Classic Shell Scripting

Simple Execution Tracing

Because program development is a human activity, there will be times when your script just doesn't do what you want it to do. One way to get some idea of what your program is doing is to turn on execution tracing. This causes the shell to print out each command as it's executed, preceded by "+ "—that is, a plus sign followed by a space. (You can change what gets printed by assigning a new value to the PS4 shell variable.) For example:

$ sh -x nusers                          
            Run with tracing on
+ who                                   Traced commands
+ wc -l
      7                                 Actual output

You can turn execution tracing on within a script by using the command set -x, and turn it off again with set +x. This is more useful in fancier scripts, but here's a simple program to demonstrate:

$ cat > trace1.sh                       
            Create script
            #! /bin/sh

            set -x                                  
            Turn on tracing
            echo 1st echo                           
            Do something

            set +x                                  
            Turn off tracing
            echo 2nd echo                           
            Do something else
            ^D                                      
            Terminate with end-of-file

$ chmod +x trace1.sh                    
            Make program executable

$ ./trace1.sh                           
            Run it
+ echo 1st echo                         First traced line
1st echo                                Output from command
+ set +x                                Next traced line
2nd echo                                Output from next command

When run, the set -x is not traced, since tracing isn't turned on until after that command completes. Similarly, the set +x is traced, since tracing isn't turned off until after it completes. The final echo isn't traced, since tracing is turned off at that point.