In this tutorial on Linux bash shell scripting, we are going to learn how to compare numbers, strings and files in shell script using if statement. Comparisons in a script are very useful & after comparison result, script will execute the commands and we must know how we can use them to our advantage.
Syntax of comparisons in shell script
if [ conditions/comparisons] then commands fi
if [2 -gt 3] then print "2 is greater" else print "2 is not greater" fi
This was just a simple example of numeric comparison & we can use more complex statement or conditions in our scripts. Now let’s learn numeric comparisons in bit more detail.
Compare Numbers in Linux Shell Script
This is one the most common evaluation method i.e. comparing two or more numbers. We will now create a script for doing numeric comparison, but before we do that we need to know the parameters that are used to compare numerical values . Below mentioned is the list of parameters used for numeric comparisons
- num1 -eq num2 check if 1st number is equal to 2nd number
- num1 -ge num2 checks if 1st number is greater than or equal to 2nd number
- num1 -gt num2 checks if 1st number is greater than 2nd number
- num1 -le num2 checks if 1st number is less than or equal to 2nd number
- num1 -lt num2 checks if 1st number is less than 2nd number
- num1 -ne num2 checks if 1st number is not equal to 2nd number
Now that we know all the parameters that are used for numeric comparisons, let’s use these in a script,
#!/bin/bash # Script to do numeric comparisons var1=10 var2=20 if [ $var2 -gt $var1 ] then echo "$var2 is greater than $var1" fi # Second comparison If [ $var1 -gt 30] then echo "$var is greater than 30" else echo "$var1 is less than 30" fi
This is the process to do numeric comparison, now let’s move onto string comparisons.
Compare Strings in Linux Shell Script
When creating a bash script, we might also be required to compare two or more strings & comparing strings can be a little tricky. For doing strings comparisons, parameters used are
- var1 = var2 checks if var1 is the same as string var2
- var1 != var2 checks if var1 is not the same as var2
- var1 < var2 checks if var1 is less than var2
- var1 > var2 checks if var1 is greater than var2
- -n var1 checks if var1 has a length greater than zero
- -z var1 checks if var1 has a length of zero
Note :- You might have noticed that greater than symbol (>) & less than symbol (<) used here are also used for redirection for stdin or stdout in Linux. This can be a problem when these symbols are used in our scripts, so what can be done to address this issue.
Solution is simple , when using any of these symbols in scripts, they should be used with escape character i.e. use it as “/>” or “/<“.
Now let’s create a script doing the string comparisons.
In the script, we will firstly be checking string equality, this script will check if username & our defined variables are same and will provide an output based on that. Secondly, we will do greater than or less than comparison. In these cases, last alphabet i.e. z will be highest & alphabet a will be lowest when compared. And capital letters will be considered less than a small letter.
#!/bin/bash # Script to do string equality comparison name=linuxtechi if [ $USER = $name ] then echo "User exists" else echo "User not found" fi # script to check string comparisons var1=a var2=z var3=Z if [ $var1 \> $var2 ] then echo "$var1 is greater" else echo "$var2 is greater" fi # Lower case & upper case comparisons if [ $var3 \> $var1 ] then echo "$var3 is greater" else echo "$var1 is greater" fi
We will now be creating another script that will use “-n” & “-z” with strings to check if they hold any value
#!/bin/bash # Script to see if the variable holds value or not var1=" " var2=linuxtechi if [ -n $var1 ] then echo "string is not empty" else echo "string provided is empty" fi
Here we only used ‘-n’ parameter but we can also use “-z“. The only difference is that with ‘-z’, it searches for string with zero length while “-n” parameter searches for value that is greater than zero.
File comparison in Linux Shell Script
This might be the most important function of comparison & is probably the most used than any other comparison. The Parameters that are used for file comparison are
- -d file checks if the file exists and is it’s a directory
- -e file checks if the file exists on system
- -w file checks if the file exists on system and if it is writable
- -r file checks if the file exists on system and it is readable
- -s file checks if the file exists on system and it is not empty
- -f file checks if the file exists on system and it is a file
- -O file checks if the file exists on system and if it’s is owned by the current user
- -G file checks if the file exists and the default group is the same as the current user
- -x file checks if the file exists on system and is executable
- file A -nt file B checks if file A is newer than file B
- file A -ot file B checks if file A is older than file B
Here is a script using the file comparison
#!/bin/bash # Script to check file comparison dir=/home/linuxtechi if [ -d $dir ] then echo "$dir is a directory" cd $dir ls -a else echo "$dir is not exist" fi
Similarly we can also use other parameters in our scripts to compare files. This completes our tutorial on how we can use numeric, string and file comparisons in bash scripts. Remember, best way to learn is to practice these yourself.