The bash while loop is a control flow statement that allows code or commands to be executed repeatedly based on a given condition. For example, run echo command 5 times or read text file line by line or evaluate the options passed on the command line for a script.
bash while loop syntax
The syntax is as follows:
while [ condition ] do command1 command2 command3 done
command1 to command3 will be executed repeatedly till condition is true. The argument for a while loop can be any boolean expression. Infinite loops occur when the conditional never evaluates to false. Here is the while loop one-liner syntax:
while [ condition ]; do commands; done while control-command; do COMMANDS; done
For example following while loop will print welcome 5 times on screen:
#!/bin/bash x=1 while [ $x -le 5 ] do echo "Welcome $x times" x=$(( $x + 1 )) done
And here is above code as a bash while one liner:
x=1; while [ $x -le 5 ]; do echo "Welcome $x times" $(( x++ )); done
Here is a sample shell code to calculate factorial using while loop:
#!/bin/bash counter=$1 factorial=1 while [ $counter -gt 0 ] do factorial=$(( $factorial * $counter )) counter=$(( $counter - 1 )) done echo $factorial
To run just type:
$ chmod +x script.sh
$ ./script.sh 5
The while loop in action on my Ubuntu Linux desktop
#!/bin/bash FILE=$1 # read $FILE using the file descriptors exec 3<&0 exec 0<$FILE while read line do # use $line variable to process line echo $line done exec 0<&3
You can easily evaluate the options passed on the command line for a script using while loop:
...... .. while getopts ae:f:hd:s:qx: option do case "${option}" in a) ALARM="TRUE";; e) ADMIN=${OPTARG};; d) DOMAIN=${OPTARG};; f) SERVERFILE=$OPTARG;; s) WHOIS_SERVER=$OPTARG;; q) QUIET="TRUE";; x) WARNDAYS=$OPTARG;; \?) usage exit 1;; esac done ....... ..
How do I use while as infinite loops?
Infinite for while can be created with empty expressions, such as:
#!/bin/bash while : do echo "infinite loops [ hit CTRL+C to stop]" done
Conditional while loop exit with break statement
You can do early exit with the break statement inside the whil loop. You can exit from within a WHILE using break. General break statement inside the while loop is as follows:
while [ condition ] do statements1 #Executed as long as condition is true and/or, up to a disaster-condition if any. statements2 if (disaster-condition) then break #Abandon the while lopp. fi statements3 #While good and, no disaster-condition. done
In this example, the break statement will skip the while loop when user enters -1, otherwise it will keep adding two numbers:
#!/bin/bash while : do read -p "Enter two numnbers ( - 1 to quit ) : " a b if [ $a -eq -1 ] then break fi ans=$(( a + b )) echo $ans done
Early continuation with the continue statement
To resume the next iteration of the enclosing WHILE loop use the continue statement as follows:
while [ condition ] do statements1 #Executed as long as condition is true and/or, up to a disaster-condition if any. statements2 if (condition) then continue #Go to next iteration of I in the loop and skip statements3 fi statements3 done
Recommended readings:
- See all sample shell script in our bash shell directory
- Bash loops from our Linux shell scripting tutorial guide
- man bash
- help while
- help {
- help break
- help continue
🐧 Get the latest tutorials on SysAdmin, Linux/Unix, Open Source/DevOps topics:
- RSS feed or Weekly email newsletter
- Share on Twitter • Facebook • 26 comments... add one ↓
| Category | List of Unix and Linux commands |
|---|---|
| File Management | cat |
| Firewall | Alpine Awall • CentOS 8 • OpenSUSE • RHEL 8 • Ubuntu 16.04 • Ubuntu 18.04 • Ubuntu 20.04 |
| Network Utilities | dig • host • ip • nmap |
| OpenVPN | CentOS 7 • CentOS 8 • Debian 10 • Debian 8/9 • Ubuntu 18.04 • Ubuntu 20.04 |
| Package Manager | apk • apt |
| Processes Management | bg • chroot • cron • disown • fg • jobs • killall • kill • pidof • pstree • pwdx • time |
| Searching | grep • whereis • which |
| User Information | groups • id • lastcomm • last • lid/libuser-lid • logname • members • users • whoami • who • w |
| WireGuard VPN | Alpine • CentOS 8 • Debian 10 • Firewall • Ubuntu 20.04 |
Nice to see the Bash shell,
How do you break out of a loop
is there a break statement, or do you have to use a goto?
