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====== Debugging a script ======
{{keywords&gt;bash shell scripting bug debug debugging}}
These few lines are not intended as a full-fledged debugging tutorial, but as hints and comments about debugging a Bash script.
===== Use a unique name for your script =====
Do **not** name your script ''test'', for example! //Why?// ''test'' is the name of a UNIX(r)-command, and __most likely built into your shell__ (it's a built-in in Bash) - so you won't be able to run a script with the name ''test'' in a normal way.
**Don't laugh!** This is a classic mistake :-)
===== Read the error messages =====
Many people come into IRC and ask something like //&quot;Why does my script fail? I get an error!&quot;//. And when you ask them what the error message is, they don't even know. Beautiful.
Reading and interpreting error messages is 50% of your job as debugger! Error messages actually **mean** something. At the very least, they can give you hints as to where to start debugging. **READ YOUR ERROR MESSAGES!**
You may ask yourself why is this mentioned as debugging tip? Well, __you would be surprised how many shell users ignore the text of error messages!__ When I find some time, I'll paste 2 or 3 IRC log-snips here, just to show you that annoying fact.
===== Use a good editor =====
Your choice of editor is a matter of personal preference, but one with **Bash syntax highlighting** is highly recommended! Syntax highlighting helps you see (you guessed it) syntax errors, such as unclosed quotes and braces, typos, etc.
From my personal experience, I can suggest ''vim'' or ''GNU emacs''.
===== Write logfiles =====
For more complex scripts, it's useful to write to a log file, or to the system log. Nobody can debug your script without knowing what actually happened and what went wrong.
An available syslog interface is ''logger'' ([[http://unixhelp.ed.ac.uk/CGI/man-cgi?logger+1 | online manpage]]).
===== Inject debugging code =====
Insert **echos** everywhere you can, and print to ''stderr'':
&lt;code&gt;
echo &quot;DEBUG: current i=$i&quot; &gt;&amp;2
&lt;/code&gt;
If you read input from **anywhere**, such as a file or [[syntax:expansion:cmdsubst | command substitution]], print the debug output with literal quotes, to see leading and trailing spaces!
&lt;code&gt;
pid=$(&lt; fooservice.pid)
echo &quot;DEBUG: read from file: pid=\&quot;$pid\&quot;&quot; &gt;&amp;2
&lt;/code&gt;
Bash's [[commands:builtin:printf | printf]] command has the ''%q'' format, which is handy for verifying whether strings are what they appear to be.
&lt;code&gt;
foo=$(&lt; inputfile)
printf &quot;DEBUG: foo is |%q|\n&quot; &quot;$foo&quot; &gt;&amp;2
# exposes whitespace (such as CRs, see below) and non-printing characters
&lt;/code&gt;
===== Use shell debug output =====
There are two useful debug outputs for that task (both are written to ''stderr''):
* ''set -v'' mode (''set -o verbose'')
* print commands to be executed to ''stderr'' as if they were read from input (script file or keyboard)
* print everything **before** any ([[syntax:expansion:intro | substitution and expansion]], ...) is applied
* ''set -x'' mode (''set -o xtrace'')
* print everything as if it were executed, after [[syntax:expansion:intro | substitution and expansion]] is applied
* indicate the depth-level of the subshell (by default by prefixing a ''+'' (plus) sign to the displayed command)
* indicate the recognized words after [[syntax:expansion:wordsplit | word splitting]] by marking them like ''&lt;nowiki&gt;'x y'&lt;/nowiki&gt;''
* in shell version 4.1, this debug output can be printed to a configurable file descriptor, rather than sdtout by setting the [[syntax:shellvars#BASH_XTRACEFD|BASH_XTRACEFD]] variable.
**__Hint:__** These modes can be entered when calling Bash:
* from commandline: ''bash -vx ./myscript''
* from shebang (OS dependant): ''#!/bin/bash -vx''
==== Simple example of how to interpret xtrace output ====
Here's a simple command (a string comparison using the [[commands:classictest | classic test command]]) executed while in ''set -x'' mode:
&lt;code&gt;
set -x
foo=&quot;bar baz&quot;
[ $foo = test ]
&lt;/code&gt;
That fails. Why? Let's see the ''xtrace'' output:
&lt;code&gt;
+ '[' bar baz = test ']'
&lt;/code&gt;
And now you see that it's (&quot;bar&quot; and &quot;baz&quot;) recognized as two separate words (which you would have realized if you READ THE ERROR MESSAGES ;) ). Let's check it...
&lt;code&gt;
# next try
[ &quot;$foo&quot; = test ]
&lt;/code&gt;
''xtrace'' now gives
&lt;code&gt;
+ '[' 'bar baz' = test ']'
^ ^
word markers!
