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<div class="dw-content-page "><!-- content --><div class="dw-content"><p>
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This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong.
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</p>
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<form id="dw__editform" method="post" action="" accept-charset="utf-8" class=" form-inline"><div class="no">
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<input type="hidden" name="sectok" value=""/><input type="hidden" name="id" value="scripting:newbie_traps"/><input type="hidden" name="rev" value="0"/><input type="hidden" name="date" value="0"/><input type="hidden" name="prefix" value="."/><input type="hidden" name="suffix" value=""/><input type="hidden" name="changecheck" value="746997842deca7c872801cb59497e425"/><input type="hidden" name="target" value="section"/><textarea name="wikitext" id="wiki__text" dir="auto" class="edit form-control" cols="80" rows="10" tabindex="1" readonly="readonly">
|
||
|
|
====== Beginner Mistakes ======
|
||
|
|
|
||
|
|
{{keywords>bash shell scripting pitfalls traps beginners}}
|
||
|
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|
||
|
|
Here are some typical traps:
|
||
|
|
|
||
|
|
===== Script execution =====
|
||
|
|
|
||
|
|
==== Your perfect Bash script executes with syntax errors ====
|
||
|
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|
||
|
|
If you write Bash scripts with Bash specific syntax and features, run them with __Bash__, and run them with Bash in __native mode__.
|
||
|
|
|
||
|
|
**Wrong**:
|
||
|
|
* no shebang
|
||
|
|
* the interpreter used depends on the OS implementation and current shell
|
||
|
|
* **can** be run by calling bash with the script name as an argument, e.g. ''bash myscript''
|
||
|
|
* ''#!/bin/sh'' shebang
|
||
|
|
* depends on what ''/bin/sh'' actually is, for a Bash it means compatiblity mode, **not** native mode
|
||
|
|
|
||
|
|
See also:
|
||
|
|
* [[scripting:bashbehaviour#sh_mode | Bash startup mode: SH mode]]
|
||
|
|
* [[scripting:bashbehaviour#posix_run_mode | Bash run mode: POSIX mode]]
|
||
|
|
|
||
|
|
==== Your script named "test" doesn't execute ====
|
||
|
|
|
||
|
|
Give it another name. The executable ''test'' already exists.
|
||
|
|
|
||
|
|
In Bash it's a builtin. With other shells, it might be an executable file. Either way, it's bad name choice!
|
||
|
|
|
||
|
|
Workaround: You can call it using the pathname:
|
||
|
|
<code>
|
||
|
|
/home/user/bin/test
|
||
|
|
</code>
|
||
|
|
|
||
|
|
===== Globbing =====
|
||
|
|
|
||
|
|
==== Brace expansion is not globbing ====
|
||
|
|
|
||
|
|
The following command line is not related to globbing (filename expansion):
|
||
|
|
<code>
|
||
|
|
# YOU EXPECT
|
||
|
|
# -i1.vob -i2.vob -i3.vob ....
|
||
|
|
|
||
|
|
echo -i{*.vob,}
|
||
|
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|
||
|
|
# YOU GET
|
||
|
|
# -i*.vob -i
|
||
|
|
</code>
|
||
|
|
**Why?** The brace expansion is simple text substitution. All possible text formed by the prefix, the postfix and the braces themselves are generated. In the example, these are only two: ''-i*.vob'' and ''-i''. The filename expansion happens **after** that, so there is a chance that ''-i*.vob'' is expanded to a filename - if you have files like ''-ihello.vob''. But it definitely doesn't do what you expected.
|
||
|
|
|
||
|
|
Please see:
|
||
|
|
* [[syntax:expansion:brace]]
|
||
|
|
|
||
|
|
|
||
|
|
===== Test-command =====
|
||
|
|
|
||
|
|
* ''if [ $foo ] ...''
|
||
|
|
* ''if [-d $dir] ...''
|
||
|
|
* ...
|
||
|
|
Please see:
|
||
|
|
* [[commands:classictest#pitfalls_summarized|The classic test command - pitfalls]]
|
||
|
|
|
||
|
|
===== Variables =====
|
||
|
|
|
||
|
|
==== Setting variables ====
|
||
|
|
|
||
|
|
=== The Dollar-Sign ===
|
||
|
|
|
||
|
|
There is no ''$'' (dollar-sign) when you reference the **name** of a variable! Bash is not PHP!
