15.5. Tracing with Shell Options and the set Command

By using the –n option to the sh command, you can check the sytnax 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 to use the set command with the –x option (set –x), or to use the –x option as an argument to the sh command (sh –x) followed by the script name. See Table 15.7 for a list of debugging options. These options allow an execution trace of your script. Each command from the 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 15.7. Bourne Shell Debugging Options
Command	Option	What It Does
`sh –x scriptname`	Echo option	Displays each line of script after variable substitutions and before execution.
`sh –v scriptname`	Verbose option	Displays each line of script before execution, just as you typed it.
`sh –n scriptname`	Noexec option	Interprets but does not execute commands.
`set –u`	Unbound variables	Flags variables that have not been set.
`set –x`	Turns on echo	Traces execution in a script.
`set +x`	Turns off echo	Turns off tracing.

With the verbose option (set –v) turned on, or by invoking the Bourne shell with the –v option (sh –v), each line of the script will be displayed just as it was typed in the script, and then executed.

The set command allows you to turn echoing on or off in a script in order to debug a portion of the script, not the whole program.

Example 15.22.

The script is called todebug. You can watch the script run with the –x switch turned on. Each iteration of the loop is displayed and the values of variables are printed as they are set and when they change.
The sh command starts the Bourne shell with the –x option. Echoing is turned on. Each line of the script will be displayed to the screen prepended with a plus sign (+). Variable substitution is performed before the line is displayed. The result of the execution of the command appears after the line has been displayed.

15.5.2 Debugging C/TC Shell Scripts

C shell scripts often fail because of some simple syntax error or logic error. Options to the csh command are provided to help you debug your programs. See Table 15.8.

Table 15.8. `echo (–x)` and verbose `(–v)`
As Options to `csh` (Works the Same in `tcsh`)
`csh –x scriptname`	Display each line of script after variable substitution and before execution.
`csh –v scriptname`	Display each line of script before execution, just as you typed it.
`csh –n scriptname`	Interpret but do not execute commands.
As Arguments to the `set` Command
`set echo`	Display each line of script after variable substitution and before execution.
`set verbose`	Display each line of script before execution, just as you typed it.
As the First Line in a Script
`#!/bin/csh –xv`	Turns on both `echo` and `verbose`. These options can be invoked separately or combined with other `csh/tcsh` invocation arguments.

Example 15.23.

The contents of the C shell script are displayed. Variable and command substitution lines are included so that you can see how echo and verbose differ.
The –v option to the csh command causes the verbose feature to be enabled. Each line of the script is displayed as it was typed in the script, and then the line is executed.
The –x option to the csh command enables echoing. Each line of the script is displayed after variable and command substitution are performed, and then the line is executed. Because this feature allows you to examine what is being replaced as a result of command and variable substitution, it is used more often than the verbose option.

Example 15.24.

The echo option is set and unset within the script. This enables you to debug certain sections of your script where you have run into a bottleneck, rather than echoing each line of the entire script.
The ––> marks where the echoing was turned on. Each line is printed after variable and command substitution and then executed.

Example 15.25.

The verbose option is set and unset within the script.
The ––> marks where verbose was turned on. The lines are printed just as they were typed in the script and then executed.

15.5.3 Debugging Korn Shell Scripts

By turning on the noexec option or using the –n argument to the ksh 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 to turn on the xtrace option or to use the ksh command with the –x option. These options allow an execution trace of your script. Each command from your script is displayed after variable substitution has been performed, and then the command is executed. When a line from your script is displayed, it is preceded with the value of the PS4 prompt, a plus (+) sign. The PS4 prompt can be changed.

With the verbose option turned on, or by invoking the Korn shell with the –v option (ksh –v scriptname), each line of the script will be displayed, just as it was typed in the script, and then executed. See Table 15.9 for debug commands.

