Category: Tech

Bash arrays

Bash arrays are a lot like Bash Associative Arrays, but with numbers as keys.

Here’s a quick reference.

Basics

$ declare -a MYARR  # Create an array
$ MYARR[3]=foo      # Put a value into an array
$ echo ${MYARR[3]}  # Get a value out of an array
foo
$ echo MYARR[3]     # WRONG
MYARR[0]
$ echo $MYARR[3]]   # WRONG
[3]

Creating, adding

$ declare -a MYARR    # Explicitly declare
$ MYARR[3]=foo        # Or this line implicitly makes it an array
$ MYARR[4]=bar        # Can add values one by one

$ declare -a MYARR=(a b c)   # Initialise all at once
$ echo ${MYARR[0]}
a
$ echo ${MYARR[1]}
b
$ echo ${MYARR[2]}
c

$ declare -a MYARR   # Or declare separately
$ MYARR=(a b c)      # Then initialise
$ echo ${MYARR[0]}
a
$ echo ${MYARR[1]}
b
$ echo ${MYARR[2]}
c

$ declare -a MYARR=(a b c)
$ MYARR=("${MYARR[@]}" d)  # Add an element
$ echo ${MYARR[@]}
a b c d
$ declare -a MYARR2=(e f g)
$ MYARR=("${MYARR[@]}" "${MYARR2[@]}")  # Concatenate arrays
$ echo ${MYARR[@]}
a b c d e f g

Keys/Indices

$ declare -a MYARR
$ MYARR[3]=foo
$ echo ${MYARR[0]}  # Unassigned values are empty

$ echo ${MYARR[4]}  # Unassigned values are empty

$ MYARR[seven]=bar     # A text index is treated as 0
$ echo ${MYARR[0]}
bar
$ echo ${MYARR[seven]} # A text index is treated as 0
bar

$ K=3
$ MYARR[$K]=baz      # Variables containing numbers work like numbers
$ echo ${MYARR[$K]}
baz
$ echo ${MYARR[3]}   # Obviously the value is accessible via the actual index
baz
$ K=foo
$ MYARR[$K]=bash     # Variables containing text are treated as 0
$ echo ${MYARR[0]}
bash

Length

$ declare -a MYARR=(a b c)
$ echo ${#MYARR[@]}  # Length of an array
3
$ echo $#MYARR[@]  # WRONG
0MYARR[@]
$ echo ${#MYARR}   # WRONG
1

$ MYARR[7]=x
$ echo ${#MYARR[@]}  # Only existing indices count in the length
4

$ declare -a MYARR=(a bb ccc)
$ echo ${#MYARR[0]}   # Length of an individual element
1
$ echo ${#MYARR[1]}
2
$ echo ${#MYARR[2]}
3

Looping

$ declare -a MYARR=("a 1" b c)
$ # Loop through array values
$ for V in "${MYARR[@]}"; do echo $V; done
a 1
b
c
$ for V in ${MYARR[@]}; do echo $V; done  #WRONG
a
1
b
c

$ echo "${!MYARR[@]}"  # Print all indices - quoted, but quotes removed by echo
0 1 2
$ echo "${MYARR[@]}"   # Print all values - quoted, but quotes removed by echo
a 1 b c

Clearing

$ declare -a MYARR
$ MYARR[3]=x

$ echo ${MYARR[3]}
x
$ unset MYARR
$ declare -a MYARR
$ echo ${MYARR[3]}

Deleting

$ MYARR[2]=foo
$ echo ${MYARR[2]}
foo
$ unset ${MYARR[2]} # WRONG
$ echo ${MYARR[2]}
foo
$ unset MYARR[2]    # To delete from an array, use "unset" with similar syntax to assigning
$ echo ${MYARR[2]}

$ MYARR[3]=quux
$ echo ${MYARR[3]}
quux
$ K=3
$ unset MYARR[$K]   # Can unset using a variable for the key too
$ echo ${MYARR[3]}


$ declare -a MYARR=(a b c d e f)
$ MYARR=("${MYARR[@]:0:3}" "${MYARR[@]:4}")  # Remove element 3, leaving no gap
$ echo ${MYARR[@]}

Cool stuff

$ declare -a MYARR=(a b c d e f g)
$ echo ${MYARR[@]:2:3}              # Extract a sub-array
c d e

$ declare -a MYARR=(a b c d e f g)
$ echo ${MYARR[@]/d/FOO}            # Replace elements that match
a b c FOO e f g

Scope

$ unset MYARR
$ function createmap() { MYARR[5]=bar; }  # Implicit creation puts it in the global scope
$ echo ${MYARR[5]}

$ createmap
$ echo ${MYARR[5]}
bar

$ unset MYARR
$ function createmaplocal() { declare -a MYARR; MYARR[3]=bar; }  # Explicit creation puts it in the local scope
$ echo ${MYARR[3]}

$ createmaplocal
$ echo ${MYARR[3]}

Links

Using if:set, unless:set etc. with macrodefs in Ant

In a previous post I outlined how to use if:set and unless:set to execute tasks conditionally in Ant 1.9.1.

Unfortunately, this does not work with macrodefs. When I try to execute a macrodef conditionally like this:

<project
    xmlns:if="ant:if"
    xmlns:unless="ant:unless"
    default="build"
>

    <macrodef name="mymacro">
        <sequential>
            <echo message="inside mymacro"/>
        </sequential>
    </macrodef>

    <target name="build">
        <mymacro if:set="doit"/>
    </target>

</project>

When I set the “doit” property and run like this, it fails:

$ ant -Ddoit=true
Buildfile: build.xml

build:

BUILD FAILED
build.xml:14: Unknown attribute [ant:if:if:set]

Total time: 0 seconds

It looks to me like this is a bug: the if:set attribute is getting passed into the macro, which is complaining that it doesn’t expect an attribute with that name. (If you try to create an attribute with that name, you’ll find that “if:set” is an illegal name…)

However, there is a workaround. You can wrap the call to your macrodef in a <sequential> tag:

<project
    xmlns:if="ant:if"
    xmlns:unless="ant:unless"
    default="build"
>

    <macrodef name="mymacro">
        <sequential>
            <echo message="inside mymacro"/>
        </sequential>
    </macrodef>

    <target name="build">
        <sequential if:set="doit">
            <mymacro/>
        </sequential>
    </target>

</project>

And now it works!

$ ant -Ddoit=true
Buildfile: build.xml

build:
     [echo] inside mymacro

BUILD SUCCESSFUL
Total time: 0 seconds

$ ant
Buildfile: build.xml

build:

BUILD SUCCESSFUL
Total time: 0 seconds

Conditional execution even when calling a macrodef. Enjoy.