Shell Script that Logs Stuff

[CrusaderAD]

I built a shell script that fetches a file via ftp, then appends a line to a text file logging it. I have a feeling it would write the line regardless of the success of the previous command. How do I write it so I can get some more details? Write one line if successful, another line if it fails? Is that possible?

[DaithiF]

something like:

somecommand
[[ $? -eq 0 ]] && echo "It worked" || echo "It failed"

[epicoder]

EDIT: Looks like someone beat me to it, but I'll just leave mine here. :P

There is a variable, $?, which is the exit status of the previous command. Most programs exit with a status of 0 on success, so you could add something like this immediately after the ftp get command:

if [ "$?" == "0" ]
then
echo "Success"
else
echo "Failure"
fi

[CrusaderAD]

Excellent! I'll give these a shot and post my results. Thanks!

[CrusaderAD]

I tried both recommdations. With this shell script:


#!/bin/sh
ping -c 2 downloads.com
if [ "$?" == "0" ]
then
echo "Success"
else
echo "Failure"
fi
>> /home/user/pingtest.log


I get this message:


2 packets transmitted, 2 received, 0% packet loss, time 5041ms
rtt min/avg/max/mdev = 24.000/24.433/24.866/0.433 ms
[: 8: 0: unexpected operator
Failure

With Daith's solution, I get this:


2 packets transmitted, 2 received, 0% packet loss, time 5058ms
rtt min/avg/max/mdev = 21.859/33.293/44.728/11.435 ms
/bin/pingtest: 3: [[: not found


Any ideas?

[DaithiF]

you're running it under the dash shell rather than bash.

ssh228's solution fails under dash because == is not a supported comparison operator for the [ test command in dash and mine fails because [[ is a bash construct only, it doesn't exist in dash.

make things easier on yourself and use #!/bin/bash as your shebang.

[epicoder]

My tendency to use "==" instead of "=" comes from my days of using python and php. :P "=" works in both bash and sh, so you should use that instead.

if [ "$?" = "0" ]
then
echo "Success"
else
echo "Failure"
fi

[CrusaderAD]

Awesome! It works great, thanks! I don't suppose either of you knows how to append the line to the top of the text file when I do this?


>> /home/user/pingtest.log

Rather than it appending to the bottom?

[DaithiF]

I don't suppose either of you knows how to append the line to the top of the text file when I do this?
Rather than it appending to the bottom?
Thats usually not a good idea ... log files almost always grow from the bottom. May I ask why you want to add it to the top? Usually you can achieve what you want using tools like tail or tac without changing the order the file is written.

[CrusaderAD]

I just want to read the latest line first without having to scroll down. Is there another way to achieve this?

[DaithiF]

tail -1 thefile

also very useful is:

tail -f thefile
which watches the file continuously (-f means 'follow') and outputs any changes as the happne.

[epicoder]

Using sed, you can prepend lines to a file by using an insert command restricted to the first line. Say the text I want to prepend is in variable $text:

sed -i "1 i$text" logfile.txt
Note that the i command in sed automatically adds a newline, so don't put one in the variable.

[CrusaderAD]

Both work great. I ended up using tail. Thanks for all the help guys!