CODEWITHSUNDEEP
bash
Robottinosino
Robottinosino

Reputation: 10882

Bash split string on delimiter, assign segments to array

In , I would like to transform a PATH-like environment variable that may contain space-separated elements into an array, making sure elements bearing spaces do not cause word-splitting, appearing as "multiple elements".

Let PATH_VARIABLE be the variable in question.

Let un:dodecaedro:per:tirare:per:i danni be the content of the variable.

It is intended for the desired array _to have 6 elements, not 7.

0) un
1) dodecaedro
2) per
3) tirare
4) per
5) i danni

The "tricky" entry may be the space-separated value: i danni.

I am looking for the absolute most elegant and correct way to achieve this.

Limitation: it must work with my version: v3.2.48(1)-release

In this is done just beautifully as so:

>>> v='un:dodecaedro:per:tirare:per:i danni'
>>> len(v.split(':'))
6

Works. Shows what I am looking for.

What's the best way to do this in our beloved ?

Can you specifically improve on my attempt 4?

Here my attempts

#!/bin/bash

PATH_VARIABLE='un:dodecaedro:per:tirare:per:i danni'

# WRONG
a1=($(echo $PATH_VARIABLE | tr ':' '\n'))

# WRONG
a2=($(
  while read path_component; do
  echo "$path_component"
  done < <(echo "$PATH_VARIABLE" | tr ':' '\n')
))

# WORKS, it is elegant.. but I have no bash 4!
# readarray -t a3 < <(echo "$PATH_VARIABLE" | tr ':' '\n')

# WORKS, but it looks "clunky" to me :(
i=0
while read line; do
  a4[i++]=$line
done < <(echo "$PATH_VARIABLE" | tr ':' '\n')

n=${#a4[@]}
for ((i=0; i < n; i++)); do
  printf '%2d) %s\n' "$i" "${a4[i]}"
done

My environment

v3.2.48(1)-release

OS X v10.8.3 (build 12D78)

Upvotes: 14

Views: 21249

Answers (3)

kojiro
kojiro

Reputation: 77107

f() {
  local IFS=:
  local foo
  set -f # Disable glob expansion
  foo=( $@ ) # Deliberately unquoted 
  set +f
  printf '%d\n' "${#foo[@]}"
  printf '%s\n' "${foo[@]}"
}

f 'un:dodecaedro:per:tirare:per:i danni'
6
un
dodecaedro
per
tirare
per
i danni

Modifying Jim McNamara's answer, you could just reset IFS:

oIFS="$IFS"
foo='un:dodecaedro:per:tirare:per:i danni'
IFS=: arr=( $foo )
IFS="$oIFS"

I prefer the function scope because it protects IFS changes from bleeding into the global scope without requiring special care to reset it.

Edits and explanations:

As a matter of clarification: In the second example, the IFS setting does change the global variable. The salient difference between this:

IFS=: arr=( $foo )

and this:

IFS=: read -a arr <<< "$foo"

is that the former is two variable assignments and no commands, and the latter is a simple command (see simple command in man (1) bash.)

Demonstration:

$ echo "$BASH_VERSION"
3.2.48(1)-release
$ echo "$IFS"


$ foo='un:dodecaedro:per:tirare:per:i danni'
$ IFS=: read -a arr <<< "$foo"
$ echo "${#arr[@]}"
6
$ echo "$IFS"


$ IFS=: arr1=( $foo )
$ echo "${#arr1[@]}"
6
$ echo "$IFS"
:

Upvotes: 9

ormaaj
ormaaj

Reputation: 6577

# Right. Add -d '' if PATH members may contain newlines.
IFS=: read -ra myPath <<<"$PATH"

# Wrong!
IFS=: myPath=($PATH)

# Wrong!
IFS=:
for x in $PATH; do ...

# How to do it wrong right...
# Works around some but not all word split problems
# For portability, some extra wrappers are needed and it's even harder.
function stupidSplit {
    if [[ -z $3 ]]; then
        return 1
    elif [[ $- != *f* ]]; then
        trap 'trap RETURN; set +f' RETURN
        set -f
    fi
    IFS=$3 command eval "${1}=(\$${2})"
}

function main {
    typeset -a myPath
    if ! stupidSplit myPath PATH :; then
        echo "Don't pass stupid stuff to stupidSplit" >&2
        return 1
    fi
}

main

Rule #1: Don't cram a compound data structure into a string or stream unless there's no alternative. PATH is one case where you have to deal with it.

Rule #2: Avoid word / field splitting at all costs. There are almost no legitimate reasons to apply word splitting on the value of a parameter in non-minimalist shells such as Bash. Almost all beginner pitfalls can be avoided by just never word splitting with IFS. Always quote.

Upvotes: 7

jim mcnamara
jim mcnamara

Reputation: 16379

Consider:

$ foo='1:2 3:4 5:6'
$ IFS=':'; arr=($foo)
$ echo "${arr[0]}"
1
$ echo "${arr[1]}"
2 3
$ echo "${arr[2]}"
4 5
$ echo "${arr[3]}"
6

Oh well - took me too long to format an answer... +1 @kojiro.

Upvotes: 6

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