Table 15.9. Korn Shell Debugging Commands and Options
Command	Function/How It Works
`export PS4='$LINENO '`	The `PS4` prompt by default is a `+`. You can reset the prompt. In this example, a line number will be printed for each line.
`ksh –n scriptname`	Invokes `ksh` with `noexec` option. Interprets but does not execute commands.
`ksh -u scriptname`	Checks for unset variables. Displays an error when expanding a variable that has not been set.
`ksh –v scriptname`	Invokes `ksh` with `verbose` option. Displays each line of the script before execution, just as you typed it.
`ksh –x scriptname`	Invokes `ksh` with `echo` option. Displays each line of the script after variable substitution and before execution.
`set +x`	Turns off `echo`. Turns off tracing.
`set –x` or `set –o xtrace`	Turns on `echo` option. Traces execution in a script.
`trap 'print $LINENO ' DEBUG`	For each script command, the trap action is performed. See format for `trap`. Prints value of `$LINENO` for each line in the script.
`trap 'print Bad input' ERR`	If a nonzero exit status is returned, the `trap` is executed.
`trap 'print Exiting from $0' EXIT`	Prints message when script or function exits.
`typeset –ft`	Turns on tracing. Traces execution in a function.

Example 15.26.

The Korn shell is invoked with the –x option. Echoing is turned on. Each line of the script will be displayed on the screen, followed by the result of executing that line. Variable substitution is performed. Alternatively, the –x option can be used in the script instead of at the command line (i.e., #!/bin/ksh –x).
The lines are preceded by the plus (+) sign, the PS4 prompt.
The while loop is entered. It will loop 4 times.
The value of num is incremented by by 1.
After the while loop exits, this line is printed.

Example 15.27.

LINENO is a special Korn shell variable that holds the number of the current script line. The DEBUG signal, used with the trap command, causes the string enclosed in single quotes to be executed every time a command in the script is executed.
As the while loop executes, the value of the variable num and the line of the script are displayed.

15.5.4 Debugging Bash Scripts

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 15.10 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 15.10. Bash 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.

Bash Invocation Options

When the shell is started using the bash command, it can take options to modify its behavior. There are two types of options: single-character options and multicharacter options. The single-character options consist of a single leading dash followed by a single character. The multicharacter options consist of two leading dashes and any number of characters. Multicharacter options must appear before single-character options. An interactive login shell normally starts up with –i (start an interactive shell), –s (read from standard input), and –m (enable job control). See Table 15.11.

Table 15.11. Additional `bash` Options to Assist in Debugging
Option	Meaning
`–i`	Shell is in the interactive mode. `TERM`, `QUIT`, and `INTERRUPT` are ignored.
`–r`	Starts a restricted shell.
`––`	Signals the end of options and disables further option processing. Any arguments after `––` or `–` are treated as filenames and arguments.
`––help`	Displays a usage message for a built-in command and exits.
`––noprofile`	When starting up, `bash` does not read the initialization files:`/etc/profile`, `~/.bash_profile`, `~/.bash_login`, or `~/.profile`.
`––norc`	For interactive shells, `bash` will not read the `~/.bashrc` file. Turned on by default, if running shell as `sh`.
`––posix`	Changes the behavior of `bash` to match the POSIX 1003.2 standard, if otherwise it wouldn't.
`––quiet`	Displays no information at shell startup, the default.
`––rcfile file`	If `bash` is interactive, uses this intialization file instead of `~/.bashrc`.
`––restricted`	Starts a restricted shell.
`––verbose`	Turns on `verbose`; same as `–v`.
`––version`	Displays version information about this `bash` shell and exit.

The set Command and Options

The set command can be used to turn shell options on and off, as well as for handling command-line arguments. To turn an option on, the dash (–) is prepended to the option; to turn an option off, the plus sign (+) is prepended to the option.

