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| Meta Title | Bash For Loop usage guide for absolute beginners | GoLinuxCloud |
| Meta Description | For loop in bash is used to iterate a certain command or task for a definite or infinite range. You can also use this in C programmer style, combine with continue and break statement |
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| Boilerpipe Text | I get this question a lot from many users, how can I run a certain
command in a loop for a defined period of time? Or how can we iterate a
task over a range only for n number of times. All these can be achieved
using bash for loop in Linux and Unix
From
What is Loop
,
a loop is a sequence of
instructions that is continually repeated until a certain condition is
reached
. So we know that a loop is a situation where we can perform a
certain task repeatedly for a certain pre-defined period of time or may
be some times infinite.
Using such loops in our day to day task are very useful to automate
stuffs using scripts. The loop can be configured using for, while, until
etc depending upon individualâs requirement. In this tutorial we will
understand in detail about bash for loop, and itâs usage across Linux
environment for different types of automation shell scripts.
I will also share different shell script examples using for loop to help
you understand itâs usage.
Basic for loop syntax in Bash
The syntax of for loop would vary based on the programming language you
choose such as C, perl, python, go etc. The provided
syntax
can be
used only with bash and shell scripts
for {ELEMENT} in ${ARRAY[@]}
do
{COMMAND}
done
Understanding the syntax
Here the
ARRAY
can contain any type of element such as strings,
integers.
Although it is important that
you pass an
ARRAY
to the loop, as
if you pass a
VARIABLE
then the loop with count all the
ELEMENTS
from the
VARIABLE
as single
ELEMENT
.
I will show you what this
means later in this article
Loop will iterate each
ELEMENT
from the
ARRAY
and a certain
command can be assigned to these
ELEMENT
The loop will run until the last
ELEMENT
from
ARRAY
EX_1: Loop over a range of numbers
In our first shell script we will iterate over a range of numbers and
print the number
for num in 1 2 3 4 5 6
do
echo "Found Element: $num"
done
Here we have passed a series of numbers separated by white space
The output from this script
Found Element: 1
Found Element: 2
Found Element: 3
Found Element: 4
Found Element: 5
Found Element: 6
EX_2: Loop over a series of strings
In this shell script we will iterate over a series of strings
for string in STRING1 STRING2 STRING3
do
echo "Found Element: $string"
done
The output from this script:
Found Element: STRING1
Found Element: STRING2
Found Element: STRING3
Let us also take a
practical example for Linux
Administrator
. In this script we will write a small script which
will search for
file1
,
file2
and
file3
under
/tmp
Now if you are not using for loop then you have to manually search for
these files.
You may think, so what it is easier to find files
manually?
Yes,
TRUE
but what if you have 100 files or 1000 files? Would you
find them manually?
for file in file1 file2 file3
do
echo "Searching for file: $file"
file_path=`find /tmp -name $file`
if [ ! -z $file_path ];then
echo "Found: $file at $file_path"
else
echo "Not Found: $file"
fi
done
The output from the script will tell us the files which were found and
the files which were not found.
Bash for loop in a range
EX_3: Use for loop with an array
Before we go ahead it is important that you understand the different
between ARRAY and Variable
Array vs Variable
It is
important
that when you use loop, you use an
ARRAY
which
contains elements separated by white character
A variable will always contain a single element while an array can
contain multiple number of elements
An array can also store multiple variables
You can
convert your
VARIABLE into an ARRAY using different methods
.
For example:
VAR="My name is Deepak"
Here we have given white space separated values, so does this become an
ARRAY
?
NO, this is a VARIABLE
Check the length of this
VARIABLE
echo "Length: ${#VAR[@]}"
Length: 1
Bash could only count
single
element
here, so if we try to iterate using this
VAR
then we
get only
1
iteration
You should know
how to declare an array in shell script
:
Now I will declare the same content as an array
VAR=(My name is Deepak)
Here I have added the elements inside
parenthesis
, now let us check
the length of this variable
# echo "Length: ${#VAR[@]}"
Length: 4
So now bash considers this as an
ARRAY
with
4
ELEMENTS
1st - My
2nd - name
3rd - is
4th - Deepak
We can
access these individual elements
using
${VAR[$num]}
Here
$num
should be replaced by the element number you want to access,
for example
:
#!/bin/bash
VAR=(My name is Deepak)
echo "First Element: ${VAR[0]}"
echo "First Element: ${VAR[1]}"
echo "First Element: ${VAR[2]}"
echo "First Element: ${VAR[3]}"
The output of this script:
Bash for loop array
Let us take a
practical example
to understand loop with array in
real time environment.
