bash -x / trace / sdout

odimachkie · 02-19-2009, 01:50 PM

i've been going through some tutorials and one of the suggestions was to use the following at the beginning of your script to trace/debug:

Code:

#!/bin/bash -x

im not sure what exactly the desired effect is, but im pretty sure im not getting it. all im getting is what i would normally get if i ran

Code:

bash myscript.sh

i am new at this so i could be overlooking something simple, and it probably doesn't help that im not really sure what exactly this code:

Code:

#!/bin/bash

is doing...is it not commented out?

any feedback would help, thanks.

armandrix · 02-19-2009, 02:30 PM

The "#!"sign in the first line first character in a file is a special character, wich is intended to give shell tips about the archive.

When you put "#! /bin/bash" you are telling shell to execute this file with bash, wich is a shell of coarse, or virtual shell wich mimics shell in graphical environ.

"#! /usr/bin/env bash" do the same but thru env utility.

Scripting file cannot have those lines, but it can cause troubles or unexpected behavior in certain cases.

In python scripting one can put "#! /usr/bin/env python" so the shell won't try to execute this file sending it for python interpreter.

The option -x in this case make bash more verbose, printing aliases and many stuffs in the prompt.

Hope it helps

odimachkie · 02-19-2009, 02:58 PM

That does help with the #! issue, i assumed something like that, but i wasnt sure.
but my -x issue is still there, maybe it would help if copied the script im trying to run.

Code:

#!/bin/bash -x

echo -n "Enter some text > "
if read -st 2; 
then
  echo "Your wrote: $REPLY"
else
  echo "To slow suckah"
fi

the script works. but as far as i know, using the -x should trace through the script, which is not what im getting when i run the script.
ive tried using -x on other more complicated scripts and still, i can detect no change in the operation.

im running debian if that has anything to do with anything.

ArfaSmif · 02-19-2009, 08:02 PM

All I've ever done is use "set -x" after the #!/bin/bash first line. That will display everything as it is being processed. For example :-

#!/bin/bash

set -x

echo -n "Enter some text > "
if read -st 2;
then
echo "Your wrote: $REPLY"
else
echo "Too slow suckah"
fi

armandrix · 02-19-2009, 10:28 PM

Hi odimachkie

About -x in your script, traces its exec steps if you want some more advanced debugging try strace, ltrace or other tools.

cheers

odimachkie · 02-19-2009, 11:02 PM

thanks for the help everyone, i will be playing with ltrace and strace tonight, as armandrix mentioned.

but, for the record, i did figure out what my problem was.

ive been running the scripts via the bash command, like so:

Code:

odimachkie@LinuxBox:~/bin$bash myscript.sh

the reason i was doing this was that just typing the filename was not working as it would with other scripts. this is because the file permissions did not allow for execution. a little chmod 755 did the trick and now -x works like a dream.

i'm very obviously new at this, so thanks for the input everyone. every little bit helps.

chrism01 · 02-20-2009, 06:48 AM

Actually, for full debugging, try

#!/bin/bash
set -xv