now let’s see the numeric comparison in action but first let’s check the parameter that we can use for numeric comparisons,
|
Numeric Comparisons |
|
|
n1 -eq n2 |
Checks if n1 is equal to n2 |
|
n1 -gt n2 |
checks if n1 is greater than n2 |
|
n1 -ge n2 |
checks if n1 is greater than or equal to n2 |
|
n1 -le n2 |
checks if n1 is less than or equal to n2 |
|
n1 -lt n2 |
Checks if n1 is less than n2 |
|
n1 -ne n2 |
checks if n1 is not equal to n2 |
Now let’s see an example to demonstrate it,
#!/bin/bash
num1=15
num2=30
if [ $num2 –gt $num1 ]
then
echo “$num2 is greater than $num1”
fi
If [ $num1 –gt 30]
then
echo “$num1 is greater than 30”
else
echo “$num1 is less than 30”
fi
String Comparisons
Now is the time to discuss string comparisons, so let’s start with list of the parameters that are used for performing string comparisons,
|
String Comparisons |
|
|
str1 = str2 |
check if string 1 is equal to string 2 |
|
str1 != str2 |
check if string 1 is not equal to string 2 |
|
str1 \< str2 |
check if string 1 is less than string 2 |
|
str1 \> str2 |
check if string 1 is greater than string 2 |
|
-n str1 |
check if the string 1 has a length greater than zero |
|
-z str1 |
check if the string 1 has a length equal to zero |
You might have noticed that we have used ‘\<’ & ‘\>’ instead of using ‘<’ & ‘>’, that is because both these symbols are used for redirection in Linux as well. So we have to use the escape symbol i.e. \. to use them for string comparisons.
Now let’s have look at some examples to demonstrate the string comparisons,
#!/bin/bash
name=”ec2-user”
if [ $USER = $name ]
then
echo “User exists”
else
echo “User not found”
fi
This is a simple script to check string comparisons, here we are checking a string i.e. a user name & checking it with provided name in the variable. Let’s see another example,
#! /bin/bash
str1=b
str2=y
str3=Y
if [ $str1 \> $str2 ]
then
echo “$str1 is greater”
else
echo “$str2 is greater”
fi
If [ $str3 \> $str1]
then
echo “$str3 is greater”
else
echo “$str1 is greater”
fi
Here we are comparing 3 strings with each other, we also compared a small letter with capital one. Keep in mind capital letters will be less than small letters & ‘z’ will be highest with ‘a’ as lowest letter.
We now end this tutorial on file, numeric & string comparisons. Please do send us any suggestions, queries or questions using the comment box below.