Assume you have to
create 5 users
and
assign a password to these
users
. In this shell script I will assign â
Passw0rd
â as password to
all the users
#!/bin/bash
USERS=(user1 user2 user3 user4 user5)
for user in ${USERS[@]}
do
echo "Creating user $user"
useradd $user
echo "Assigning password for $user"
echo "Passw0rd" | passwd "$user" --stdin
done
EX_4: Using for loops in bash as C programmers
Bash extends the
for
keyword to provide functionality similar to the
three-argument
for loop used in C using double parenthesis
We can use the above examples when we have a small number of range to
iterate over but what if we have a
larger range of items
?
Or if we do not know the number of times the loop has to run and
instead we get this value internally from the script?
Syntax:
for ((ASSIGN_VALUE; ASSIGN_LIMIT ; STEP_UP))
do
[COMMANDS]
done
The
syntax
would be more clear in this sample script.
The
first expression
is run before the loop starts: We assign the
i
value to zero to start it off.
The
second expression
is the test used to determine whether the
loop should continue or stop: We test whether the value of
i
is less
than 5.
The
third expression
runs after each instance of the loop: We add
one to the value of
i
.
#!/bin/bash
for (( i=0; i<=5; i++ ))
do
echo "Iteration $1"
done
The output from this script:
Iteration 0
Iteration 1
Iteration 2
Iteration 3
Iteration 4
Iteration 5
Now you must be wondering
do we have any use case for such of âfor
loopâ in real time scenario
.
The answer would be
YES
, very much. I will share some real time
scenarios to help you understand better:
In this example I will search for all files with the syntax matching
file*
under
/tmp
Then append some content to these files
Now the loop should iterate based on the number of files found
Since the iteration starts from
0
we have given variable
i
to be
one less than the number of files which is found
#!/bin/bash
file_path=(`find /tmp -name "file*" -type f`)
echo "Found ${#file_path[@]} files"
for (( i=1; i<${#file_path[@]}; i++ ))
do
echo "Adding content to ${file_path[$i]}"
echo "add some content" >> "${file_path[$i]}"
done
The output from this script:
bash for loop example
Define multiple variables
We can also use
multiple variables
in single for loop. In this
example I have defined
two variables
where the loop will iterate until
i
is less than equal to
j
variable
#!/bin/bash
for (( i=1,j=5; i<=j; i++,j-- ))
do
echo "Iteration for i=$i"
echo "Iteration for j=$j"
done
The output from this script
Iteration for i=1
Iteration for j=5
Iteration for i=2
Iteration for j=4
Iteration for i=3
Iteration for j=3
In this script I will perform some more operations using
two
variables
in single for loop with bash
for (( i=0, j=0 ; i+j < 5 ; i++, j++ ))
do
echo "Value for i=$i"
echo "Value for j=$j"
echo "Multiply: $((i*j))"
echo "add: $((i+j))"
done
The output from this script
Value for i=0
Value for j=0
Multiply: 0
add: 0
Value for i=1
Value for j=1
Multiply: 1
add: 2
Value for i=2
Value for j=2
Multiply: 4
add: 4
EX_5: Use continue statement
Now it is not mandatory that you would want to perform task for complete
iteration, it is possible you may have a specific requirement to
ignore certain value
during the iteration run
I have this sample script where the loop iterates for 5 times
for (( i=0 ; i<=5 ; i++ ))
do
if [ $i -eq 2 ];then
echo "Don't do anything when i=2"
fi
echo "Doing something when i=$i"
done
Now I have an additional check when value of the variable
i
is equal
to
2
, so when this matches the
script should do nothing and
continue with the remaining iteration
But let us check the output from this script
Doing something when i=0
Doing something when i=1
Don't do anything when i=2
Doing something when i=2
Doing something when i=3
Doing something when i=4
Doing something when i=5
Even when I added a check to ignore
i=2
match, still we see
â
Doing something when i=2
â, so
how to fix this?
We need to use
continue
statement for such case
The
continue
statement will exit the current iteration of the loop
and will continue with the next iteration in the loop
for (( i=0 ; i<=5 ; i++ ))
do
if [ $i -eq 2 ];then
echo "Don't do anything when i=2"
continue
fi
echo "Doing something when i=$i"
done
The output from this script
Doing something when i=0
Doing something when i=1
Don't do anything when i=2
Doing something when i=3
Doing something when i=4
Doing something when i=5
Now we see that when variable of
i
is equal to
2
,
the iteration is skipped and
the loop continues with remaining iteration
EX_6: Use break statement
With
continue
we were able to skip the loop for single iteration but
what if we want to
completely come out of the loop with a certain
condition?
