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This article is about using strings as commands in Bash scripts. For this purpose, the eval command is used.

Eval Command in Bash Scripts

In some Bash scripts, you have to create a string using variables or input values ​​(for example) and run it as a command at the end. For such cases, evalis the solution.

In Bash, evalcommand evaluation parameters are similar to shell commands. The parameters are combined into a string and sent to the shell command, which is then executed.

Runs a command in the current shell eval. This command is convenient when running commands with specific operators or reserved terms.

This command can also be used in any script where the variable names are unknown before the script is run. This guide will help Linux users understand how to use this command.

syntax:

eval [arguments...]

Arguments are passed to this command as string literals which are then sent to Bash for execution. This command returns the exit status after executing the command.

If no arguments are passed to this command or null is passed, it returns 0 as the exit status.

Counting the number of lines in a file using eval

Suppose we have a file sample.txt containing several lines of text. If we need to count the number of lines in this file, we can use the wc -l command.

We will use the eval command to perform this task. The command will be:

myComm="wc -l sample.txt"
eval $myComm

Let's look at the output of this.

Using eval to solve arithmetic operations

Suppose we need to perform some arithmetic operations. We can do this using the eval command.

To do this, we will create a Bash script.

#!/bin/bash
a=4
b=5
comm1="`expr $a + $b`"
comm2="echo"

eval $comm2 $comm1

In this script, we have created two variables containing two commands, one for performing arithmetic operations and another for echoing commands. In the last line, we pass these two variables as arguments to the eval command.

eval will combine these two variables and construct the following command.

echo 'expr 4 + 5'

After executing this script, we will get the following output.

Use eval to print variable values

In the following example, we will print the value of a variable containing another variable.

#!/bin/bash
str1="my script"
str2=str1
comm="echo"
eval $comm \${$str2}

The output of the above script will be:

Printing the sum of numbers using eval

In the following example, we will use a for loop in a script to print the sum of numbers in the range of 1 to 4. We will then use the eval command to print the sum.

The script for this question is:

#!/bin/bash
sum=0
for n in {1..4}
do
sum=$(($sum+$n))
done
comm="echo 'The result of sum from 1-4 is: '"
eval $comm $sum

The output of the above script will be:

Summarize

evalCommands can be run by expressing any Bash command as a string. In this lesson, evalcommands run several Bash built-in tasks and create a series of variables.

After reading this article, the user will evalhave a better understanding of the command and will be able to use it for various applications.