Shell Scripting: Generate or Print Range of Numbers ( Sequence of Numbers for Loop )

One our reader asks:

How do I generate a range of numbers under BASH for loop command? For example, I need to run particular command inside loop 100 or 500 times. Basically I want function that counts FROM and TO a range of numbers like 50-10.

To print a sequence of numbers use GNU seq command. In olden days we use our own shell script. But no more dirty shell script, just use good seq command.

This is quite handy when you want to writing shell scripts that requires loop-using range of numbers.

seq command syntax

seq LAST
seq FIRST LAST
seq FIRST INCREMENT LAST

If FIRST or INCREMENT is omitted, it defaults to 1. That is, an omitted INCREMENT defaults to 1 even when LAST is smaller than FIRST. FIRST, INCREMENT, and LAST are interpreted as floating point values. INCREMENT is usually positive if FIRST is smaller than LAST, and INCREMENT is usually negative if FIRST is greater than LAST.

Try out following examples:
$ seq 5
Output:

1
2
3
4
5

Few more examples:$ seq 5 10
$ seq 0 2 10
$ seq 5 -1 1


seq command and shell script (for loop)

You can use seq command in bash for loop as follows:

#!/bin/bash
for i in $(seq 5)
do
  echo "Welcome $i times"
done
 

Bash for loop to generate a sequence of numbers

You can also use bash for loop as follows:

#!/bin/bash
for ((a=1; a <= 5 ; a++))
do
   echo "Welcome $i times"
done

Real World Experience

#!/bin/bash
# Script to move topics from old shared hosting to new lighttpd vps server
#
U="http://forum.cyberciti.biz/viewtopic.php?t="
rdu=""
for n in $(seq 1 521)
do
  rdu="$(lynx --dump $U$n | egrep "vt([0-9]+)\.html$" | awk '{ print $2}' | uniq)"
  if [ "$rdu" == "" ]; then
    :
  else
    echo "$n => \"$rdu\","
  fi
  rdu=""
done

Above script generates a new php code (array) to match with variable t so that we can send 301 redirect. So anyone visiting old forum url http://www.cyberciti.biz/nixcraft/forum/viewtopic.php?t=513 will be redirected to new url :D
Next, I just need to copy and paste the array generated by shell script into viewtopic.php and all users are redirected to new url with 301 http code.

<?php
$URLS = array (
....
..
3 => "http://forum.cyberciti.biz/mysql-cluster-vt3.html",
4 => "http://forum.cyberciti.biz/nice-look-and-feel-vt4.html",
5 => "http://forum.cyberciti.biz/squid-monitoring-using-mrtg-vt5.html"
....
);
$top=$_GET['t'];
$u=$URLS[$top];
if ( isset($u)) {
  @include_once("../../include.genlib.php");
  movePage(301, "$u");
}
else
{
  @include_once("../../include.genlib.php");
  movePage(301, "http://forum.cyberciti.biz");
}
?>

Set a step value and generate a sequence number

In case, you are interested in my old (perhaps dirty) shell script ;)

#!/bin/bash
# Usage:
# ./script 1 5 +
# ./script 100 50 -
# where    START END +/-
FROM=$1
TO=$2
ACTION="$3"
if [ "$ACTION" == "+" ];
then
while [ $FROM -le $TO ]
do
  echo -n $FROM " "
  (( FROM++ ))
done
echo ""
else
while [ $FROM -ge $TO ]
do
  echo -n $FROM " "
  (( FROM-- ))
done
echo " "
fi

Run script as follows to set a step value and generate a sequence:
$ chmod +x script
$ ./script 5 10 +
$ ./script 20 10 -