Yes there is a break and continue statement. man bash has more information.
Very useful examples, thank you!
Thank you so much .
you define very nicly all exampls regarding while loop secquence
statement….
could u please give the shell script for printing the fibonocci series and to know if the number is palindrome and also if it an armstrong number
Does anyone know how to write a script that echoes three different arguments three times using a loop
shift command to traverse through arguments
12969574905f9bfeffe896d_000010
does anyone can give an example of a while loop within a loop?
12969574905f9bfeffe896d_000011
The script “test” should set variable “filter_mode” to FALSE if there are no lines in the file “switches” and to TRUE if there exists at least one line in the file “switches”.
Why do I get “filter_mode : FALSE” eventually?
gg@GeorgSimon:~$ cat testAnd my script:
12969574905f9bfeffe896d_000012
Run as:
gg@GeorgSimon:~$ cat switchesswitch
gg@GeorgSimon:~$ sh testswitch : switch
filter_mode : TRUE
filter_mode : FALSE
how to write a program in bash for displaying table using until statement? table should be like
1 2 3 4
2 4 6 8
3 6 9 12
4 8 12 16
plz reply soon….thanks
12969574905f9bfeffe896d_000013
#!/bin/bash
for (( i=1; i<=4; i++))
do
for (( j=0; j<=3; j++ ))
do
a=$(( $i + $j ))
echo -n " $a "
done
echo
done
#!/bin/bash
for (( i=1; i<=4; i++))
do
flag=0
for (( j=0; j<=3; j++ ))
do
if [ $flag -eq 0 ];then
b=$i
echo -n " $i "
flag=1
else
b=$(( $b + $i ))
echo -n " $b "
fi
done
echo ""
done
#!/bin/bash
#Print multiplication tables from I till X
i=1
x=10
while [ $i -le $x ]
do
t=1
str=””
while [ $t -le 10 ]
do
str=$str” “$(( t*i ))
$((t++))
done
echo $str
$((i++))
done
x=1;while [ $x -le 10 ]; do let y=x*2; let z=x*3; let a=x*4; echo $x $y $z $a ; sleep 1; let x=x+1;done
#!/bin/bash
for (( i=1; i<=4; i++))
do
for (( j=0; j<=3; j++ ))
do
a=$(( $i + $j ))
echo -n " $a "
done
echo
done
Hi, anybody can help me with my code below? I got error:
./while_do: line 4: [1: command not found
thanks..
——————————————————-
#!/bin/bash
x=1
while [$x -le 5];
do
echo “bil $x”
((x++))
done
You made a little mistake with the “while” loop.
You have to put a space between bracket and statement. Also, semicolon isn’t necessary here.
So here is the correct “while” statement : while [ $x -le 5 ]
Thanks Robin. Sorry for my late reply.
I want to print this message block line by line and it will count opening braces n closing braces also…i.e if i use 3 opening braces then it should print 3 closing braces…
djghdkjghjghfghhgklfjhgjkdfhgjkfhg jdhfgjlsdkhgklsdgjklsdjgkldjg djkfhdjklldgklsjh message { abc_djh = 300 path_jksdfh { path_state = normal gamil.com } system data = { 8, 999} squre = { 10, 0x00 00xxxxxx } } } dghfdkwgfjkdgfjkd dgfsdghfjkdhf 3657865894275I need a Script that wait for 1 hr checking for file ,If file is there it displays elase after 1 hr it has to display .. File not found
Thank you! Very easy to understand exemples and effective :)
Thanks again
Thanks man. Very easy to follow and set up. Kudos.
sometimes the condition is inside [ ] sometimes it is not, why?
why the double parenthesis? ans=$(( a + b ))
Thanks
codes like a ab ac ad
all files like oneab1606.txt setab.txt
these type 100 files are there one??1606.txt
and set??.txt
i want ro write zip fill both of them