Passing parameters to a Bash function
I am trying to search how to pass parameters in a Bash function, but what comes up is always how to pass parameter from the command line. I would like to pass parameters within my script. I tried:
myBackupFunction("..", ". ", "xx") function myBackupFunction($directory, $options, $rootPassword) «. but what comes up is always how to pass parameter from the command line» — Yes! That’s because Bash scripts are basically sequences of command lines — invoke a function in a Bash script exactly as if it was a command on the command line! 🙂 Your call would be myBackupFunction «..» «. » «xx»; no parenthesis, no commas.
7 Answers 7
There are two typical ways of declaring a function. I prefer the second approach.
To call a function with arguments:
The function refers to passed arguments by their position (not by name), that is $1 , $2 , and so forth. $0 is the name of the script itself.
Also, you need to call your function after it is declared.
#!/usr/bin/env sh foo 1 # this will fail because foo has not been declared yet. foo() < echo "Parameter #1 is $1" >foo 2 # this will work. ./myScript.sh: line 2: foo: command not found Parameter #1 is 2 Good answer. My 2 cents: in shell constructs that reside in a file that is sourced (dotted) when needed, I prefer to use the function keyword and the () . My goal (in a file, not command line) is to increase clarity, not reduce the number of characters typed, viz, function myBackupFunction() compound-statement .
@CMCDragonkai, the function keyword version is an extension; the other form works in all POSIX-compliant shells.
@RonBurk, perhaps — but even if we consider only clarity, the function keyword had guarantees in the old ksh-family shells that introduced it that modern bash don’t honor (in such shells, function made variables local-by-default; in bash, it does not). As such, its use decreases clarity to anyone who knows, and might expect, the ksh behavior. See wiki.bash-hackers.org/scripting/obsolete
Knowledge of high level programming languages (C/C++, Java, PHP, Python, Perl, etc.) would suggest to the layman that Bourne Again Shell (Bash) functions should work like they do in those other languages.
Instead, Bash functions work like shell commands and expect arguments to be passed to them in the same way one might pass an option to a shell command (e.g. ls -l ). In effect, function arguments in Bash are treated as positional parameters ( $1, $2..$9, $, $ , and so on). This is no surprise considering how getopts works. Do not use parentheses to call a function in Bash.
(Note: I happen to be working on OpenSolaris at the moment.)
# Bash style declaration for all you PHP/JavaScript junkies. :-) # $1 is the directory to archive # $2 is the name of the tar and zipped file when all is done. function backupWebRoot () < tar -cvf - "$1" | zip -n .jpg:.gif:.png "$2" - 2>> $errorlog && echo -e "\nTarball created!\n" > # sh style declaration for the purist in you. ;-) # $1 is the directory to archive # $2 is the name of the tar and zipped file when all is done. backupWebRoot () < tar -cvf - "$1" | zip -n .jpg:.gif:.png "$2" - 2>> $errorlog && echo -e "\nTarball created!\n" > # In the actual shell script # $0 $1 $2 backupWebRoot ~/public/www/ webSite.tar.zip Want to use names for variables? Just do something this.
local filename=$1 # The keyword declare can be used, but local is semantically more specific. Be careful, though. If an argument to a function has a space in it, you may want to do this instead! Otherwise, $1 might not be what you think it is.
local filename="$1" # Just to be on the safe side. Although, if $1 was an integer, then what? Is that even possible? Humm. Want to pass an array to a function by value?
callingSomeFunction "$" # Expands to all array elements. Inside the function, handle the arguments like this.
function callingSomeFunction ()
Need to pass a value and an array, but still use «$@» inside the function?
function linearSearch () < local myVar="$1" shift 1 # Removes $1 from the parameter list for value in "$@" # Represents the remaining parameters. do if [[ $value == $myVar ]] then echo -e "Found it!\t. after a while." return 0 fi done return 1 >linearSearch $someStringValue "$" In Bash 4.3 and above, you can pass an array to a function by reference by defining the parameter of a function with the -n option.