In this script we want to end the loop if the value of
i
variable is
equal to
2
so with the if condition I have added a
break
statement
for (( i=0 ; i<=5 ; i++ ))
do
if [ $i -eq 2 ];then
echo "Exiting, match sucessful"
break
fi
echo "Doing something when i=$i"
done
The output from this script
Doing something when i=0
Doing something when i=1
Exiting, match successful
As expected, our for loop terminates when the if condition is fulfilled
using the
break
statement.
EX_7: One liner for loop examples
We can write for loop in one liner commands to perform simple tasks
using below
syntax
:
for {ELEMENT} in ${ARRAY[@]}; do [COMMAND_1]; [COMMAND_2]; [COMMAND_3]; done
In this shell script I will
list all the network
devices
available on my server. here instead of defining an
ARRAY
, I have provided a
PATH
with
*
, so all the files and
directories under that
PATH
would be considered as
ELEMENT
of the
ARRAY
# for int in `ls /proc/sys/net/ipv4/conf/`;do echo "Interface: $int"; done
Interface: all
Interface: bond0
Interface: bond1
Interface: default
Interface: eth0
Interface: eth1
Interface: eth2
Interface: eth3
Interface: lo
In this script I will remove all files under
/tmp
matching regex
file*
# for file in /tmp/file*; do rm -vf $file; done
removed '/tmp/file1'
removed '/tmp/file2'
removed '/tmp/file3'
Conclusion
Even though bash is loosing itâs popularity with new programming
language coming every day but engineers and developers still prefer bash
to automate day to day tasks which consists of system commands. In such
case it is always handy for engineers to be aware of using such loop
based iteration.
I have given some very basic examples for beginners to understand the
syntax but this can be used to do complex tasks as well. Lastly I hope
this tutorial to learn bash for loop with examples to iterate over a
range and series of items on Linux and Unix was helpful. So, let me know
your suggestions and feedback using the comment section. |
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- [Bash Compare Numbers](https://www.golinuxcloud.com/bash-compare-numbers/ "Bash Compare Numbers")
- [Bash Compare Strings](https://www.golinuxcloud.com/bash-compare-strings/ "Bash Compare Strings")
- [Bash Concatenate Strings](https://www.golinuxcloud.com/bash-concatenate-strings/ "Bash Concatenate Strings")
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- [Run While Loop Until Specific Time](https://www.golinuxcloud.com/run-while-loop-until-specific-time-shell-bash/ "Run While Loop Until Specific Time")
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- [Script Arguments Guide](https://www.golinuxcloud.com/beginners-guide-to-use-script-arguments-in-bash-with-examples/ "Script Arguments Guide")
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- [Check if File Exists](https://www.golinuxcloud.com/bash-check-if-file-exists/ "Check if File Exists")
- [Check if Script is Running](https://www.golinuxcloud.com/check-if-script-is-already-running-shell-linux/ "Check if Script is Running")
- [Get Script Name and Path](https://www.golinuxcloud.com/get-script-name-get-script-path-shell-script/ "Get Script Name and Path")
- [Get Script Execution Time](https://www.golinuxcloud.com/get-script-execution-time-command-bash-script/ "Get Script Execution Time")
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# Bash For Loop usage guide for absolute beginners
By [Deepak Prasad](https://www.golinuxcloud.com/author/admin/)
Updated: Aug 4, 2020
In [programming](https://www.golinuxcloud.com/category/programming/) \| Tags: [\#bash](https://www.golinuxcloud.com/tag/bash/), [\#loop](https://www.golinuxcloud.com/tag/loop/), [\#shell script](https://www.golinuxcloud.com/tag/shell-script/)
## On this page
- [Basic for loop syntax in Bash](https://www.golinuxcloud.com/bash-for-loop/#basic-for-loop-syntax-in-bash)
- [Understanding the syntax](https://www.golinuxcloud.com/bash-for-loop/#understanding-the-syntax)
- [EX\_1: Loop over a range of numbers](https://www.golinuxcloud.com/bash-for-loop/#ex_1-loop-over-a-range-of-numbers)
- [EX\_2: Loop over a series of strings](https://www.golinuxcloud.com/bash-for-loop/#ex_2-loop-over-a-series-of-strings)
- [EX\_3: Use for loop with an array](https://www.golinuxcloud.com/bash-for-loop/#ex_3-use-for-loop-with-an-array)
- [Array vs Variable](https://www.golinuxcloud.com/bash-for-loop/#array-vs-variable)
- [EX\_4: Using for loops in bash as C programmers](https://www.golinuxcloud.com/bash-for-loop/#ex_4-using-for-loops-in-bash-as-c-programmers)
- [Define multiple variables](https://www.golinuxcloud.com/bash-for-loop/#define-multiple-variables)
- [EX\_5: Use continue statement](https://www.golinuxcloud.com/bash-for-loop/#ex_5-use-continue-statement)
- [EX\_6: Use break statement](https://www.golinuxcloud.com/bash-for-loop/#ex_6-use-break-statement)
- [EX\_7: One liner for loop examples](https://www.golinuxcloud.com/bash-for-loop/#ex_7-one-liner-for-loop-examples)
- [Conclusion](https://www.golinuxcloud.com/bash-for-loop/#conclusion)
I get this question a lot from many users, how can I run a certain command in a loop for a defined period of time? Or how can we iterate a task over a range only for n number of times. All these can be achieved using bash for loop in Linux and Unix
From [What is Loop](https://whatis.techtarget.com/definition/loop), **a loop is a sequence of instructions that is continually repeated until a certain condition is reached**. So we know that a loop is a situation where we can perform a certain task repeatedly for a certain pre-defined period of time or may be some times infinite.
