Principal Software Engineer/Architect, motorcyclist, drummer, and artist
Here is a quick snippet for checking whether or not a variable is a valid integer in BASH. It is also a howto for regular expressions in a shell script.
# Make sure that FOO is an integer
if [[ ! "$FOO" =~ ^[0-9]+$ ]]; then
echo "The FOO was NOT an integer"
fiLet’s say that you have a file and you would like to convert each line in the file to an element in an array.
The key to this is knowing about and how to manipulate the IFS (Internal Field Separator). The default IFS is whitespace (a space, tab, or newline) and if you create an array passing it a whitespace delimited list of strings, each token will be set to an element in the array.
ARRAY=(a b d Here is a quick one-liner for trimming a specific number of characters from the end of a string under bash:
# Remove the last 5 characters
$ echo "somestringwith12345" | sed "s/.....$//g"
$ somestringwith
# Remove the last 3 characters
$ echo "somestringwith12345" | sed "s/...$//g"
$ somestringwith12About a year ago I posted an article about how to split into an array of values based on a given delimiter in bash.
The following is how to take that same string and split it on the first occurrence of the same user defined delimiter.
Both use the ‘read’ command, but in a slightly different way.
Instead of passing read the -a [aname] parameter which tells it that “The words are assigned to sequential indices of the array → Continue reading “Splitting a String in Bash on the FIRST Occurrence of a Character”
Often times you will want to write a BASH script where you don’t want to have to keep track of all of the positional command line arguments and/or you might want to configure it with a set of environmental variables while having a default value for each in the script.
Following is the syntax for declaring them in the shell script, and then an example on how to invoke it.
#!/bin/bash
: ${ARG1:="somedefault_arg1"}
: ${ARG2:="10"}
echo "ARG1 = $ARG1"
echo A quick one-liner for counting the number of characters in a given field for each line in an ASCII file.
for i in `cat source.txt`; do echo $i | awk {'print length($1)}' >> counts.txtIf you want to write a HQL hive query and run it mulitple times from a shell script, each time passing it different data for the query, here is a quick example that should get you started.
The first thing to know is that by specifying n number of -hivevar key value pairs when invoking hive on the command line will allow you to pass that data into the hive process.
For example, if you do the following
$ hive If you have a long-running command on shell-script that you want to generate a beep upon completion on your PC running Linux do the following:
Make sure that the pcspkr module is loaded:
# modprobe pcspkrThen create a wrapper shell script that looks something like this:
#!/bin/bash
# Some long running command here . . .
echo -e '\a' /dev/consoleIf you would like to, in a shell script, dynamically generate SQL queries for MySQL and save the output of those queries to a variable that you can then use in the script, here is an example:
#!/bin/bash
for i in `cat tables_list.txt`
do
# Build the query
QUERY="SELECT count(*) FROM ${i}"
# Run the query from the command-line and save the
# output into the $ROW_COUNT variable
ROW_COUNT=$(echo $QUERY | mysql -u${USER_NAME} -p${PASSWORD} -h ${HOST} -P ${PORT} --skip-column-names ${DBASE})
Following is how to dynamically append (push) items to an array in a bash script.
declare -a ARRAY=()
ARRAY=("${ARRAY[@]}" "item1")
ARRAY=("${ARRAY[@]}" "item2")