Linux for-loop
We can print from 1 to 10 using the following script:
#bin/bash # loop.sh for n in 1 2 3 4 5 6 7 8 9 10 do echo "$n" done
The output looks like this:
$ ./loop.sh 1 2 3 4 5 6 7 8 9 10
We can switch the for with more compact form:
for n in {1..10}
We can setup step value using {start..end..inc}:
#bin/bash
# step.sh
for n in {1..10..2}
do
echo "$n"
done
Output:
$ ./step.sh 1 3 5 7 9
But actually, the sep size is not supported for the Bash version < 4. So, the code can be modified like this:
if [ ${BASH_VERSINFO} -ge 4 ]
then
for n in {1..10..2}
do
echo "$n"
done
else
echo "step : not supported in bash ${BASH_VERSION}"
fi
Or:
if [ ${version:=${BASH_VERSINFO}} -ge 4 ]
then
for n in {1..10..2}
do
echo "$n"
done
else
echo "step : not supported in bash $version"
fi
To make it work regardless of the Bash version:
if [ ${BASH_VERSINFO} -ge 4 ]
then
for n in {1..10..2}
do
echo "$n"
done
else
echo "step : not supported in bash ${BASH_VERSION}"
echo "using seq"
for n in $(seq 1 2 10)
do
echo "$n"
done
fi
Or:
echo "myArray"
if [ ${BASH_VERSINFO} -ge 4 ]
then
myArray=${1..10..2}
else
myArray=$(seq 1 2 10)
fi
for n in $myArray
do
echo $n
done
This type of for loop share a common syntax with the C programming language. It is characterized by a three-parameter loop control expression; consisting of an initializer (EXP1), a loop-test or condition (EXP2), and a counting expression (EXP3):
for (( EXP1; EXP2; EXP3 ))
Here is our script:
#bin/bash # C_style.sh for ((n=1; n<=10; n++)) do echo "$n" done
With the same output:
$ ./C_style.sh 1 2 3 4 5 6 7 8 9 10
If we do not want to put no new-line, we run this:
#bin/bash # no-newline.sh for ((a=1; a<=10; a++)) do echo -n "$a " done echo
Output should look like this:
$ ./no-newline.sh 1 2 3 4 5 6 7 8 9 10
Or we can get the same output by doing the last line differently (instead of putting echo at the end of the loop) :
n=10
for ((a=1; a<n; a++))
do
if [ $a -eq $((n-1)) ]
then
echo $a
else
echo -n "$a "
fi
done
This time, let's add from 1 to 10:
#bin/bash # count10.sh sum=0 for ((i=1; i<=10; i++)) do sum=$((sum+i)) done echo "sum=$sum"
The output:
$ ./count10.sh sum=55
We can do infinite loop:
#bin/bash # inf.sh count=0 for ((;;)) do count=$((count+1)) echo -n "$count " done echo
Output:
$ ./inf.sh 1 2 3 4 5 6 7 8 9 10 11 12 13 ...
We can use the for-loop to check the file in a folder:
#!/bin/bash
# pass.sh
lines=1
for file in /etc/*
do
if [ "${file}" == "/etc/passwd" ]
then
echo "passwd is the ${lines}-th file listed in /etc"
break
fi
lines=$((lines+1))
done
Output:
$ ./pass.sh passwd is the 172-th file listed in /etc
Problem : Given N integers, compute their average correct to three decimal places.
Input Format :
The first line contains an integer, N.
N lines follow, each containing a single integer.
Output Format :
Display the average of the N integers, rounded off to three decimal places.
Input Constraints :
1<=N<=500
#! /bin/bash
if [ $# -lt 2 ]; then
echo "Need 2 args at least"
exit 1
fi
if [ $1 -lt 1 -o $1 -gt 500 ]; then
echo " 1 <= N <= 500"
exit 1
fi
index=0
sum=0
for i in $@
do
if [ $index -ne 0 ]; then
array[$index]=$i
sum=$((sum+i))
fi
index=$((index+1))
done
echo "input=${array[@]}"
echo "sum=$sum"
echo "average=$((sum/$1))"
Output looks like this:
$ ./avg.sh 6 10 20 30 40 50 60 input=10 20 30 40 50 60 sum=210 average=35
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