Converting a Bash array into a delimited string

Question

I would like to know the following; - Why the given non-working example doesn't work. - If there are any other cleaner methods than those given in working example. Non-working example ``` > ids=(1 2 3 4);echo ${ids[*]// /|} 1 2 3 4 > ids=(1 2 3 4);echo ${${ids[*]}// /|} -bash: ${${ids[*]}// /|}: bad substitution > ids=(1 2 3 4);echo ${"${ids[*]}"// /|} -bash: ${"${ids[*]}"// /|}: bad substitution ``` Working example ``` > ids=(1 2 3 4);id="${ids[@]}";echo ${id// /|} 1|2|3|4 > ids=(1 2 3 4); lst=$( IFS='|'; echo "${ids[*]}" ); echo $lst 1|2|3|4 ``` In context, the delimited string to be used in a sed command for further parsing.

Accepted Answer

Array to strings in `bash` 1. Array to string, by using `$IFS` Because parentheses are used to delimit an array, not a string: ``` ids="1 2 3 4";echo ${ids// /|} 1|2|3|4 ``` Some samples: Populating `$ids` with two strings: `a b` and `c d` ``` ids=("a b" "c d" e\ f) echo ${ids[*]// /|} a|b c|d e|f IFS='|';echo "${ids[*]}";IFS=$' ' a b|c d|e f ``` ... and finally: ``` IFS='|';echo "${ids[*]// /|}";IFS=$' ' a|b|c|d|e|f ``` Where array is assembled, separated by 1st char of `$IFS`, but with space replaced by `|` in each element of array. When you do: ``` id="${ids[@]}" ``` you transfer the string build from the merging of the array `ids` by a space to a new variable of type string. Note: when `"${ids[@]}"` give a space-separated string, `"${ids[*]}"` (with a star `*` instead of the at sign `@`) will render a string separated by the first character of `$IFS`. what `man bash` says: ``` man -Len -Pcol\ -b bash | sed -ne '/^ *IFS /{N;N;p;q}' IFS The Internal Field Separator that is used for word splitting after expansion and to split lines into words with the read builtin command. The default value is ``''. ``` Playing with `$IFS`: ``` printf "%q " "$IFS" $' ' ``` Literally a `space`, a `tabulation` and (meaning or) a `line-feed`. So, while the first character is a space. the use of `*` will do the same as `@`. But: ``` { IFS=: read -a array < <(echo root:x:0:0:root:/root:/bin/bash) echo 1 "${array[@]}" echo 2 "${array[*]}" OIFS="$IFS" IFS=: echo 3 "${array[@]}" echo 4 "${array[*]}" IFS="$OIFS" } 1 root x 0 0 root /root /bin/bash 2 root x 0 0 root /root /bin/bash 3 root x 0 0 root /root /bin/bash 4 root:x:0:0:root:/root:/bin/bash ``` Note: The line `IFS=: read -a array < <(...)` will use `:` as separator, without setting `$IFS` permanently. This is because output line `#2` present spaces as separators. 1.1 Using function, localize `$IFS` To just print array ``` printArry() { local IFS="$1" shift echo "$*" } printArry @ "${ids[@]}" a b@c d@e f ``` Or to merge array in place. ``` mergeArry() { local IFS="$1" local -n _array_to_merge=$2 _array_to_merge=("${_array_to_merge[*]}") } declare -p ids declare -a ids=([0]="a b" [1]="c d" [2]="e f") mergeArry '#' ids declare -p ids declare -a ids=([0]="a b#c d#e f") ``` 2. Array of strings to array of strings (`[@]` vs `[*]`) There is a notable difference between: - `"$@"` and `"${var[@]}"` result in an array of strings - `"$*"` and `"${var[*]}"` result in an unique string Read carefully: `man '-Pless +/Special\ Parameters' bash` For this, I will quote each argument in order to not be splitted by `$IFS` at command line expansion, using double-quotes to permit variable expansion. ``` ids=('a b c' 'd e f' 'g h i') ``` ``` printf '<< %s >> ' "${ids[@]// /|}" ``` ``` << a|b|c >> << d|e|f >> << g|h|i >> ``` ``` printf '<< %s >> ' "${ids[*]// /|}" ``` ``` << a|b|c d|e|f g|h|i >> ``` Where: - All spaces where replaced by pipes, in each strings - All string where merged into one string, separated by 1st `$IFS` character. ``` ( IFS='@'; printf '<< %s >> ' "${ids[*]// /|}" ) << a|b|c@d|e|f@g|h|i >> ``` Note: `${var// /something}` will replace every spaces by something, but `${var[*]}` will merge array by using only one 1st character: ``` ( IFS='FOO'; printf '<< %s >> ' "${ids[*]// / BAR }" ) << a BAR b BAR cFd BAR e BAR fFg BAR h BAR i >> ``` And yes: by using `${var// / ... }`, you could replace 1 space by anthing you want, including more spaces. 3. Array to string, by using `printf` As we see, using `$IFS` is limited to only 1 character. If you need to use more characters to be inserted between your fields. You have to use `printf`: ``` ids=("a b" "c d" e\ f) sep=" long separator " printf -v string "%s$sep" "${ids[@]}" echo "${string%$sep}" a b long separator c d long separator e f ``` Note: this syntax work but is something limited, see further! 3.1 Array to string, by using `printf`, into a function In order to support special characters as `%` or `*` in `separator`, the function have to prevents - `%` to be interpreted by `printf` (`printf '%%'` will render a `%`) and - `*` to be interpreted by parameter expansion, for this `$sep` have to be double quoted. ``` printArry() { local sep=$1 string shift printf -v string "%s${sep//%/%%}" "$@" echo "${string%"$sep"}" } ``` ``` printArry ' long separator ' "${ids[@]}" a b long separator c d long separator e f printArry '*' "${ids[@]}" a b*c d*e f printArry '%' "${ids[@]}" a b%c d%e f ``` Or to merge array in place. ``` mergeArry() { local sep=$1 string local -n _array_to_merge=$2 printf -v string "%s${sep//%/%%}" "${_array_to_merge[@]}" _array_to_merge=("${string%"$sep"}") } mergeArry ' another separator ' ids declare -p ids declare -a ids=([0]="a b another separator c d another separator e f") i…

Converting a Bash array into a delimited string

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