====== Beginner Mistakes ====== {{keywords>bash shell scripting pitfalls traps beginners}} Here are some typical traps: ===== Script execution ===== ==== Your perfect Bash script executes with syntax errors ==== 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: /home/user/bin/test ===== Globbing ===== ==== Brace expansion is not globbing ==== The following command line is not related to globbing (filename expansion): # YOU EXPECT # -i1.vob -i2.vob -i3.vob .... echo -i{*.vob,} # YOU GET # -i*.vob -i **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! # THIS IS WRONG! $myvar="Hello world!" 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... ="Hello world!" ...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: $picture" === Whitespace === Putting spaces on either or both sides of the equal-sign (''='') when assigning a value to a variable **will** fail. # INCORRECT 1 example = Hello # INCORRECT 2 example= Hello # INCORRECT 3 example =Hello The only valid form is **no spaces between the variable name and assigned value**: # CORRECT 1 example=Hello # CORRECT 2 example=" Hello" ==== Expanding (using) variables ==== A typical beginner's trap is quoting. As noted above, when you want to **expand** a variable i.e. "get the content", the variable name needs to be prefixed with a dollar-sign. But, since Bash knows various ways to quote and does word-splitting, the result isn't always the same. Let's define an example variable containing text with spaces: example="Hello world" ^Used form^result^number of words^ |''$example'' |''Hello world''|2| |''"$example"'' |''Hello world''|1| |''\$example'' |''$example''|1| |'''$example''' |''$example''|1| If you use parameter expansion, you **must** use the **name** (''PATH'') of the referenced variables/parameters. i.e. **not** (''$PATH''): # WRONG! echo "The first character of PATH is ${$PATH:0:1}" # CORRECT echo "The first character of PATH is ${PATH:0:1}" Note that if you are using variables in [[syntax:arith_expr | arithmetic expressions]], then the bare **name** is allowed: ((a=$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. 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''): $ cat script.sh export hello=world $ ./script.sh $ echo $hello $ 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: $ source ./script.sh $ echo $hello world $ 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: grep ^root: /etc/passwd >/dev/null 2>&1 if [ $? -ne 0 ]; then echo "root was not found - check the pub at the corner" fi This can be simplified to: if ! grep ^root: /etc/passwd >/dev/null 2>&1; then echo "root was not found - check the pub at the corner" fi Or, simpler yet: grep ^root: /etc/passwd >/dev/null 2>&1 || echo "root was not found - check the pub at the corner" If you need the specific value of ''$?'', 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: $(ls -l /tmp) newvariable=$(printf "foo") 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: if grep someuser /etc/passwd ; then # do something fi if ( w | grep someuser | grep sqlplus ) ; then # someuser is logged in and running sqlplus fi Make sure you're using the form you intended: # WRONG! if $(grep ERROR /var/log/messages) ; then # send alerts fi Please see: * [[syntax:ccmd:intro]] * [[syntax:expansion:cmdsubst]]