|
||
|
|
<code>
|
||
|
|
# THIS IS WRONG!
|
||
|
|
$myvar="Hello world!"
|
||
|
|
</code>
|
||
|
|
|
||
|
|
A variable name preceeded with a dollar-sign always means that the variable gets **expanded**. In the example above, it might expand to nothing (because it wasn't set), effectively resulting in...
|
||
|
|
<code>
|
||
|
|
="Hello world!"
|
||
|
|
</code>
|
||
|
|
...which **definitely is wrong**!
|
||
|
|
|
||
|
|
When you need the **name** of a variable, you write **only the name**, for example
|
||
|
|
* (as shown above) to set variables: ''picture=/usr/share/images/foo.png''
|
||
|
|
* to name variables to be used by the ''read'' builtin command: ''read picture''
|
||
|
|
* to name variables to be unset: ''unset picture''
|
||
|
|
|
||
|
|
When you need the **content** of a variable, you prefix its name with **a dollar-sign**, like
|
||
|
|
* echo "The used picture is: $picture"
|
||
|
|
|
||
|
|
=== Whitespace ===
|
||
|
|
|
||
|
|
Putting spaces on either or both sides of the equal-sign (''='') when assigning a value to a variable **will** fail.
|
||
|
|
|
||
|
|
<code>
|
||
|
|
# INCORRECT 1
|
||
|
|
example = Hello
|
||
|
|
|
||
|
|
# INCORRECT 2
|
||
|
|
example= Hello
|
||
|
|
|
||
|
|
# INCORRECT 3
|
||
|
|
example =Hello
|
||
|
|
</code>
|
||
|
|
|
||
|
|
The only valid form is **no spaces between the variable name and assigned value**:
|
||
|
|
<code>
|
||
|
|
# CORRECT 1
|
||
|
|
example=Hello
|
||
|
|
|
||
|
|
# CORRECT 2
|
||
|
|
example=" Hello"
|
||
|
|
</code>
|
||
|
|
|
||
|
|
==== Expanding (using) variables ====
|
||
|
|
|
||
|
|
A typical beginner's trap is quoting.
|
||
|
|
|
||
|
|
As noted above, when you want to **expand** a variable i.e. "get the content", the variable name needs to be prefixed with a dollar-sign. But, since Bash knows various ways to quote and does word-splitting, the result isn't always the same.
|
||
|
|
|
||
|
|
Let's define an example variable containing text with spaces:
|
||
|
|
<code>
|
||
|
|
example="Hello world"
|
||
|
|
</code>
|
||
|
|
|
||
|
|
^Used form^result^number of words^
|
||
|
|
|''$example'' |''Hello world''|2|
|
||
|
|
|''"$example"'' |''Hello world''|1|
|
||
|
|
|''\$example'' |''$example''|1|
|
||
|
|
|''<nowiki>'$example'</nowiki>'' |''$example''|1|
|
||
|
|
|
||
|
|
If you use parameter expansion, you **must** use the **name** (''PATH'') of the referenced variables/parameters. i.e. **not** (''$PATH''):
|
||
|
|
<code>
|
||
|
|
# WRONG!
|
||
|
|
echo "The first character of PATH is ${$PATH:0:1}"
|
||
|
|
|
||
|
|
# CORRECT
|
||
|
|
echo "The first character of PATH is ${PATH:0:1}"
|
||
|
|
</code>
|
||
|
|
|
||
|
|
Note that if you are using variables in [[syntax:arith_expr | arithmetic expressions]], then the bare **name** is allowed:
|
||
|
|
<code>
|
||
|
|
((a=$a+7)) # Add 7 to a
|
||
|
|
((a = a + 7)) # Add 7 to a. Identical to the previous command.
|
||
|
|
((a += 7)) # Add 7 to a. Identical to the previous command.
|
||
|
|
|
||
|
|
a=$((a+7)) # POSIX-compatible version of previous code.