Using such loops in our day to day task are very useful to automate stuffs using scripts. The loop can be configured using for, while, until etc depending upon individualâs requirement. In this tutorial we will understand in detail about bash for loop, and itâs usage across Linux environment for different types of automation shell scripts.
I will also share different shell script examples using for loop to help you understand itâs usage.
***
## Basic for loop syntax in Bash
The syntax of for loop would vary based on the programming language you choose such as C, perl, python, go etc. The provided **syntax** can be used only with bash and shell scripts
```
for {ELEMENT} in ${ARRAY[@]}
do
{COMMAND}
done
```
### Understanding the syntax
- Here the `ARRAY` can contain any type of element such as strings, integers.
- Although it is important that **you pass an `ARRAY`** to the loop, as if you pass a `VARIABLE` then the loop with count all the `ELEMENTS` from the `VARIABLE` as single `ELEMENT`. *I will show you what this means later in this article*
- Loop will iterate each `ELEMENT` from the `ARRAY` and a certain command can be assigned to these `ELEMENT`
- The loop will run until the last `ELEMENT` from `ARRAY`
***
## EX\_1: Loop over a range of numbers
In our first shell script we will iterate over a range of numbers and print the number
```
for num in 1 2 3 4 5 6
do
echo "Found Element: $num"
done
```
Here we have passed a series of numbers separated by white space
The output from this script
```
Found Element: 1
Found Element: 2
Found Element: 3
Found Element: 4
Found Element: 5
Found Element: 6
```
***
## EX\_2: Loop over a series of strings
In this shell script we will iterate over a series of strings
```
for string in STRING1 STRING2 STRING3
do
echo "Found Element: $string"
done
```
The output from this script:
```
Found Element: STRING1
Found Element: STRING2
Found Element: STRING3
```
Let us also take a
[practical example for Linux Administrator](https://www.golinuxcloud.com/linux-commands-cheat-sheet/). In this script we will write a small script which will search for `file1`, `file2` and `file3` under `/tmp`
Now if you are not using for loop then you have to manually search for these files. **You may think, so what it is easier to find files manually?**
Yes, **TRUE** but what if you have 100 files or 1000 files? Would you find them manually?
```
for file in file1 file2 file3
do
echo "Searching for file: $file"
file_path=`find /tmp -name $file`
if [ ! -z $file_path ];then
echo "Found: $file at $file_path"
else
echo "Not Found: $file"
fi
done
```
The output from the script will tell us the files which were found and the files which were not found.
*Bash for loop in a range*
***
## EX\_3: Use for loop with an array
Before we go ahead it is important that you understand the different between ARRAY and Variable
### Array vs Variable
- It is **important** that when you use loop, you use an `ARRAY` **which contains elements separated by white character**
- A variable will always contain a single element while an array can contain multiple number of elements
- An array can also store multiple variables
- You can [convert your VARIABLE into an ARRAY using different methods](https://www.golinuxcloud.com/bash-split-string-into-array-linux/ "Bash split string into array using 4 simple methods").
*For example:*
```
VAR="My name is Deepak"
```
Here we have given white space separated values, so does this become an `ARRAY`? **NO, this is a VARIABLE**
Check the length of this `VARIABLE`
```
echo "Length: ${#VAR[@]}"
Length: 1
```
Bash could only count single element here, so if we try to iterate using this `VAR` then we get only `1` iteration
You should know **how to declare an array in shell script**:
Now I will declare the same content as an array
```
VAR=(My name is Deepak)
```
Here I have added the elements inside *parenthesis*, now let us check the length of this variable
```
# echo "Length: ${#VAR[@]}"
Length: 4
```
So now bash considers this as an `ARRAY` with **4** `ELEMENTS`
```
1st - My
2nd - name
3rd - is
4th - Deepak
```
We can **access these individual elements** using `${VAR[$num]}`
Here `$num` should be replaced by the element number you want to access, *for example*:
```
#!/bin/bash
VAR=(My name is Deepak)
echo "First Element: ${VAR[0]}"
echo "First Element: ${VAR[1]}"
echo "First Element: ${VAR[2]}"
echo "First Element: ${VAR[3]}"
```
The output of this script:
*Bash for loop array*
Let us take a **practical example** to understand loop with array in real time environment.
