Keeping You in the Loop – Bash For, While, Until Loop Examples

Looping statements are used to force a program to repeatedly execute a statement. The executed statement is called the loop body.

Loops execute until the value of a controlling expression is 0. The controlling expression may be any scalar data type.

The shell language also provide several iteration or looping statements. In this article let us review the looping statements which bash provides using some examples.

Bash supports following three types of looping statement

1. For loop
2. While loop
3. Until loop

This article is part of the on-going Bash Tutorial series.

Loops can be nested. Like any other programming language, bash also supports break statement to exit the current loop, and continue statement to resume the next iteration of the loop statement.

Bash For Loop – First Method

For loops are typically used when the number of iterations is known before entering the bash loop. Bash supports two kinds of for loop. The first form of bash for loop is:

for varname in list
	do
		commands ##Body of the loop
	done

In the above syntax:

• for, in, do and done are keywords
• List is any list which has list of items
• varname is any Bash variable name.

In this form, the for statement executes the command enclosed in a body, once for each item in the list. The current item from the list will be stored in a variable “varname” each time through the loop. This varname can be processed in the body of the loop. This list can be a variable that contains several words separated by spaces. If list is missing in the for statement, then it takes the positional parameter that were passed into the shell.

Bash For Loop Example 1. Unzip all the Zip file

The following example finds the list of files which matches with “*.zip*” in the root directory, and creates a new directory in the same location where the zip file exists, and unzip the zip file content.

# cat zip_unzip.sh
#! /bin/bash
# Find files which has .zip
for file in `find /root -name "*.zip*" -type f`
do

# Skip the extension .zip
dirname=`echo ${file} | awk -F'.' '{print $1}'`

# Create the directory
mkdir $dirname

# Copy the zip file
cp ${file} ${dirname}
cd $dirname

# Unzip the zip file from newly created directory
unzip ${dirname}/$(echo ${file##/*/})
done

• In this example find command returns the list of files, from which each file will be processed through a loop.
• For each item, it creates the directory with the name of the zip file, and copies the zip file to the newly created directory and unzip the zip file from there.
• The echo statement, echo ${file##/*/} gives you only the file name not the path.

# ./zip_unzip.sh
Archive:  /root/test2/test2.zip
 extracting: t1/p
 extracting: t1/q
 extracting: t1/r
 extracting: t1/s
 extracting: t1/t
 extracting: t1/u
 extracting: t1/v
Archive:  /root/test1/test1.zip
 extracting: t/a
 extracting: t/b
 extracting: t/c
 extracting: t/d
 extracting: t/e

Similar to the Bash loop, Awk also provides for loop and while loop as we discussed in our Awk While and For Loop article.

Bash For Loop – Second Method

The second form of for loop is similar to the for loop in ‘C’ programming language, which has three expression (initialization, condition and updation).

for (( expr1; expr2; expr3 ))
         do
		commands
         done

• In the above bash for command syntax, before the first iteration, expr1 is evaluated. This is usually used to initialize variables for the loop.
• All the statements between do and done is executed repeatedly until the value of expr2 is TRUE.
• After each iteration of the loop, expr3 is evaluated. This is usually use to increment a loop counter.

The following example generates the n number of random numbers.

Bash For Example 2. Generate n random numbers

$ cat random.sh
#! /bin/bash

echo -e "How many random numbers you want to generate"
read max

for (( start = 1; start <= $max; start++ ))
do
        echo -e $RANDOM
done

$ ./random.sh
How many random numbers you want to generate
5
6119
27200
1998
12097
9181

In the above code snippet, the for loop generates the random number at max number of times. RANDOM is an internal bash function that returns a random integer at each invocation.

Bash While Loop

Another iteration statement offered by the shell programming language is the while statement.

Syntax:
while expression
do
	commands
done

In the above while loop syntax:

• while, do, done are keywords
• Expression is any expression which returns a scalar value
• While statement causes a block of code to be executed while a provided conditional expression is true.

Bash While Example 3. Write contents into a file

The following example reads the data from the stdout and writes into a file.

$ cat writefile.sh
#! /bin/bash

echo -e "Enter absolute path of the file name you want to create"
read file
while read line
do
echo $line >> $file
done

$ sh writefile.sh
Enter absolute path of the file name you want to create
/tmp/a
while
for
until

$ cat /tmp/a
while
for
until

The above example, reads the filename from the user, and reads the lines of data from stdin and appends each line to a given filename. When EOF enters, read will fail, so the loop ends there.

If you are writing lot of bash scripts, you can use Vim editor as a Bash IDE using the Vim bash-support plugin as we discussed earlier.

Bash While Example 4. Read the contents of a file

In the previous example, it reads the data from stdout and write it into a file. In this example, it reads the file
content and write it into a stdout.

$ cat read.sh
#! /bin/bash
echo -e "Enter absolute path of the file name you want to read"
read file
exec <$file # redirects stdin to a file
while read line
do
echo $line
done

$ ./read.sh

Enter absolute path of the file name you want to read
/tmp/a
while
for
until

In this example, it gets the filename to read, and using exec it redirects stdin to a file. From that point on, all stdin comes from that file, rather than from keyboard. read command reads the line from stdin, so while loop reads the stdin, till EOF occurs.

Bash Until Loop

The until statement is very similar in syntax and function to the while statement. The only real difference between the two is that the until statement executes its code block while its conditional expression is false, and the while statement executes its code block while its conditional expression is true.

syntax:

until expression
	do
	   commands #body of the loop
	done

In the above bash until syntax:
where until, do, done are keywords
expression any conditional expression

Bash Until Example 5. Monitor the logfile

This example monitors the size of the logfile, once the logfile size reaches 2000bytes, it takes the copy of that logfile.

$ cat monitor.sh

file=/tmp/logfile
until [ $(ls -l $file | awk '{print $5}') -gt 2000 ]
        do
            echo "Sleeping for next 5 seconds"
            sleep 5
        done
date=`date +%s`
cp $file "$file-"$date.bak

$ ./monitor.sh
Sleeping for next 5 seconds
Sleeping for next 5 seconds

$ ls -l /tmp/logfile*
-rw-r--r-- 1 sss sss      2010 Jun 24 12:29 logfile
-rw-r--r-- 1 sss sss      2005 Jun 24 16:09 logfile-1277474574.bak

The until statement continues to execute the body of the loop, till the condition becomes true. In this example condition is size of the file greater than 2000 bytes, so it copies the file once it reaches the 2000 bytes.

Also, make sure to refer to our earlier Bash Array examples.

Bash Until Example 6. Waiting for the Machine to come up

This example is used to wait till the machine comes up before doing a ssh to that machine. The until loop statement ends only when the ping gives the responses.

$ cat mac_wait.sh
#! /bin/bash

read -p "Enter IP Address:" ipadd
echo $ipadd
until ping -c 1 $ipadd
do
        sleep 60;
done
ssh $ipadd

$./mac_wait.sh
Enter IP Address:192.143.2.10
PING 192.143.2.10 (192.143.2.10) 56(84) bytes of data.

--- 192.143.2.10 ping statistics ---
1 packets transmitted, 0 received, 100% packet loss, time 0ms

PING 192.143.2.10 (192.143.2.10) 56(84) bytes of data.
64 bytes from 192.143.2.10: icmp_seq=1 ttl=64 time=0.059 ms

--- 192.143.2.10 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.059/0.059/0.059/0.000 ms
The authenticity of host '192.143.2.10 (192.143.2.10)' can't be established.
Are you sure you want to continue connecting (yes/no)? yes

Until loop is quite useful at the command line, as a way of waiting for certain events to occur.