14.9. Debugging

By using the –n option to the bash command, you can check the syntax of your scripts without really executing any of the commands. If there is a syntax error in the script, the shell will report the error. If there are no errors, nothing is displayed.

The most commonly used method for debugging scripts is the set command with the –x option, or bash invoked with the –x option and the script name. See Table 14.7 for a list of debugging options. These options allow an execution trace of your script. Each command from your script is displayed after substitution has been performed, and then the command is executed. When a line from your script is displayed, it is preceded with a plus (+) sign.

Table 14.7. Debugging Options

Command	Option	What It Does
bash –x scriptname	Echo option	Displays each line of script after variable substitutions and before execution.
bash –v scriptname	Verbose option	Displays each line of script before execution, just as you typed it.
bash –n scriptname	Noexec option	Interprets but does not execute commands.
set –x	Turns on echo	Traces execution in a script.
set +x	Turns off echo	Turns off tracing.

With the verbose option turned on, or by invoking the shell with the –v option (bash –v scriptname), each line of the script will be displayed just as it was typed in the script, and then executed. (See Chapter 15, "Debugging Shell Scripts," on page 967 for details.)

Example 14.69.

(The Script)

    #!/bin/bash

    # Scriptname: todebug



1   name="Joe Shmoe"

    if [[ $name == "Joe Blow" ]]

    then

        printf "Hello $name\n"

    fi



    declare -i num=1

    while (( num < 5 ))

    do

       let num+=1

    done

    printf "The total is %d\n", $num



(The Output)

2 bash -x todebug

+ name=Joe Shmoe

+ [[ Joe Shmoe == \J\o\e\ \B\l\o\w ]]

+ declare -i num=1

+ ((  num < 5  ))

+ let num+=1

+ ((  num < 5  ))

+ let num+=1

+ ((  num < 5  ))

+ let num+=1

+ ((  num < 5  ))

+ let num+=1

+ ((  num < 5  ))

+ printf 'The total is %d\n,' 5

The total is 5

EXPLANATION