So I'm trying to do something, not sure if it's possible. I have the following code:
for i in {0..5}; do
if [[ -f ./user$i ]]; then
group$i=$(grep -w "group" ./user0|awk '{print $2}'|perl -lape 's/\s+//sg')
What I want to do is assign a unique variable for each instance of the {0..5} so group1 group2 group3 group4 for each variable name. Then I would change ./user0 to ./user$i and create a dynamic list of variables based on my sequence. Is this possible? I get the following error when trying to execute this and I'm unsure of what I have actually done that bash doesn't like.
test.sh: line 16: group0=j: command not found
Kurt Stutsman provides the right pointer in a comment on the question: use Bash arrays to solve your problem.
Here's a simplified example:
groups=() # declare an empty array; same as: declare -a groups
for i in {0..5}; do
groups[i]="group $i" # dynamically create element with index $i
done
# Print the resulting array's elements.
printf '%s\n' "${groups[@]}"
See the bottom of this answer for other ways to enumerate the elements of array ${groups[@]}
.
bash
arrays can be dynamically expanded (and can even be sparse - element indices need not be contiguous)
$i
works, without prior sizing of the array.Note how $i
need not be prefixed with $
in the array subscript, because array subscripts are evaluated in an arithmetic context (the same context in which $(( ... ))
expressions are evaluated).
As for what you did wrong:
group$i=...
is not recognized as a variable assignment by Bash, because - taken literally - group$i
is not a valid identifier (variable name).
Because it isn't, Bash continues to parse until the next shell metacharacter is found, and then interprets the resulting word as a command to execute, which in your case resulted in error message group0=j: command not found
.
If, for some reason, you don't want to use arrays to avoid this problem entirely, you can work around the problem:
By involving a variable-declaring builtin [command] such as declare
, local
, or export
, you force Bash to perform expansions first, which expands group$i
to a valid variable name before passing it to the builtin.
user2683246's answer demonstrates the next best approach by using declare
(or, if local variables inside a function are desired, local
) to create the variables.
Soren's answer uses export
, but that is only advisable if you want to create environment variables visible to child processes rather than mere shell variables.
Caveat: With this technique, be sure to double-quote the RHS in order to capture the full value; to illustrate:
i=0; declare v$i=$(echo 'hi, there'); echo "$v0" # !! WRONG -> 'hi,': only UP TO 1ST SPACE
i=0; declare v$i="$(echo 'hi, there')"; echo "$v0" # OK -> 'hi, there'
Other ways to enumerate the groups
array created above:
# Enumerate array elements directly.
for element in "${groups[@]}"; do
echo "$element"
done
# Enumerate array elements by index.
for (( i = 0; i < ${#groups[@]}; i++ )); do
echo "#$i: ${groups[i]}"
done
Use declare group$i=...
instead of just group$i=...
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