Assume you have to *create 5 users* and *assign a password to these users*. In this shell script I will assign â`Passw0rd`â as password to all the users
```
#!/bin/bash
USERS=(user1 user2 user3 user4 user5)
for user in ${USERS[@]}
do
echo "Creating user $user"
useradd $user
echo "Assigning password for $user"
echo "Passw0rd" | passwd "$user" --stdin
done
```
***
## EX\_4: Using for loops in bash as C programmers
- Bash extends the `for` keyword to provide functionality similar to the three-argument **for loop used in C using double parenthesis**
- We can use the above examples when we have a small number of range to iterate over but what if we have a *larger range of items*?
- Or if we do not know the number of times the loop has to run and instead we get this value internally from the script?
**Syntax:**
```
for ((ASSIGN_VALUE; ASSIGN_LIMIT ; STEP_UP))
do
[COMMANDS]
done
```
- The **syntax** would be more clear in this sample script.
- The **first expression** is run before the loop starts: We assign the `i` value to zero to start it off.
- The **second expression** is the test used to determine whether the loop should continue or stop: We test whether the value of `i` is less than 5.
- The **third expression** runs after each instance of the loop: We add one to the value of `i`.
```
#!/bin/bash
for (( i=0; i<=5; i++ ))
do
echo "Iteration $1"
done
```
The output from this script:
```
Iteration 0
Iteration 1
Iteration 2
Iteration 3
Iteration 4
Iteration 5
```
Now you must be wondering **do we have any use case for such of âfor loopâ in real time scenario**.
The answer would be **YES**, very much. I will share some real time scenarios to help you understand better:
- In this example I will search for all files with the syntax matching `file*` under `/tmp`
- Then append some content to these files
- Now the loop should iterate based on the number of files found
- Since the iteration starts from **0** we have given variable `i` to be one less than the number of files which is found
```
#!/bin/bash
file_path=(`find /tmp -name "file*" -type f`)
echo "Found ${#file_path[@]} files"
for (( i=1; i<${#file_path[@]}; i++ ))
do
echo "Adding content to ${file_path[$i]}"
echo "add some content" >> "${file_path[$i]}"
done
```
The output from this script:
*bash for loop example*
### Define multiple variables
We can also use **multiple variables** in single for loop. In this example I have defined *two variables* where the loop will iterate until `i` is less than equal to `j` variable
```
#!/bin/bash
for (( i=1,j=5; i<=j; i++,j-- ))
do
echo "Iteration for i=$i"
echo "Iteration for j=$j"
done
```
The output from this script
```
Iteration for i=1
Iteration for j=5
Iteration for i=2
Iteration for j=4
Iteration for i=3
Iteration for j=3
```
In this script I will perform some more operations using **two variables** in single for loop with bash
```
for (( i=0, j=0 ; i+j < 5 ; i++, j++ ))
do
echo "Value for i=$i"
echo "Value for j=$j"
echo "Multiply: $((i*j))"
echo "add: $((i+j))"
done
```
The output from this script
```
Value for i=0
Value for j=0
Multiply: 0
add: 0
Value for i=1
Value for j=1
Multiply: 1
add: 2
Value for i=2
Value for j=2
Multiply: 4
add: 4
```
***
## EX\_5: Use continue statement
Now it is not mandatory that you would want to perform task for complete iteration, it is possible you may have a specific requirement to **ignore certain value** during the iteration run
I have this sample script where the loop iterates for 5 times
```
for (( i=0 ; i<=5 ; i++ ))
do
if [ $i -eq 2 ];then
echo "Don't do anything when i=2"
fi
echo "Doing something when i=$i"
done
```
Now I have an additional check when value of the variable `i` is equal to `2`, so when this matches the script should do nothing and continue with the remaining iteration
But let us check the output from this script
```
Doing something when i=0
Doing something when i=1
Don't do anything when i=2
Doing something when i=2
Doing something when i=3
Doing something when i=4
Doing something when i=5
```
Even when I added a check to ignore `i=2` match, still we see â`Doing something when i=2`â, so **how to fix this?** We need to use `continue` statement for such case
**The `continue` statement will exit the current iteration of the loop and will continue with the next iteration in the loop**
```
for (( i=0 ; i<=5 ; i++ ))
do
if [ $i -eq 2 ];then
echo "Don't do anything when i=2"
continue
fi
echo "Doing something when i=$i"
done
```
The output from this script
```
Doing something when i=0
Doing something when i=1
Don't do anything when i=2
Doing something when i=3
Doing something when i=4
Doing something when i=5
```
Now we see that when variable of `i` is equal to `2`, the iteration is skipped and the loop continues with remaining iteration
***
## EX\_6: Use break statement
With `continue` we were able to skip the loop for single iteration but what if we want to **completely come out of the loop with a certain condition?**
In this script we want to end the loop if the value of `i` variable is equal to `2` so with the if condition I have added a `break` statement
```
for (( i=0 ; i<=5 ; i++ ))
do
if [ $i -eq 2 ];then
echo "Exiting, match sucessful"
break
fi
echo "Doing something when i=$i"
done
```
The output from this script
```
Doing something when i=0
Doing something when i=1
Exiting, match successful
```
As expected, our for loop terminates when the if condition is fulfilled using the `break` statement.
