linux-repeat-command

Posted on: 2019-10-08 2019-10-08
Tags: Linux

Linux Commands – Repeat a Command n Times

Linux

1. Introduction

Repeating a command is one of the most commonly used operations when
working with Linux scripting.

In this quick tutorial, we’ll present how to perform an operation n
times using different approaches.

**2. Using a for Loop**

Let’s start by defining a task that we’ll use throughout our examples.
To make things simple, we’ll print the word “Command” out to the
console five times. We’ll also include an iteration number wherever
possible.

Let’s jump straight into it and start with the good old “for-loop”:

for i in {1..5}
do
  echo "Command no. $i"
done

The above example results in the output:

Command no. 1
Command no. 2
Command no. 3
Command no. 4
Command no. 5

We can also condense those operations into a one-line solution:

for i in {1..5}; do echo "Command no. $i"; done

For the scenarios where we need the condition-based expressions, we can
use a more programming-style approach as well:

for ((i=0;i<5;i++)); do echo "Command no. $i"; done

**3. Using a while Loop**

To implement a repeatable command, we can also use a “while-loop”:

i=1
while [[ $i -le 5 ]]; do
  echo "Command no. $i"
  let ++i;
done

As in the previous example, we can implement this loop in just one line:

i=1; while [[ $i -le 5 ]]; do echo "Command no. $i"; let ++i; done

4. Using seq with xargs

Let’s now use a less straightforward approach that is proven to be
pretty efficient.

According to Linux documentation, seq prints sequences of numbers
and *xargs* constructs an argument list and executes the specified
command on each argument.

With that in mind, we can pipe those two operations to create a
repeatable command:

seq 5 | xargs -I{} echo "Command no. {}"

In the above example, we used the -I flag with placeholder string
“\{}”. The placeholder in the defined command is then replaced with a
sequenced number, resulting in the same output we saw above.

5. Bash Functions

Every solution described so far requires some amount of boilerplate
code.

If we want to use repeatable commands often, we can rewrite one of our
examples into a bash function:

function repeat(){
  for ((i=0;i<$1;i++)); do
    eval ${*:2}
  done
}

We can now use our function in the shell:

repeat 5 echo "Command"

In the above example, we used eval to be able to use commands that are
not executables, like variable assignments.

We also introduced $\{*:2} which evaluates to an array of all the
command input variables, starting from the index 2.

As our implementation accepts multiple arguments, we can also pass
multiple commands to our repeat function:

repeat 5 echo "Command" ";" echo "Another command"

6. ZSH

Previously described examples are valid for every bash shell, but there
are other solutions out there like ZSH.

This shell has a simple built-in function for repeating a command n
times:

repeat 5 echo "Command"

And for multiple commands:

repeat 5 {echo "Command1"; echo "Command2"}

7. Conclusion

In this article, we explored several ways of repeating a Linux command
multiple times.

We looked at examples of using vanilla bash scripting and showed the
predefined repeat function of custom shell ZSH.

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