Newbie Bash Program Problem with Redirection

LNewbie · 03-06-2007, 03:31 PM

Hello,

I am new to this forum and I am working on a lab assignment that supplies the program and then asks for me to run the program from command line, then use at command, diff and cron.

The problem that I am having is that I can't get program to run correctly. It doesn't redirect the stdout to the file. I know that I must have a syntax error. I am running Fedora Core 3.

Any hints or suggestions would be appreciated.

Thank you.

This is the first few lines of the program.

#!/bin/sh
OUTFILE="/tmp/lab6.$$"
exec >$OUTFILE

cfaj · 03-06-2007, 03:39 PM

Quote:

Originally Posted by LNewbie

Hello,

I am new to this forum and I am working on a lab assignment that supplies the program and then asks for me to run the program from command line, then use at command, diff and cron.

The problem that I am having is that I can't get program to run correctly. It doesn't redirect the stdout to the file. I know that I must have a syntax error. I am running Fedora Core 3.

Any hints or suggestions would be appreciated.

Thank you.

This is the first few lines of the program.

#!/bin/sh
OUTFILE="/tmp/lab6.$$"
exec >$OUTFILE

There's nothing wrong with that portion of your script. Are you sure you can write to /tmp?

LNewbie · 03-06-2007, 07:40 PM

Thank you for the reply.

I assumed that I could write to /tmp because I am logged in as root. I am using root to run the chron jobs after I get this program running.

If I remove the $ just before OUTFILE, it creates a file named OUTFILE with the correct output.

----------

#!/bin/sh
OUTFILE="/tmp/lab6.$$"
exec >$OUTFILE

cfaj · 03-06-2007, 09:52 PM

Quote:

Originally Posted by LNewbie

Thank you for the reply.

I assumed that I could write to /tmp because I am logged in as root. I am using root to run the chron jobs after I get this program running.

Don't login as root. Use sudo, or su if you really need to. Never run anything as root that can be done by an ordinary user, especially writing and testing scripts.

Quote:

If I remove the $ just before OUTFILE, it creates a file named OUTFILE with the correct output.

What system are you using, and which shell (/bin/sh can be any one of several shells)?

Quote:

#!/bin/sh
OUTFILE="/tmp/lab6.$$"
exec >$OUTFILE

Did you check the value of $OUTFILE? Add a line to echo it, so that you can read its value.

LNewbie · 03-06-2007, 11:27 PM

Quote:

Don't login as root. Use sudo, or su if you really need to. Never run anything as root that can be done by an ordinary user, especially writing and testing scripts.

I am using root because that is the instruction for this lab. I am using a standalone home test pc for the lab. Once I get the program working, I have to run it from command line, then use the at command and compare the files using diff. Then run it from cron as root.

Quote:

What system are you using, and which shell (/bin/sh can be any one of several shells)?

The system is a standalone test home pc running Fedora Core 3 2.6.9-1.667 and bash shell.

Quote:

Did you check the value of $OUTFILE? Add a line to echo it, so that you can read its value.

I will try this.

Thank you

slantoflight · 03-07-2007, 12:24 AM

Code:

#!/bin/sh
OUTFILE="/tmp/lab6.$$"
exec >$OUTFILE

Once you launch a program, your bash variables become pretty irrelevant outside the script.anything you do environmentally is lost once you leave #/bin/sh

in fact you'll notice anything after exec doesn't even run..
unless you fork it.. like this exec >$OUTFILE &&

but you're still back to square one.

So the solution.

remove exec and replace it with the name of your program.

cfaj · 03-07-2007, 10:22 AM

Quote:

Originally Posted by slantoflight

Code:

#!/bin/sh
OUTFILE="/tmp/lab6.$$"
exec >$OUTFILE

Once you launch a program, your bash variables become pretty irrelevant outside the script.anything you do environmentally is lost once you leave #/bin/sh

in fact you'll notice anything after exec doesn't even run..
unless you fork it.. like this exec >$OUTFILE &&

That is not correct.

1. The line, "exec >$OUTFILE", redirects the script's stdout to $OUTFILE. The script then continues.

2. You do not use && to put a command into the background; you use &.

druuna · 03-07-2007, 11:18 AM

Hi,

The rest of the program could be the problem. If output is redirected elsewhere in the script, $OUTFILE will be empty (but it will be created!).

I.e:

Code:

#!/bin/sh
OUTFILE="/tmp/lab6.$$"
exec >$OUTFILE

ls -l 1>/tmp/NOT.lab6.$$ 2>&1

If you run this, /tmp/lab6.<pid> will be created, but empty. /tmp/NOT.lab6.<pid> will hold all the output.

If /tmp/lab6.<pid> isn't even created, the problem lies elsewhere.

Hope this helps.

LNewbie · 03-08-2007, 05:51 AM

Thank you for the help. This is the program supplied by the professor but I can't get it working.

The next part that I am marked on asks for me to run the program from the command line then use the AT command and compare using diff. Then run the command from cron using root. Run the command using a different user. Try to set up a cron tab with error. Then try to submit the job to run in 2029. Then clean-up and remove your jobs.

I thought if you knew what needs to be done after the program is working. It might help. I am not asking for help or assistance with the part that I am being marked on.

