simple for loop

hatfani · 03-02-2020, 11:00 AM

HEllo guys,

I'm trying to take my output into a txt file but something is wrong.

Code:

H=$(head -n1 SAMPLE.txt) ## = 19990

for ((i = $H  ; i <= 19999 ; i++))
  do
  echo $i > FILL2.txt
 done

for some reason it gives me only 19999 in FILL2.txt
while the output of echo $i is:
19990
19991
19992
19993
19994
and so on until 19999

what is wrong? I don't understand.

teckk · 03-02-2020, 11:08 AM

head -n1 SAMPLE.txt will give you the first line of SAMPLE.txt

You want to append not overwrite.

Code:

echo $i >> FILL2.txt

hatfani · 03-02-2020, 11:16 AM

Quote:

Originally Posted by teckk

head -n1 SAMPLE.txt will give you the first line of SAMPLE.txt

You want to append not overwrite.

Code:

echo $i >> FILL2.txt

I already tried it, when I do it

Code:

echo $i >> FILL2.txt

it gives me the next output:
19999
19990
19991
19992
19993
19994
19995
19996
19997
19998
19999

hence - 19999 is two times in FILL2.txt
I want to overwrite, not to append.

teckk · 03-02-2020, 11:29 AM

If you want the file to be empty at the start of each loop then clear the file.

Code:

for i in {1..10}; do
    echo "$i" >> file.txt
done

cat file.txt
1
2
3
4
5
6
7
8
9
10

> file.txt
or
truncate -s 0 file.txt

for i in {a..j}; do
    echo "$i" >> file.txt
done

cat file.txt
a
b
c
d
e
f
g
h
i
j

hatfani · 03-02-2020, 11:35 AM

Quote:

Originally Posted by teckk

If you want the file to be empty at the start of each loop then clear the file.

Code:

for i in {1..10}; do
    echo "$i" >> file.txt
done

cat file.txt
1
2
3
4
5
6
7
8
9
10

> file.txt
or
truncate -s 0 file.txt

for i in {a..j}; do
    echo "$i" >> file.txt
done

cat file.txt
a
b
c
d
e
f
g
h
i
j

Those things I already knew before, I appreciate your attention but it's not exactly what I wanted to do.
I wanted to overwrite it, appending interrupting me when I run the script once again - cause it's appending more unnecessary data again.

scasey · 03-02-2020, 12:28 PM

Quote:

Originally Posted by hatfani

Those things I already knew before, I appreciate your attention but it's not exactly what I wanted to do.
I wanted to overwrite it, appending interrupting me when I run the script once again - cause it's appending more unnecessary data again.

It's not at all clear what you're trying to accomplish, to me.

Your original post says read one line from a file that has the value of 19990, then loop from 19990 to 19999 and save the loop value into a text file, overwriting each time. That wasn't doing what you wanted, as it always showed 19999.
It was however, doing what you told it to do, which is the nature of programming...the darn machines will always only do what you tell them to do.

You were told how to avoid overwriting by appending, but that wasn't what you wanted either.

Please try to explain exactly what you're trying to accomplish, because I, for one, don't understand.

MadeInGermany · 03-02-2020, 01:13 PM

Redirect the whole loop:

Code:

for ((i = $H  ; i <= 19999 ; i++))
do
  echo $i
done > FILL2.txt

The shell opens the file when the loop is read into memory.
Then the loop is processed and standard output goes to the file.
The file is closed when the loop ends.

thw1959 · 03-02-2020, 06:47 PM

One suggestion:
Or use -v or -x to debug.
Script = t1.sh
run: bash -x t1.sh

OR:

Add #!/bin/bash -x as first line to script
make executable: chmod u+x t1.sh
Run: ./t1.sh

Beware that depending on sh or bash, variables in for loops have different requirements.
I always use bash..