read write in bash script

yhus · 09-16-2006, 05:04 AM

Hi,

I can use a "read" command to read a file in bash script, is there a "write" command I can write a value to the file? If not, how can I write a numerical value to a file?

Thank you.

Jim

gilead · 09-16-2006, 05:19 AM

Do you mean something like this?

Code:

echo -n "Type in a number: "
read ANSWER
echo $ANSWER >> afile

/bin/bash · 09-17-2006, 04:04 AM

Quote:

is there a "write" command I can write a value to the file? If not, how can I write a numerical value to a file?

No there is a write command to send a message to another user. To write to a file there are many options:

Code:

# Write a number to a file. Will destroy any existing file.
$NUM=1000;echo $NUM >num;cat num                
1000
# Same thing but will NOT destroy existing file instead will append to the end.
$NUM=2000;echo $NUM >>num;cat num               
1000
2000
# Edit an existing file using sed, also you can make a backup using -i.bak.
$sed -i 's/$/.00/g' num;cat num                 
1000.00
2000.00
# Open a file for R/W access. Close file when done.
$exec 3<> num;read -n 1 <&3;echo -n 9 >&3;read -n 7 <&3;echo -n 3 >&3;exec 3>&-;cat num
1900.00
3000.00

jlliagre · 09-17-2006, 09:22 AM

Quote:

Originally Posted by yhus

Hi,

I can use a "read" command to read a file in bash script

Not exactly, read is meant to read one ore more variables from the shell standard input.
The standard input can be a file with redirections or a pipe though.

The "command" to read a file with the shell is

Code:

< somefile

This command is mostly useless, as nothing is done with the file.

Another way to read a file with the shell can be:

Code:

content=$(<somefile)

Then, you'll have everything the file contains in the "content" variable.

Quote:

is there a "write" command I can write a value to the file? If not, how can I write a numerical value to a file?

There are plenty of ways, here is one:

Code:

value=12.3456
printf "%.2f\n"  $value >> /tmp/somefile

Kristijan · 09-17-2006, 10:18 PM

Is there a way I can write to certian sections of the file?

Example, just say I wanted to write a script that would write to a pf.conf file for packet filtering. Now PF reads from top to bottom, so I want to be able to order the rules how I'd like them.

Is there a way to insert rules into certian sections of a file?

The above is just an example...Just say I wanted to write text to a certian line in a file etc...

Is this possible with bash scripting?

-Kristijan

chrism01 · 09-18-2006, 02:40 AM

To be honest, to do that in general you'd have to write a script to read in the file, then re-write it (to a new version when testing).
It is do-able in shell using the above techniques, although I'd prob use Perl for something like that, cetainly if you want to be able to interact with the prog eg try out different rule set orders over and over.
Shell is a bit clunky for that.

/bin/bash · 09-18-2006, 12:23 PM

It's really not that hard to edit a file from shell. That is exactly whet sed was written to do. I'm not sure exactly what you are wanting to do but I think it's something like this:

$ cat file
Rule No. 1
Rule No. 2
Rule No. 4
Rule No. 5

# Lets insert Rule No. 3
$ sed -i 's/^Rule No. 2$/Rule No. 2\nRule No. 3/' file
$ cat file
Rule No. 1
Rule No. 2
Rule No. 3
Rule No. 4
Rule No. 5