Shulou(Shulou.com)06/01 Report--

Debugging means of BASH

Echo/print (General Technology)

Print some variables, or prompt messages. This should be a general method. In BASH, we can simply use echo, or print to output some log, or add some loglevel to filter some log. Here is a post of my usual function:

_ loglevel=2

DIE () {

Echo "Critical: $1 >" > & 2

Exit 1

}

INFO () {

[$_ loglevel-ge 2] & & echo "INFO: $1 >" > & 2

}

ERROR () {

[$_ loglevel-ge 1] & & echo "ERROR: $1 >" > & 2

}

The implementation here simply adds a loglevel, but you can actually output the log to a file or color the log. For example:

# add color

[$_ loglevel-ge 1] & & echo-e "\ 033 [31m ERROR:\ 033 [0m $1 >" > & 2

# redirect to file

[$_ loglevel-ge 1] & & echo "ERROR: $1 >" > / var/log/xxx_log.$BASHPID

Set-x (rare technology)

The-x (xtrace) option causes BASH to print out the command to be executed before executing it. This option is useful for debugging some command errors.

Sometimes, because the parameters passed in contain some special characters, the BASH parsing is not carried out as we expected. At this point, turn on-x and you can print out the expanded command before the command is executed. For example, based on the function written earlier:

Set-x

INFO "this is an info log >"

ERROR "this is an error log >"

Set + x

If you want to open xtrace all the way, you can add the-x parameter when you execute the script.

Trap/bashdb (epic technique)

To facilitate debugging, BASH also provides a trap mechanism. This is a lot more advanced than the two methods introduced earlier. We can use the built-in command trap to specify which commands should be executed by each sigspec. Specific uses of trap are as follows:

Trap [- lp] [[arg] sigspec.]

Sigspec includes each signal, EXIT,ERR,RETURN, and DEBUG defined in.

The various signal will not be introduced here. EXIT executes the specified command when shell exits. If a command execution returns a non-zero value in the current shell, the command associated with the ERR is executed. RETURN, on the other hand, is for source and., and each execution triggers a RETURN trap. If you bind a command to DEBUG, the trap DEBUG will be executed before each command is executed. Note here that ERR and DEBUG are only valid for the current shell. If you want functions and child shell to automatically inherit these trap, you can set-T (DEBUG/RETURN) and-E (ERR).

For example, the following script executes echo when exiting:

#! / bin/bash

Trap "echo this is an exit echo >" EXIT

Echo "this is a normal echo >"

Or, let the command in the script print out when something goes wrong:

#! / bin/bash

Trap 'echo $BASH_COMMAND return err' ERR

Echo this is a normal test

UnknownCmd

Alternatively, let the script's commands step through:

#! / bin/bash

Trap'(read-p "[$0: $LINENO] $BASH_COMMAND? >") 'DEBUG

Echo this is a test

ITunes 0

While [true]

Do

Echo $I

((iTunes +))

Done

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