&lt;/code&gt;
==== Making xtrace more useful ====
(by AnMaster)
''xtrace'' output would be more useful if it contained source file and line number. Add this assignment [[syntax:shellvars#PS4|PS4]] at the beginning of your script to enable the inclusion of that information:
&lt;code&gt;
export PS4='+(${BASH_SOURCE}:${LINENO}): ${FUNCNAME[0]:+${FUNCNAME[0]}(): }'
&lt;/code&gt;
**Be sure to use single quotes here!**
The output would look like this when you trace code //outside a function//:
&lt;code&gt;
+(somefile.bash:412): echo 'Hello world'
&lt;/code&gt;
...and like this when you trace code //inside a function//:
&lt;code&gt;
+(somefile.bash:412): myfunc(): echo 'Hello world'
&lt;/code&gt;
That helps a lot when the script is long, or when the main script sources many other files.
=== Set flag variables with descriptive words ===
If you test variables that flag the state of options, such as with ''%%if [[ -n $option ]];%%'', consider using descriptive words rather than short codes, such as 0, 1, Y, N, because xtrace will show ''%%[[ -n word ]]%%'' rather than ''%%[[ -n 1 ]]%%'' when the option is set.
===== Debugging commands depending on a set variable =====
For general debugging purposes you can also define a function and a variable to use:
&lt;code&gt;
debugme() {
[[ $script_debug = 1 ]] &amp;&amp; &quot;$@&quot; || :
# be sure to append || : or || true here or use return 0, since the return code
# of this function should always be 0 to not influence anything else with an unwanted
# &quot;false&quot; return code (for example the script's exit code if this function is used
# as the very last command in the script)
}
&lt;/code&gt;
This function does nothing when ''script_debug'' is unset or empty, but it executes the given parameters as commands when ''script_debug'' is set. Use it like this:
&lt;code&gt;
script_debug=1
# to turn it off, set script_debug=0
debugme logger &quot;Sorting the database&quot;
database_sort
debugme logger &quot;Finished sorting the database, exit code $?&quot;
&lt;/code&gt;
Of course this can be used to execute something other than echo during debugging:
&lt;code&gt;
debugme set -x
# ... some code ...
debugme set +x
&lt;/code&gt;
===== Dry-run STDIN driven commands =====
Imagine you have a script that runs FTP commands using the standard FTP client:
&lt;code bash&gt;
ftp user@host &lt;&lt;FTP
cd /data
get current.log
dele current.log
FTP
&lt;/code&gt;
A method to dry-run this with debug output is:
&lt;code bash&gt;
if [[ $DRY_RUN = yes ]]; then
sed 's/^/DRY_RUN FTP: /'
else
ftp user@host
fi &lt;&lt;FTP
cd /data
get current.log
dele current.log
FTP
&lt;/code&gt;
This can be wrapped in a shell function for more readable code.
===== Common error messages =====
==== Unexpected end of file ====
&lt;code&gt;
script.sh: line 100: syntax error: unexpected end of file
&lt;/code&gt;
Usually indicates exactly what it says: An unexpected end of file. It's unexpected because Bash waits for the closing of a [[syntax:ccmd:intro | compound command]]:
* did you close your ''do'' with a ''done''?
* did you close your ''if'' with a ''fi''?
* did you close your ''case'' with a ''esac''?
* did you close your ''{'' with a ''}''?
* did you close your ''('' with a '')''?
**__Note:__** It seems that here-documents (tested on versions ''1.14.7'', ''2.05b'', ''3.1.17'' and ''4.0'') are correctly terminated when there is an EOF before the end-of-here-document tag (see [[syntax:redirection | redirection]]). The reason is unknown, but it seems to be deliberate. Bash 4.0 added an extra message for this: ''warning: here-document at line &lt;N&gt; delimited by end-of-file (wanted `&lt;MARKER&gt;')''
==== Unexpected end of file while looking for matching ... ====
&lt;code&gt;
script.sh: line 50: unexpected EOF while looking for matching `&quot;'
script.sh: line 100: syntax error: unexpected end of file
&lt;/code&gt;
This one indicates the double-quote opened in line 50 does not have a matching closing quote.
These //unmatched errors// occur with:
* double-quote pairs
* single-quote pairs (also ''&lt;nowiki&gt;$'string'&lt;/nowiki&gt;''!)
* missing a closing ''}'' with [[syntax:pe | parameter expansion syntax]]
==== Too many arguments ====
&lt;code&gt;
bash: test: too many arguments
&lt;/code&gt;
You most likely forgot to quote a variable expansion somewhere. See the example for ''xtrace'' output from above. External commands may display such an error message though in our example, it was the **internal** test-command that yielded the error.