Code:

#!/bin/sh
OUTFILE="/tmp/lab6.$$"
# This command redirects stdout to the file
exec >$OUTFILE
# These are back quotes around 'date' in the line below
echo "running at `date`"
echo -n "My parent is PID $PPID, "
# use echo "My parent is PID $PPID,\c" on a System V system
ps ax | grep $PPID | grep -v grep | cut -c 27-
# You'll need to modify the above line if you're on a system
# with an AT & T ps. You may also need to change the column in
# the cut command if you're using anything but Redhat 6.0
echo "=============="
echo "My environment"
echo "============="
env

druuna · 03-08-2007, 06:55 AM

Hi,

Could you be a bit clearer about what works and what doesn't?
- Do you get any error messages when running the script?
- Is the outfile (lab6.<pid>) created if you run the script?
- If so: Is there anything in it?

In general about the script you work with:

First thing I would change is this: #!/bin/sh => #!/bin/bash
On a linux box it will probably make no difference (most of the time sh is linked to bash), but on a unix box it will make a big difference (sh is not the same as bash). PPID for example is not a known variable in sh, it is in bash (on a unix box that is).

ps ax is also not known on all linux/unix flavors (as stated in the script itself), you probably need to change this to ps -ef (or alike) and change the cut statement accordingly.

After some minor changes (sh->bash and the ps ax line), the script does work on a SUN box running SunOS 5.9.

Hope this gets you going again.

LNewbie · 03-09-2007, 12:12 AM

Hi,

Quote:

Could you be a bit clearer about what works and what doesn't?
- Do you get any error messages when running the script?

No, error message

Quote:

- Is the outfile (lab6.<pid>) created if you run the script?
- If so: Is there anything in it?

No, the lab6.<pid> file doesn't get created. That is the part that doesn't work. I kept changing and testing the script and at one point, the files were created correctly. I deleted them and cleaned up. I went to run the lab again from the start and it doesn't create the files anymore.

I made the suggested changes

Quote:

#!/bin/bash and changed ps ax

and the outfile (lab6.<pid>) didn't get created

Thanks

druuna · 03-09-2007, 03:01 AM

Hi again,

A few more questions, since we still haven't found the problem:

- What are the permissions of the script (post a ls -l <scriptname>)?
- How do you execute the script?
- Could you post the output of the following script:

Code:

#!/bin/sh
set -x
OUTFILE="/tmp/lab6.$$"
echo $OUTFILE
exec >$OUTFILE
ls -l /tmp/

Lets hope this gets us a clue.

hoodedmanwithsythe · 03-09-2007, 01:46 PM

the problem is this

Code:

exec >$OUTFILE

put the whole script(excluding the env variable assignment for $outfile) in a function then you can do this instead

Code:

function1 > $outfile

which is a much better way of doing things
like this
for example

Code:

#!/bin/sh
OUTFILE="/home/timothy/lab6.txt"


function1()
{


# These are back quotes around 'date' in the line below
echo "running at `date`"
echo -n "My parent is PID $PPID, "
# use echo "My parent is PID $PPID,\c" on a System V system
ps ax | grep $PPID | grep -v grep | cut -c 27-
# You'll need to modify the above line if you're on a system
# with an AT & T ps. You may also need to change the column in
# the cut command if you're using anything but Redhat 6.0
echo "=============="
echo "My environment"
echo "============="
env 

}

function1 > $OUTFILE

cat $OUTFILE

LNewbie · 03-14-2007, 11:37 PM

Thank you. This program worked and I finally have some output to read. I made a mistake because the output showed up in the terminal window. But, I will fix that

Quote:

put the whole script(excluding the env variable assignment for $outfile) in a function then you can do this instead. For example:

#!bin/sh
OUTFILE="/home/timothy/lab6.txt"

function1()
{

# These are back quotes around 'date' in the line below
echo "running at `date`"
echo -n "My parent is PID $PPID, "
# use echo "My parent is PID $PPID,\c" on a System V system
ps ax | grep $PPID | grep -v grep | cut -c 27-
# You'll need to modify the above line if you're on a system
# with an AT & T ps. You may also need to change the column in
# the cut command if you're using anything but Redhat 6.0
echo "=============="
echo "My environment"
echo "============="
env

}

function1 > $OUTFILE

cat $OUTFILE

hoodedmanwithsythe · 03-18-2007, 10:25 AM

Quote:

Originally Posted by LNewbie

Thank you. This program worked and I finally have some output to read. I made a mistake because the output showed up in the terminal window. But, I will fix that

the script I have written does both it outputs to the terminal as well as outputting to the filethats what the

Code:

cat $outfile

does it reads the file
i had changed the outfile location to suit me so here is the corrected version of your script with the original outfile location

Code:

#!bin/sh
OUTFILE="/tmp/lab6.$$"


function1()
{


# These are back quotes around 'date' in the line below
echo "running at `date`"
echo -n "My parent is PID $PPID, "
# use echo "My parent is PID $PPID,\c" on a System V system
ps ax | grep $PPID | grep -v grep | cut -c 27-
# You'll need to modify the above line if you're on a system
# with an AT & T ps. You may also need to change the column in
# the cut command if you're using anything but Redhat 6.0
echo "=============="
echo "My environment"
echo "============="
env

}

function1 > $OUTFILE

cat $OUTFILE

sorry i should have told you about that earlier rather than assuming you knew what i did there