bash error checking

mreff555 · 06-23-2013, 01:04 PM

I want to write a script which will be configuring and making a series of programs. I would like to have some form of error checking. It doesn't have to be that sophistocated. Just something that will stop the script on errors.
How can I do this?

lleb · 06-23-2013, 01:16 PM

typically in BASH, and mind you im still relatively new to BASH myself, if there is an error in the script it will crash at that point.

you can always use test statements to verify before you continue, that is one form of error checking. what exactly are you looking for and what have you written so far? please place your script in "code" flags. you get this by going to advance posting options or by placing code /code inside of []

druuna · 06-23-2013, 02:25 PM

Quote:

Originally Posted by mreff555

Just something that will stop the script on errors.

If you just want the script to stop when an error occurs you can use bash's -e option.

Code:

#!/bin/bash -e

..code goes here ...

# or

#!/bin/bash
set -e

... code goes here ...

Both stop executing the script when an error occurs.

chrism01 · 06-23-2013, 06:23 PM

If you want to check cmds individually

Code:

#!/bin/bash

cmd_here
if [[ $? -ne 0 ]]
then
    echo "cmd errored .."
    exit
fi

Depending on cmd, you may be able to capture output and echo that, or just catch the whole script

Code:

./script.sh >scriptlog 2>&1

http://rute.2038bug.com/index.html.gz

mreff555 · 06-25-2013, 09:46 AM

ok, here is where I'm getting a little fuzzy with command line operators.

2>&1

Could someone explain this to me?

As I understand it, it pushes stderr as well as stdout to the screen/log whatever is this correct. could someone break this down?

Madhu Desai · 06-25-2013, 09:50 AM

Quote:

Originally Posted by mreff555

ok, here is where I'm getting a little fuzzy with command line operators.

2>&1

Could someone explain this to me?

As I understand it, it pushes stderr as well as stdout to the screen/log whatever is this correct. could someone break this down?

Redirects error to where output is going

Here both means same

Code:

./script.sh > scriptlog 2>&1
./script.sh >& scriptlog

chrism01 · 06-25-2013, 08:19 PM

All Unix processes are automatically given 3 i/o channels on creation/startup

chan 0 = stdin
chan 1 = stdout
chan 2 = stderr

So we're directing '1' (stdout) to logfile and directing 2 (stderr) to the same address (&) as 1 ie the logfile.
Its possible to have separate output and err files, but its tricky to match them up afterwards, so usually you put them both to the same file.

Code:

#separate logs
./script.sh >script.log 2>script.err

Note that the '1' is optional here (and above), as its 'assumed' by the shell.

You can in fact capture the output/err of any prog launched from the shell the same way, UNLESS its already coded internally to put the o/p or err info elsewhere.

http://rute.2038bug.com/index.html.gz

mreff555 · 06-26-2013, 06:10 AM

Ok, thats kinda what I thought was going on. As for the ampersand.(&)
Since that only appears to be used when directing error to the output stream, would that be akin to a reference operator in c?

chrism01 · 06-26-2013, 07:28 PM

Kind of ...

BUT only in this context

(although in C its called the address operator iirc. The Ref (aka ptr) operator is '*' )

Used like this

Code:

./myscript &

'&' puts the script into the background.
Basically, like all (most?) of the punct symbols, it has multiple 'definitions' dependent on context

Code:

./myscript >myscript.log 2>&1 &

Can you guess what's going on here ?

See the link in my prev post.

In a sed cmd '&' has yet another meaning, and so it goes...

mreff555 · 06-27-2013, 05:42 AM

Code:

(although in C its called the address operator iirc. The Ref (aka ptr) operator is '*' )

I always get those two mixed up. My reference was here: www.cplusplus.com/doc/tutorial/pointers/

Code:

Can you guess what's going on here ? ;)

I believe that it would run the script in the background, sending both stdio and stderror to the log file.

Code:

See the link in my prev post.

In a sed cmd '&' has yet another meaning, and so it goes...[/QUOTE]

Thanks for the link.

David the H. · 06-28-2013, 09:49 AM

This link has a great explanation on how file descriptors work in the shell:

redirections and file descriptors explained