***
## EX\_7: One liner for loop examples
We can write for loop in one liner commands to perform simple tasks using below **syntax**:
```
for {ELEMENT} in ${ARRAY[@]}; do [COMMAND_1]; [COMMAND_2]; [COMMAND_3]; done
```
In this shell script I will list all the network devices available on my server. here instead of defining an `ARRAY`, I have provided a `PATH` with `*`, so all the files and directories under that `PATH` would be considered as `ELEMENT` of the `ARRAY`
```
# for int in `ls /proc/sys/net/ipv4/conf/`;do echo "Interface: $int"; done
Interface: all
Interface: bond0
Interface: bond1
Interface: default
Interface: eth0
Interface: eth1
Interface: eth2
Interface: eth3
Interface: lo
```
In this script I will remove all files under `/tmp` matching regex `file*`
```
# for file in /tmp/file*; do rm -vf $file; done
removed '/tmp/file1'
removed '/tmp/file2'
removed '/tmp/file3'
```
***
## Conclusion
Even though bash is loosing itâs popularity with new programming language coming every day but engineers and developers still prefer bash to automate day to day tasks which consists of system commands. In such case it is always handy for engineers to be aware of using such loop based iteration.
I have given some very basic examples for beginners to understand the syntax but this can be used to do complex tasks as well. Lastly I hope this tutorial to learn bash for loop with examples to iterate over a range and series of items on Linux and Unix was helpful. So, let me know your suggestions and feedback using the comment section.
Deepak Prasad
R\&D Engineer
Founder of GoLinuxCloud with over a decade of expertise in Linux, Python, Go, Laravel, DevOps, Kubernetes, Git, Shell scripting, OpenShift, AWS, Networking, and Security. With extensive experience, he excels across development, DevOps, networking, and security, delivering robust and efficient solutions for diverse projects.
[View all posts â](https://www.golinuxcloud.com/author/admin/)
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| Readable Markdown | I get this question a lot from many users, how can I run a certain command in a loop for a defined period of time? Or how can we iterate a task over a range only for n number of times. All these can be achieved using bash for loop in Linux and Unix
From [What is Loop](https://whatis.techtarget.com/definition/loop), **a loop is a sequence of instructions that is continually repeated until a certain condition is reached**. So we know that a loop is a situation where we can perform a certain task repeatedly for a certain pre-defined period of time or may be some times infinite.
Using such loops in our day to day task are very useful to automate stuffs using scripts. The loop can be configured using for, while, until etc depending upon individualâs requirement. In this tutorial we will understand in detail about bash for loop, and itâs usage across Linux environment for different types of automation shell scripts.
I will also share different shell script examples using for loop to help you understand itâs usage.
***
## Basic for loop syntax in Bash
The syntax of for loop would vary based on the programming language you choose such as C, perl, python, go etc. The provided **syntax** can be used only with bash and shell scripts
```
for {ELEMENT} in ${ARRAY[@]}
do
{COMMAND}
done
```
### Understanding the syntax
- Here the `ARRAY` can contain any type of element such as strings, integers.
- Although it is important that **you pass an `ARRAY`** to the loop, as if you pass a `VARIABLE` then the loop with count all the `ELEMENTS` from the `VARIABLE` as single `ELEMENT`. *I will show you what this means later in this article*
- Loop will iterate each `ELEMENT` from the `ARRAY` and a certain command can be assigned to these `ELEMENT`
- The loop will run until the last `ELEMENT` from `ARRAY`
***
## EX\_1: Loop over a range of numbers
In our first shell script we will iterate over a range of numbers and print the number
```
for num in 1 2 3 4 5 6
do
echo "Found Element: $num"
done
```
Here we have passed a series of numbers separated by white space
The output from this script
```
Found Element: 1
Found Element: 2
Found Element: 3
Found Element: 4
Found Element: 5
Found Element: 6
```
***
## EX\_2: Loop over a series of strings
In this shell script we will iterate over a series of strings
```
for string in STRING1 STRING2 STRING3
do
echo "Found Element: $string"
done
```
The output from this script:
```
Found Element: STRING1
Found Element: STRING2
Found Element: STRING3
```
Let us also take a
[practical example for Linux Administrator](https://www.golinuxcloud.com/linux-commands-cheat-sheet/). In this script we will write a small script which will search for `file1`, `file2` and `file3` under `/tmp`
Now if you are not using for loop then you have to manually search for these files. **You may think, so what it is easier to find files manually?**
Yes, **TRUE** but what if you have 100 files or 1000 files? Would you find them manually?
