Converting a Bash array into a delimited string

Question

I would like to know the following;

1. Why the given non-working example doesn't work.
2. 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.

Answer 1

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")  echo ${ids[*]// /|} a|b c|d  IFS='|';echo "${ids[*]}";IFS=$' \t\n' a b|c d 

... and finally:

IFS='|';echo "${ids[*]// /|}";IFS=$' \t\n' a|b|c|d 

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 ``<space><tab><newline>''. 

Playing with $IFS:

declare -p IFS declare -- IFS="  " printf "%q\n" "$IFS" $' \t\n' 

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.

Answer 2

You can use printf too, without any external commands or the need to manipulate IFS:

ids=(1 2 3 4)                     # create array printf -v ids_d '|%s' "${ids[@]}" # yields "|1|2|3|4" ids_d=${ids_d:1}                  # remove the leading '|'