|
||
|
|
</code>
|
||
|
|
|
||
|
|
|
||
|
|
Please see:
|
||
|
|
* [[syntax:words]]
|
||
|
|
* [[syntax:quoting]]
|
||
|
|
* [[syntax:expansion:wordsplit]]
|
||
|
|
* [[syntax:pe]]
|
||
|
|
|
||
|
|
==== Exporting ====
|
||
|
|
|
||
|
|
Exporting a variable means giving **newly created** (child-)processes a copy of that variable. It does **not** copy a variable created in a child process back to the parent process. The following example does **not** work, since the variable ''hello'' is set in a child process (the process you execute to start that script ''./script.sh''):
|
||
|
|
|
||
|
|
<code>
|
||
|
|
$ cat script.sh
|
||
|
|
export hello=world
|
||
|
|
|
||
|
|
$ ./script.sh
|
||
|
|
$ echo $hello
|
||
|
|
$
|
||
|
|
</code>
|
||
|
|
|
||
|
|
Exporting is one-way. The direction is from parent process to child process, not the reverse. The above example **will** work, when you don't execute the script, but include ("source") it:
|
||
|
|
<code>
|
||
|
|
$ source ./script.sh
|
||
|
|
$ echo $hello
|
||
|
|
world
|
||
|
|
$
|
||
|
|
</code>
|
||
|
|
In this case, the export command is of no use.
|
||
|
|
|
||
|
|
Please see:
|
||
|
|
* [[scripting:processtree]]
|
||
|
|
|
||
|
|
===== Exit codes =====
|
||
|
|
==== Reacting to exit codes ====
|
||
|
|
|
||
|
|
If you just want to react to an exit code, regardless of its specific value, you **don't need** to use ''$?'' in a test command like this:
|
||
|
|
|
||
|
|
<code bash>
|
||
|
|
grep ^root: /etc/passwd >/dev/null 2>&1
|
||
|
|
|
||
|
|
if [ $? -ne 0 ]; then
|
||
|
|
echo "root was not found - check the pub at the corner"
|
||
|
|
fi
|
||
|
|
</code>
|
||
|
|
|
||
|
|
This can be simplified to:
|
||
|
|
<code bash>
|
||
|
|
if ! grep ^root: /etc/passwd >/dev/null 2>&1; then
|
||
|
|
echo "root was not found - check the pub at the corner"
|
||
|
|
fi
|
||
|
|
</code>
|
||
|
|
|
||
|
|
Or, simpler yet:
|
||
|
|
<code bash>
|
||
|
|
grep ^root: /etc/passwd >/dev/null 2>&1 || echo "root was not found - check the pub at the corner"
|
||
|
|
</code>
|
||
|
|
|
||
|
|
If you need the specific value of ''$?'', there's no other choice. But if you need only a "true/false" exit indication, there's no need for ''$?''.
|
||
|
|
|
||
|
|
See also:
|
||
|
|
* [[scripting:basics#exit_codes | Exit codes]]
|
||
|
|
|
||
|
|
==== Output vs. Return Value ====
|
||
|
|
|
||
|
|
It's important to remember the different ways to run a child command, and whether you want the output, the return value, or neither.
|
||
|
|
|
||
|
|
When you want to run a command (or a pipeline) and save (or print) the **output**, whether as a string or an array, you use Bash's ''$(command)'' syntax:
|
||
|
|
<code>
|
||
|
|
$(ls -l /tmp)
|
||
|
|
newvariable=$(printf "foo")
|
||
|
|
</code>
|
||
|
|
|
||
|
|
When you want to use the **return value** of a command, just use the command, or add ( ) to run a command or pipeline in a subshell:
|
||
|
|
<code>
|
||
|
|
if grep someuser /etc/passwd ; then
|
||
|
|
# do something
|
||
|
|
fi
|
||
|
|
|
||
|
|
if ( w | grep someuser | grep sqlplus ) ; then
|
||
|
|
# someuser is logged in and running sqlplus
|
||
|
|
fi
|
||
|
|
</code>
|
||
|
|
|
||
|
|
Make sure you're using the form you intended:
|
||
|
|
<code>
|
||
|
|
# WRONG!
|
||
|
|
if $(grep ERROR /var/log/messages) ; then
|
||
|
|
# send alerts
|
||
|
|
fi
|
||
|
|
</code>
|
||
|
|
|
||
|
|
Please see:
|
||
|
|
* [[syntax:ccmd:intro]]
|
||
|
|
* [[syntax:expansion:cmdsubst]]
|
||
|
|
* [[syntax:ccmd:grouping_subshell]]</textarea>
|
||
|
|
<div id="wiki__editbar" class="editBar">
|
||
|
|
<div id="size__ctl">
|
||
|
|
</div>
|
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|
|
</div>
|
||
|
|
</div></form>
|
||
|
|
</div>
|
||
|
|
</div><!-- /content --></div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
<div class="small text-right">
|
||
|
|
|
||
|
|
<span class="docInfo">
|
||
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