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Wednesday, November 17, 2010

bash loops


Simple ways to loop in bash shell scripts, or one-liners.

Note that the variable in bash is declared without a "$", and called with it, but in perl the variable is declared with and called with a "$" at the beginning of the name.

The Bash "for" Loop

Loop a number of times


for i in {1..10}; do echo -n "$i ";done;echo

Output:

1 2 3 4 5 6 7 8 9 10


For short lists, the items can be entered manually.

for num in 1 2 3;do echo -n $num;done;echo

Output:

1 2 3


Loop Through Items on a List

for host in `cat all.txt `;do ping -c 1 $host; done
See also: How to Run a Bash Command on All Items in a List
and: Ping Multiple Hosts Using Bash Nmap and Fping


Loop using Perl

This syntax should be familiar to C programmers. In Perl. the scalar variable does not have to be declared as it would in C (int $counter). Note that the variable name starts with "$" to indicate a scalar.


#!/usr/bin/perl

for($counter = 1; $counter <= 10; $counter++){
print "for loop #$counter\n";
}

Wait a minute. Isn't this article about bash and one-liners?

O.K., you can run perl for loops from a one-liner at a bash prompt, or within a shell script.

perl -e for($counter = 1; $counter <= 10; $counter++){
print "for loop #$counter\n";
}


For more detailed scripting ideas, check out the Linux Documentation Project's "Advanced Bash Scripting Guide" -- http://www.tldp.org/LDP/abs/html

1 comment:

Custom LAN/WAN Applications said...

Interesting post..thank you for posting it here.