Principal Software Engineer/Architect, motorcyclist, drummer, and artist
If you would like to join an array of elements with a defined delimiter in Bash there is an easy way to go about it by using printf. Following is an example
#!/bin/bash
declare -a arr=()
for i in `seq 1 5`
do
arr=("${arr[@]}" $i)
done
# Generate a single string joined by a comma. The printf string can contain
# any arbitrary delimiter.
printf -v joined '%s,' "${arr[@]}"
# Print out the string minus the trailing comma
echo The GNU diff command on most Linux and UNIX systems will diff the contents of two files. With Bash, you can, using process substitution, take the output of any arbitrary command and process its input, or output, as a file descriptor. In this way, you can then use diff against the output of two commands as follows
diff <(cmd1) <(cmd2)Both cmd1 and cmd2 will appear as a file name/file descriptor. The < character indicates that the file descriptor should → Continue reading “Diffing the output of two commands”
If you want to dynamically define and export variable names in Bash here is the TLDR;
# Define the name of the variable
key="my-dynamic-var-name"
# Declare it and export it
declare -gx "$key"="some-value"To then access that value via a dynamically generated variable name
# Create a variable that contains the variable name
var_to_access="my-dynamic-var-name"
# Read the value
my_var_value=${!var_to_access}Read the man page for declare for more details and read this article for a really good explanation and further examples.→ Continue reading “Declaring, Exporting, and Reading Dynamic Variables in Bash”
Sometimes you will want to ensure that a file is sourced instead of executed. This ensures, among other things, that any environment variables that the script defines remain in your current shell after the script completes.
To do so, use the following to check whether the file was sourced or run in a sub-shell
(return 0 2/dev/null) && sourced=1 || sourced=0
echo "sourced=$sourced"Bash allows return statements only from functions and in a scripts top level scope IF it → Continue reading “How to check if a file is sourced in Bash”
I have no idea why, but for some reason I always have a hard time remembering the exact syntax for find when I want to prune some list of directories from a search.
Let’s say that you want to execute a find in a directory where there are a lot of .git directories and you don’t want to search through the guts of the repo directories. With the following command we specify the prune predicate ahead of the search for → Continue reading “Pruning directories from find”
The TLDR; is to first use tr to replace all occurrences of any horizontal whitespace character with a single space, and then squeeze down any number of spaces to a single space and then define the delimiter for cut as a single space. The following example assumes that you want to see from the 5th column to the end of the line.
<do-something-to-generate-input| tr '[:blank:]' ' ' | tr -s ' ' | cut -d ' ' -f5-The → Continue reading “Using cut with a delimiter of any amount of whitespace”
I recently learned about the Bash built-in fc. It is a great tool that enables you to edit and re-execute commands from your bash history.
Oftentimes there is a command in your history that instead of just grepping through the history and then re-executing as-is you’ll want to make a modification or two. With fc you can first edit it in your favorite editor and then when closing the editor fc will execute the command.
For me, vim is my → Continue reading “Using fc to Edit and Re-execute Bash Commands”
I cannot take credit for figuring this one out. Original post is here.
TLDR; is to add the following to your ~/.bashrc
export PROMPT_COMMAND='history -a'Following are the history configs that I use
######################################################################
shopt -s histappend
HISTSIZE=-1
HISTFILESIZE=-1
HISTCONTROL=ignoreboth
HISTTIMEFORMAT="[%F %T] "
export PROMPT_COMMAND='history -a'
######################################################################In general, I have moved to writing all of my applications to write their log output to STDOUT. This makes running them on the command line, in an IDE, on a bare metal box, VM, or in a container completely decoupled from how you store and view the logs. No more having multiple logging configs for each flavor of deployment.
In this particular case, I am running an application in a container (but it isn’t necessary that it is in → Continue reading “Configuring rsyslog to rotate log files from log messages streamed to it from a Systemd service”
If you want to just redirect STDERR to tee, run the following command.
some-command 2> >(tee /var/tmp/output)