```
for file in file1 file2 file3
do
echo "Searching for file: $file"
file_path=`find /tmp -name $file`
if [ ! -z $file_path ];then
echo "Found: $file at $file_path"
else
echo "Not Found: $file"
fi
done
```
The output from the script will tell us the files which were found and the files which were not found.
*Bash for loop in a range*
***
## EX\_3: Use for loop with an array
Before we go ahead it is important that you understand the different between ARRAY and Variable
### Array vs Variable
- It is **important** that when you use loop, you use an `ARRAY` **which contains elements separated by white character**
- A variable will always contain a single element while an array can contain multiple number of elements
- An array can also store multiple variables
- You can [convert your VARIABLE into an ARRAY using different methods](https://www.golinuxcloud.com/bash-split-string-into-array-linux/ "Bash split string into array using 4 simple methods").
*For example:*
```
VAR="My name is Deepak"
```
Here we have given white space separated values, so does this become an `ARRAY`? **NO, this is a VARIABLE**
Check the length of this `VARIABLE`
```
echo "Length: ${#VAR[@]}"
Length: 1
```
Bash could only count single element here, so if we try to iterate using this `VAR` then we get only `1` iteration
You should know **how to declare an array in shell script**:
Now I will declare the same content as an array
```
VAR=(My name is Deepak)
```
Here I have added the elements inside *parenthesis*, now let us check the length of this variable
```
# echo "Length: ${#VAR[@]}"
Length: 4
```
So now bash considers this as an `ARRAY` with **4** `ELEMENTS`
```
1st - My
2nd - name
3rd - is
4th - Deepak
```
We can **access these individual elements** using `${VAR[$num]}`
Here `$num` should be replaced by the element number you want to access, *for example*:
```
#!/bin/bash
VAR=(My name is Deepak)
echo "First Element: ${VAR[0]}"
echo "First Element: ${VAR[1]}"
echo "First Element: ${VAR[2]}"
echo "First Element: ${VAR[3]}"
```
The output of this script:
*Bash for loop array*
Let us take a **practical example** to understand loop with array in real time environment.
Assume you have to *create 5 users* and *assign a password to these users*. In this shell script I will assign â`Passw0rd`â as password to all the users
```
#!/bin/bash
USERS=(user1 user2 user3 user4 user5)
for user in ${USERS[@]}
do
echo "Creating user $user"
useradd $user
echo "Assigning password for $user"
echo "Passw0rd" | passwd "$user" --stdin
done
```
***
## EX\_4: Using for loops in bash as C programmers
- Bash extends the `for` keyword to provide functionality similar to the three-argument **for loop used in C using double parenthesis**
- We can use the above examples when we have a small number of range to iterate over but what if we have a *larger range of items*?
- Or if we do not know the number of times the loop has to run and instead we get this value internally from the script?
**Syntax:**
```
for ((ASSIGN_VALUE; ASSIGN_LIMIT ; STEP_UP))
do
[COMMANDS]
done
```
- The **syntax** would be more clear in this sample script.
- The **first expression** is run before the loop starts: We assign the `i` value to zero to start it off.
- The **second expression** is the test used to determine whether the loop should continue or stop: We test whether the value of `i` is less than 5.
- The **third expression** runs after each instance of the loop: We add one to the value of `i`.
```
#!/bin/bash
for (( i=0; i<=5; i++ ))
do
echo "Iteration $1"
done
```
The output from this script:
```
Iteration 0
Iteration 1
Iteration 2
Iteration 3
Iteration 4
Iteration 5
```
Now you must be wondering **do we have any use case for such of âfor loopâ in real time scenario**.
