Substring extraction (Bash)
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How to retrieve a substring, similar to the SQL command substring_index? In Sep. 2022, I ran into this problem for retrieving parts of the index of an associate array.
The problem
I use an associative array to emulate a multidimensional array. An example of two arrays:
# Assigning values ######################################## # imwiz[393,title]="Hello, world!" imwiz[393,name]="hello-world"
Inside a loop, this element of the array becomes available like this:
for i in "${!imwiz[@]}"
do
echo "Index (i): $i"
echo "Contents: ${imwiz[$i]}"
...
done
- How to efficiently extract the part until the comma from the index?
- How to efficiently extract the part after the comma from the index?
Inventory
- awk
- Bash parameter substitution - Preferred
- cut
- expr
- Regular expressions - icw. Bash
- sed
Parameter substitution example
string="US/Central - 10:26 PM (CST)"
etime="${string% [AP]M*}"
etime="${etime#* - }"
echo $etime
Leads to:
for i in
Nicerobot's regular expression example
I suspect that this is the best solution: No outside tools, no subshells, supposingly fast, and as an extra: The chaining is probably what I actually need. A quick test, showed that it indeed works (add a line echo $NUM). Now the details of this compact code:
FN=someletters_12345_moreleters.ext
[[ ${FN} =~ _([[:digit:]]{5})_ ]] && NUM=${BASH_REMATCH[1]}
echo $NUM
Details:
[ [...] ]: A condition=~: The condition will be a regular expression_([ [:digit:] ]{5})_: Regular expression that stores the matching string in a capture group (see below. Note that the string cannot be verbosely printed in MediaWiki like this)&&: Chains the commands: The following command will only be executed if the first command (the conditional command) was successfulNUM: Just the name of a variableBASH_REMATCH[1]: Capture group.
Current solution
########################################################################
# test_loop_through_databases()
########################################################################
#
test_loop_through_databases()
{
#
# * This is a "test function": It contains a lot of comments and
# maybe some alternative code. It's meant for learning, not for
# production
# * Keep this function: When I have to review this stuff in e.g.,
# 6 months, I might be really happy for it
# * By keeping this test code in a separate function, it isn't
# in the way of the production code (because it becomes quickly
# annoying to me, if production code contains comments that are
# obvious for me at that time)
# * When a row will be processed, use 'real' variables, not just
# "i"
#
#
echo ""; echo ""; echo "# test_loop_through_databases()"; echo ""
#
for i in ${!site[@]}
do
# Detect if we have a db entry
########################################
#
if [[ $i =~ db ]]; then
# Retrieve database name
########################################
#
site_db=${site[$i]}
echo " Database: ${site_db}"
# Retrieve url (=index)
########################################
#
# * Below, I want to retrieve the currency. To do so, I first
# need the first part of the index (=url)
# * I suspect that this code is quite essential in many
# operations around 'pseude multidimensional arrays'
#
site_index=$i
echo " site_index: $site_index"
site_url=${site_index%,*}
echo " site_url: $site_url"
# Retrieve currency
########################################
#
# * First compile a new (compounded) index for retrieving the
# currency
# * I'm surprised that I can have two parameter expansions
# in one expression. I thought that wasn't possible. This
# implies that I don't need to first explicitly create a
# new index
#
new_index=${i%,*},currency
echo " new_index: $new_index"
site_currency=${site[${site_url},currency]}
#
echo " Currency (direct): ${site[${i%,*},currency]}"
echo " Currency (new_index): ${site[$new_index]}"
echo " Currency (real vars): ${site[${site_url},currency]}"
echo " site_currency: $site_currency"
#
# Invoke update_prices_euro_site
########################################
#
# [[ ${site[${i%,*},currency]} = "eur" ]] && update_prices_euro_site
# [[ ${site[$site_url,currency]} = "eur" ]] && update_prices_euro_site
#
[[ $site_currency = "eur" ]] && update_prices_euro_site
fi
done
}