function callingSomeFunction () < local -n someArray=$1 # also $to make the parameter mandatory. for value in "$" # Nice! do : done > callingSomeFunction myArray # No $ in front of the argument. You pass by name, not expansion / value. The last example posted does not work as far as I can tell. I tried to run it on bash v5+ and it is just returning me the full array in the loop as opposed to each item
after testing again, I found that it was my error as I was declaring the array in line instead of declaring it before
@iomv Nonetheless, do be careful of the «circular variable reference» problem. Whatever name you declare the array as inside of the function, DO NOT name your array argument in the calling context / client code the same name. Notice how I changed the last example to help people avoid the «circular name reference» problem. Good call, even though you made an error on your own. 🙂
If you prefer named parameters, it’s possible (with a few tricks) to actually pass named parameters to functions (also makes it possible to pass arrays and references).
The method I developed allows you to define named parameters passed to a function like this:
function example < args : string firstName , string lastName , integer age > < echo "My name is $$ and I am $ years old." > You can also annotate arguments as @required or @readonly, create . rest arguments, create arrays from sequential arguments (using e.g. string[4] ) and optionally list the arguments in multiple lines:
function example < args : @required string firstName : string lastName : integer age : string[] . favoriteHobbies echo "My name is $$ and I am $ years old." echo "My favorite hobbies include: $" > In other words, not only you can call your parameters by their names (which makes up for a more readable core), you can actually pass arrays (and references to variables — this feature works only in Bash 4.3 though)! Plus, the mapped variables are all in the local scope, just as $1 (and others).
The code that makes this work is pretty light and works both in Bash 3 and Bash 4 (these are the only versions I’ve tested it with). If you’re interested in more tricks like this that make developing with bash much nicer and easier, you can take a look at my Bash Infinity Framework, the code below is available as one of its functionalities.
shopt -s expand_aliases function assignTrap < local evalString local -i paramIndex=$local initialCommand="$" if [[ "$initialCommand" != ":" ]] then echo "trap - DEBUG; eval \"$\"; unset __previousTrap; unset __paramIndex;" return fi while [[ "$" == "," || "$" == "$" ]] || [[ "$" -gt 0 && "$paramIndex" -eq 0 ]] do shift # First colon ":" or next parameter's comma "," paramIndex+=1 local -a decorators=() while [[ "$" == "@"* ]] do decorators+=( "$1" ) shift done local declaration= local wrapLeft='"' local wrapRight='"' local nextType="$1" local length=1 case $ in string | boolean) declaration="local " ;; integer) declaration="local -i" ;; reference) declaration="local -n" ;; arrayDeclaration) declaration="local -a"; wrapLeft= ; wrapRight= ;; assocDeclaration) declaration="local -A"; wrapLeft= ; wrapRight= ;; "string["*"]") declaration="local -a"; length="$" ;; "integer["*"]") declaration="local -ai"; length="$" ;; esac if [[ "$" != "" ]] then shift local nextName="$1" for decorator in "$" do case $ in @readonly) declaration+="r" ;; @required) evalString+="[[ ! -z \$$ ]] || echo \"Parameter '$nextName' ($nextType) is marked as required by '$' function.\"; " >&2 ;; @global) declaration+="g" ;; esac done local paramRange="$paramIndex" if [[ -z "$length" ]] then # . rest paramRange="" # trim leading . nextName="$" if [[ "$" -gt 1 ]] then echo "Unexpected arguments after a rest array ($nextName) in '$' function." >&2 fi elif [[ "$length" -gt 1 ]] then paramRange="" paramIndex+=$((length - 1)) fi evalString+="$ $=$\$$$; " # Continue to the next parameter: shift fi done echo "$ local -i __paramIndex=$;" > alias args='local __previousTrap=$(trap -p DEBUG); trap "eval \"\$(assignTrap \$BASH_COMMAND)\";" DEBUG;'