The answer would be **YES**, very much. I will share some real time scenarios to help you understand better:
- In this example I will search for all files with the syntax matching `file*` under `/tmp`
- Then append some content to these files
- Now the loop should iterate based on the number of files found
- Since the iteration starts from **0** we have given variable `i` to be one less than the number of files which is found
```
#!/bin/bash
file_path=(`find /tmp -name "file*" -type f`)
echo "Found ${#file_path[@]} files"
for (( i=1; i<${#file_path[@]}; i++ ))
do
echo "Adding content to ${file_path[$i]}"
echo "add some content" >> "${file_path[$i]}"
done
```
The output from this script:
*bash for loop example*
### Define multiple variables
We can also use **multiple variables** in single for loop. In this example I have defined *two variables* where the loop will iterate until `i` is less than equal to `j` variable
```
#!/bin/bash
for (( i=1,j=5; i<=j; i++,j-- ))
do
echo "Iteration for i=$i"
echo "Iteration for j=$j"
done
```
The output from this script
```
Iteration for i=1
Iteration for j=5
Iteration for i=2
Iteration for j=4
Iteration for i=3
Iteration for j=3
```
In this script I will perform some more operations using **two variables** in single for loop with bash
```
for (( i=0, j=0 ; i+j < 5 ; i++, j++ ))
do
echo "Value for i=$i"
echo "Value for j=$j"
echo "Multiply: $((i*j))"
echo "add: $((i+j))"
done
```
The output from this script
```
Value for i=0
Value for j=0
Multiply: 0
add: 0
Value for i=1
Value for j=1
Multiply: 1
add: 2
Value for i=2
Value for j=2
Multiply: 4
add: 4
```
***
## EX\_5: Use continue statement
Now it is not mandatory that you would want to perform task for complete iteration, it is possible you may have a specific requirement to **ignore certain value** during the iteration run
I have this sample script where the loop iterates for 5 times
```
for (( i=0 ; i<=5 ; i++ ))
do
if [ $i -eq 2 ];then
echo "Don't do anything when i=2"
fi
echo "Doing something when i=$i"
done
```
Now I have an additional check when value of the variable `i` is equal to `2`, so when this matches the script should do nothing and continue with the remaining iteration
But let us check the output from this script
```
Doing something when i=0
Doing something when i=1
Don't do anything when i=2
Doing something when i=2
Doing something when i=3
Doing something when i=4
Doing something when i=5
```
Even when I added a check to ignore `i=2` match, still we see â`Doing something when i=2`â, so **how to fix this?** We need to use `continue` statement for such case
**The `continue` statement will exit the current iteration of the loop and will continue with the next iteration in the loop**
```
for (( i=0 ; i<=5 ; i++ ))
do
if [ $i -eq 2 ];then
echo "Don't do anything when i=2"
continue
fi
echo "Doing something when i=$i"
done
```
The output from this script
```
Doing something when i=0
Doing something when i=1
Don't do anything when i=2
Doing something when i=3
Doing something when i=4
Doing something when i=5
```
Now we see that when variable of `i` is equal to `2`, the iteration is skipped and the loop continues with remaining iteration
***
## EX\_6: Use break statement
With `continue` we were able to skip the loop for single iteration but what if we want to **completely come out of the loop with a certain condition?**
In this script we want to end the loop if the value of `i` variable is equal to `2` so with the if condition I have added a `break` statement
```
for (( i=0 ; i<=5 ; i++ ))
do
if [ $i -eq 2 ];then
echo "Exiting, match sucessful"
break
fi
echo "Doing something when i=$i"
done
```
The output from this script
```
Doing something when i=0
Doing something when i=1
Exiting, match successful
```
As expected, our for loop terminates when the if condition is fulfilled using the `break` statement.
***
## EX\_7: One liner for loop examples
We can write for loop in one liner commands to perform simple tasks using below **syntax**:
```
for {ELEMENT} in ${ARRAY[@]}; do [COMMAND_1]; [COMMAND_2]; [COMMAND_3]; done
```
In this shell script I will list all the network devices available on my server. here instead of defining an `ARRAY`, I have provided a `PATH` with `*`, so all the files and directories under that `PATH` would be considered as `ELEMENT` of the `ARRAY`
```
# for int in `ls /proc/sys/net/ipv4/conf/`;do echo "Interface: $int"; done
Interface: all
Interface: bond0
Interface: bond1
Interface: default
Interface: eth0
Interface: eth1
Interface: eth2
Interface: eth3
Interface: lo
```
In this script I will remove all files under `/tmp` matching regex `file*`
```
# for file in /tmp/file*; do rm -vf $file; done
removed '/tmp/file1'
removed '/tmp/file2'
removed '/tmp/file3'
```
***
## Conclusion
Even though bash is loosing itâs popularity with new programming language coming every day but engineers and developers still prefer bash to automate day to day tasks which consists of system commands. In such case it is always handy for engineers to be aware of using such loop based iteration.
I have given some very basic examples for beginners to understand the syntax but this can be used to do complex tasks as well. Lastly I hope this tutorial to learn bash for loop with examples to iterate over a range and series of items on Linux and Unix was helpful. So, let me know your suggestions and feedback using the comment section. |
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