To Continuously loop using for in shell scripting - shell

Question

for m in $count
do
`cat $op ${arr[$m]} > $op1`
`rm -f $op`
`touch $op`
`cat $op1 ${arr[$m+1]} > $op`
if [ $m ge $count ]; then
`rm -f $op1`
`touch $op1`
fi
m=$((m+1))
done
I wanted to continuously loop from the start count 2 till the end count 10 . The $count=10 here. But the above piece of code executes the for loop only once.

Answer 1

Rainy sunday - having much free time - long answer ;)
Many issues with your script, some recommended solutions. Because you used the construction m=$((m+1)) - will be using bash as "shell". (Consider adding the bash tag)
For the cycle - several possibilities
count=10
m=2 #start with 2
while (( $m <= $count )) #while m is less or equal to 10
do #do
echo $m #this action
let m++ #increment m (add one to m)
done #end of while
or, if the count is a constant (not a variable), you can write
for m in {2..10} #REMEMBER, will not works with a variables, like {2..$count}
do
echo "$m"
done
another variant - using the seq (man seq) command for counting
for m in $(seq 2 ${count:=10}) # ${count:=10} - defaults the $count to 10 if it is undefined
do
echo $m
done
or C-like for loop
let count=10
for ((m=2; m<=count; m++))
do
echo $m
done
All 4 loops produces:
2
3
4
5
6
7
8
9
10
so, having a right cycle now. Now add your specific actions.
The:
rm -f $op
touch $op
can be replaced by one command
echo -n > $op #echo nothing and write the "nothing" into the file
it is faster, because the echo is an bash builtin (doesn't start two external commands)
So your actions could looks like
cat $op ${arr[$m]} > $op1
echo -n > $op
cat $op1 ${arr[$m+1]} > $op
in this case, the echo is useless, because the second cat will write its output
to the $op anyway (and before write shortens the file to zero size), so this result is
identical with the above
cat $op ${arr[$m]} > $op1
cat $op1 ${arr[$m+1]} > $op
Those two cat commands can be shorted to one, using bash's >> append to file redirection
cat ${arr[$m]} ${arr[m+1]} >> $op
The whole script could look like the next
#making a testing environment
for f in $(seq 12) #create 12 files opdata-N
do
arr[$f]="opdata-$f" #store the filenames in the array "arr"
echo "data-$f" > ${arr[$f]} #each file contains one line "data-N"
done
#echo ${arr[#]}
#setting the $op and $op1 filenames
#consider choosing more descriptive variable names
op="file_op"
#op1="file_op1" #not needed
#add some initial (old) value to $op
echo "initial value" > $op
#end of creating the testing environment
#the script
count=10
for m in $(seq 2 $count)
do
cat ${arr[$m]} ${arr[m+1]} >> $op
done
at the end, file $op will contain:
initial value
data-2
data-3
data-3
data-4
data-4
data-5
data-5
data-6
data-6
data-7
data-7
data-8
data-8
data-9
data-9
data-10
data-10
data-11
BTW, are you sure about the result? Because if only want add file-2 .. file-10 to the end of $op (without duplicating entries), you can simple write:
cat file-{2..10} >> $op #the '>>' adds to the end of file...
or by using your array:
startpos=2
count=10
cat ${arr[#]:$startpos:$count} >> $op
Ufff.. ;)
Ps: usually it is good practice to enclose variables in double quotes like "$filename" - in the above examples for better readability I omitted them.

Answer 2

Any loop needs a "condition to keep looping". When you use a
for m in count
type of loop, the condition is "if there are more elements in the collection count, pick the next one and keep going". This doesn't seem to be what you want. You are looking for the bash equivalent of
for(m = 0; m < 10; m++)
I think. The best way to do this is - with exactly that kind of loop (but note - an extra pair of parentheses, and a semicolon):
#!/bin/bash
# Display message 5 times
for ((i = 0 ; i < 5 ; i++)); do
echo "Welcome $i times."
done
see nix craft for original
I think you can extend this to your situation… if I understood your question correctly you need something like this:
for ((m = 2; m <= 10; m++))
do
cat $op ${arr[$m]} > $op1
rm -f $op
touch $op
cat $op1 ${arr[$m+1]} > $op
if [ $m ge $count ]; then
rm -f $op1
touch $op1
fi
done

Answer 3

Use a while loop instead.
The for loop is when you have multiple objects to iterate against. You have only one, i.e. $count.