Table 15.12. The Built-In `set` Command Options
Name of Option	Shortcut Switch	What It Does
`allexport`	`–a`	Automatically marks new or modified variables for export from the time the option is set until unset.
`braceexpand`	`–B`	Enables brace expansion, and is a default setting.^[a]
`emacs`		For command-line editing, uses the `emacs` built-in editor, and is a default setting.
`errexit`	`–e`	If a command returns a nonzero exit status (fails), exits. Not set when reading initialization files.
`histexpand`	`–H`	Enables `!` and `!!` when performing history substitution, and is a default setting.^[a]
`history`		Enables command-line history; on by default.^[a]
`ignoreeof`		Disables EOF (Ctrl-D) from exiting a shell; must type `exit`. Same as setting shell variable, `IGNOREEOF=10`.
`keyword`	`–k`	Places keyword arguments in the environment for a command.^[a]
`interactive-comments`		For interactive shells, a leading `#` is used to comment out any text remaining on the line.
`monitor`	`–m`	Allows job control.
`noclobber`	`–C`	Protects files from being overwritten when redirection is used.
`noexec`	`–n`	Reads commands, but does not execute them. Used to check the syntax of scripts. Not on when running interactively.
`noglob`	`–d`	Disables pathname expansion (i.e., turns off wildcards).
`notify`	`–b`	Notifies user when background job finishes.
`nounset`	`–u`	Displays an error when expanding a variable that has not been set.
`onecmd`	`–t`	Exits after reading and executing one command.^[a]
`physical`	`–P`	If set, does not follow symbolic links when typing `cd` or `pwd`. The physical directory is used instead.
`posix`		Shell behavior is changed if the default operation doesn't match the POSIX standard.
`privileged`	`–p`	When set, the shell does not read the `.profile` or `ENV` file and shell functions are not inherited from the environment; automatically set for `setuid` scripts.
`verbose`	`–v`	Turns on the `verbose` mode for debugging.
`vi`		For command-line editing, uses the `vi` built-in editor.
`xtrace`	`–x`	Turns on the `echo` mode for debugging.

The shopt Command and Options

The shopt (bash 2.x) command can also be used to turn shell options on and off.

Table 15.13. The `shopt` Command Options
Option	Meaning
`cdable_vars`	If an argument to the `cd` built-in command is not a directory, it is assumed to be the name of a variable whose value is the directory to change to.
`cdspell`	Corrects minor errors in the spelling of a directory name in a `cd` command. The errors checked for are transposed characters, a missing character, and a character too many. If a correction is found, the corrected path is printed, and the command proceeds. Only used by interactive shells.
`checkhash`	`Bash` checks that a command found in the hash table exists before trying to execute it. If a hashed command no longer exists, a normal path search is performed.
`checkwinsize`	`Bash` checks the window size after each command and, if necessary, updates the values of `LINES` and `COLUMNS`.
`cmdhist`	`Bash` attempts to save all lines of a multiple-line command in the same history entry. This allows easy re-editing of multiline commands.
`dotglob`	`Bash` includes filenames beginning with a dot `(.)` in the results of filename expansion.
`execfail`	A noninteractive shell will not exit if it cannot execute the file specified as an argument to the `exec` built-in command. An interactive shell does not exit if `exec` fails.
`expand_aliases`	Aliases are expanded. Enabled by default.
`extglob`	The extended pattern matching features (regular expression metacharacters derived from Korn shell for filename expansion) are enabled.
`histappend`	The history list is appended to the file named by the value of the `HISTFILE` variable when the shell exits, rather than overwriting the file.
`histreedit`	If `readline` is being used, a user is given the opportunity to re-edit a failed history substitution.
`histverify`	If set, and `readline` is being used, the results of history substitution are not immediately passed to the shell parser. Instead, the resulting line is loaded into the `readline` editing buffer, allowing further modification.
`hostcomplete`	If set, and `readline` is being used, `bash` will attempt to perform hostname completion when a word containing an `@` is being completed. Enabled by default.
`huponexit`	If set, `bash` will send `SIGHUP` (hangup signal) to all jobs when an interactive login shell exits.
`interactive_comments`	Allows a word beginning with # to cause that word and all remaining characters on that line to be ignored in an interactive shell. Enabled by default.
`lithist`	If enabled, and the `cmdhist` option is enabled, multiline commands are saved to the history with embedded newlines rather than using semicolon separators where possible.
`mailwarn`	If set, and a file that `bash` is checking for mail has been accessed since the last time it was checked, the message `The mail in mailfile has been read is` displayed.
`nocaseglob`	If set, `bash` matches filenames in a case-insensitive fashion when performing filename expansion.
`nullglob`	If set, `bash` allows filename patterns that match no files to expand to a null string, rather than themselves.
`promptvars`	If set, prompt strings undergo variable and parameter expansion after being expanded. Enabled by default.
`restricted_shell`	The shell sets this option if it is started in restricted mode. The value may not be changed. This is not reset when the startup files are executed, allowing the startup files to discover whether or not a shell is restricted.
`shift_verbose`	If this is set, the `shift` built-in prints an error message when the shift count exceeds the number of positional parameters.
`sourcepath`	If set, the `source` built-in uses the value of `PATH` to find the directory containing the file supplied as an argument. Enabled by default.
`source`	A synonym for dot `(.)`.