==== !&quot;: event not found ====
&lt;code&gt;
$ echo &quot;Hello world!&quot;
bash: !&quot;: event not found
&lt;/code&gt;
This is not an error per se. It happens in interactive shells, when the C-Shell-styled history expansion (&quot;''!searchword''&quot;) is enabled. This is the default. Disable it like this:
&lt;code&gt;
set +H
# or
set +o histexpand
&lt;/code&gt;
==== syntax error near unexpected token `(' ====
When this happens during a script **function definition** or on the commandline, e.g.
&lt;code&gt;
$ foo () { echo &quot;Hello world&quot;; }
bash: syntax error near unexpected token `('
&lt;/code&gt;
you most likely have an alias defined with the same name as the function (here: ''foo''). Alias expansion happens before the real language interpretion, thus the alias is expanded and makes your function definition invalid.
===== The CRLF issue =====
==== What is the CRLF issue? ====
There's a big difference in the way that UNIX(r) and Microsoft(r) (and possibly others) handle the **line endings** of plain text files. The difference lies in the use of the CR (Carriage Return) and LF (Line Feed) characters.
* MSDOS uses: ''\r\n'' (ASCII ''CR'' #13 ''^M'', ASCII LF #10)
* UNIX(r) uses: ''\n'' (ASCII ''LF'' #10)
Keep in mind your script is a **plain text file**, and the ''CR'' character means nothing special to UNIX(r) - it is treated like any other character. If it's printed to your terminal, a carriage return will effectively place the cursor at the beginning of the //current// line. This can cause much confusion and many headaches, since lines containing CRs are not what they appear to be when printed. In summary, CRs are a pain.
==== How did a CR end up in my file? ====
Some possible sources of CRs:
* a DOS/Windows text editor
* a UNIX(r) text editor that is &quot;too smart&quot; when determining the file content type (and thinks &quot;//it's a DOS text file//&quot;)
* a direct copy and paste from certain webpages (some pastebins are known for this)
==== Why do CRs hurt? ====
CRs can be a nuisance in various ways. They are especially bad when present in the shebang/interpreter specified with ''#!'' in the very first line of a script. Consider the following script, written with a Windows(r) text editor (''^M'' is a symbolic representation of the ''CR'' carriage return character!):
&lt;code&gt;
#!/bin/bash^M
^M
echo &quot;Hello world&quot;^M
...
&lt;/code&gt;
Here's what happens because of the ''#!/bin/bash^M'' in our shebang:
* the file ''/bin/bash^M'' doesn't exist (hopefully)
* So Bash prints an error message which (depending on the terminal, the Bash version, or custom patches!) may or may not expose the problem.
* the script can't be executed
The error message can vary. If you're lucky, you'll get:
&lt;code&gt;
bash: ./testing.sh: /bin/bash^M: bad interpreter: No such file or directory
&lt;/code&gt;
which alerts you to the CR. But you may also get the following:
&lt;code&gt;
: bad interpreter: No such file or directory
&lt;/code&gt;
Why? Because when printed literally, the ''^M'' makes the cursor go back to the beginning of the line. The whole error message is //printed//, but you //see// only part of it!
&lt;note warning&gt;
It's easy to imagine the ''^M'' is bad in other places too. If you get weird and illogical messages from your script, rule out the possibility that''^M'' is involved. Find and eliminate it!
&lt;/note&gt;
==== How can I find and eliminate them? ====
**To display** CRs (these are only a few examples)
* in VI/VIM: '':set list''
* with ''cat(1)'': ''cat -v FILE''
**To eliminate** them (only a few examples)
* blindly with ''tr(1)'': ''tr -d &lt;nowiki&gt;'\r'&lt;/nowiki&gt; &lt;FILE &gt;FILE.new''
* controlled with ''recode(1)'': ''recode MSDOS..latin1 FILE''
* controlled with ''dos2unix(1)'': ''dos2unix FILE''
===== See also =====
* [[commands:builtin:set | the set builtin command]] (for ''-v'' and ''-x'')
FIXME
* DEBUG trap
* BASH Debugger http://bashdb.sourceforge.net/</textarea>
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<ul class="list-inline"><li><span class="iconify text-muted" data-icon="mdi:file-document-outline"></span> <span title="scripting/debuggingtips.txt">scripting/debuggingtips.txt</span></li><li><span class="iconify text-muted" data-icon="mdi:calendar"></span> Last modified: <span title="2017/06/07 02:42">2017/06/07 02:42</span></li><li class="text-muted">by <bdi>fgrose</bdi></li></ul> </span>
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