Bash – How to create a for loop with a changeable number of iterations

bashforshell-script

How do you iterate through a loop n amount of times when n is specified by the user at the beginning?

I have written a shell script and need to repeat a certain part of it n numbers of times (depending upon how many times the user wishes).

My script so far looks like this:

echo "how many times would you like to print Hello World?"
read num
for i in {1.."$num"}
do
echo "Hello World"
done

If I change "num" to a number such as "5" the loop works however I need to be able to let the user specify the amount of times to iterate through the loop.

Best Answer

You can use seq

for i in $(seq 1 "$num")

or your shell may support C-style loops e.g. in bash

for ((i=0; i<$num; i++))

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Bash – How to create a bash script that sums any number of command line arguments

No need for bash, plain sh will do as well:

#! /bin/sh - 
IFS=+; echo "$(($*))"

$* in POSIX shells, expands to the list of positional parameters (in this case, the arguments to the script) separated by the first character of $IFS (or space if $IFS is unset or nothing if $IFS is empty). $((...)) is the shell internal arithmetic expansion operator (note that it supports decimal, octal and hexadecimal numbers)

If you need floating point support, that's where you'll need a different shell like ksh93 or zsh (not bash as bash only supports integer arithmetic), though you could also use awk:

#! /usr/bin/awk -f
BEGIN {t=0; for (i in ARGV) t+=ARGV[i]; print t}

That will use long (for integer) and double (for floating point) type numbers as implemented by your system. The input numbers must be decimal floating point or engineering notation in the English style (floating point delimiter is the period character regardless of the locale). With some awk implementations, it will fail if the first number is negative as awk would try to interpret it as an option.

Some awk implementations like GNU awk when POSIXLY_CORRECT is in the environment also support hexadecimals including with binary exponent notations. Or with --non-decimal-data, it understands octals and hexadecimals:

$ POSIXLY_CORRECT=1 ./sum  0xap3 0xa
90 # (0xa * 2^3) + 0xa
$ awk --non-decimal-data -f ./sum  010
8

Best Answer

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Bash – How to create a bash script that sums any number of command line arguments

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