You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
<strong>Bash</strong> <strong>Guide</strong> for <strong>Beginners</strong><br />
Comments also make your own life easier. Say that you had to read a lot of man pages in order to achieve a<br />
particular result with some command that you used in your script. You won't remember how it worked if you<br />
need to change your script after a few weeks or months, unless you have commented what you did, how you<br />
did it and/or why you did it.<br />
Take the script1.sh example and copy it to commented-script1.sh, which we edit so that the<br />
comments reflect what the script does. Everything the shell encounters after a hash mark on a line is ignored<br />
and only visible upon opening the shell script file:<br />
#!/bin/bash<br />
# This script clears the terminal, displays a greeting and gives information<br />
# about currently connected users. The two example variables are set and displayed.<br />
clear<br />
# clear terminal window<br />
echo "The script starts now."<br />
echo "Hi, $USER!"<br />
echo<br />
# dollar sign is used to get content of variable<br />
echo "I will now fetch you a list of connected users:"<br />
echo<br />
w<br />
# show who is logged on and<br />
echo<br />
# what they are doing<br />
echo "I'm setting two variables now."<br />
COLOUR="black"<br />
VALUE="9"<br />
echo "This is a string: $COLOUR"<br />
echo "And this is a number: $VALUE"<br />
echo<br />
# set a local shell variable<br />
# set a local shell variable<br />
# display content of variable<br />
# display content of variable<br />
echo "I'm giving you back your prompt now."<br />
echo<br />
In a decent script, the first lines are usually comment about what to expect. Then each big chunk of commands<br />
will be commented as needed for clarity's sake. Linux init scripts, as an example, in your system's init.d<br />
directory, are usually well commented since they have to be readable and editable by everyone running Linux.<br />
2.3. Debugging <strong>Bash</strong> scripts<br />
2.3.1. Debugging on the entire script<br />
When things don't go according to plan, you need to determine what exactly causes the script to fail. <strong>Bash</strong><br />
provides extensive debugging features. The most common is to start up the subshell with the -x option, which<br />
will run the entire script in debug mode. Traces of each command plus its arguments are printed to standard<br />
output after the commands have been expanded but before they are executed.<br />
This is the commented-script1.sh script ran in debug mode. Note again that the added comments are<br />
not visible in the output of the script.<br />
willy:~/scripts> bash -x script1.sh<br />
+ clear<br />
+ echo 'The script starts now.'<br />
The script starts now.<br />
Chapter 2. Writing and debugging scripts 25
Change language