# Bash

The Bourne Again SHell is common on GNU/Linux systems and it is worth knowing a bit about it as it can be leveraged to great advantage with just a little knowledge.

# General Syntax

==== For loops ====

```bash
# List the file names
for i in $( ls ); do
     echo item: $i
done
# Loop over range of numbers
for i in `seq 0 9`; do
    echo $1
done
```

## Default Variables

Bash has many default variables, here are some

| Variable | Description     |
| `"${BASH_SOURCE[0]}"` | The directory from which a script is being called |

## Parsing Arguments


## Arrays

You must `declare` a variable to be an array, after doing so you can assign items to an array and subsequently access elements by index or loop over them...

```bash
declare -a AN_ARRAY
AN_ARRAY=( element1 element2 element3 element4 element5 )
# Indexing starts at zero
AN_ARRAY[0] 

# Loop over elements
for ELEMENT in  "${AN_ARRAY[@]}"; do
    echo $ELEMENT
done
```

# Archiving

Create an archive with the current date (or time if you choose to format it as such) using...

```bash
tar cjvf archive-$(date +"%Y-%m-%d").tar.bz2 *
```


# Script Tricks
## Insert date

You can insert a date in-line by using the following with the desired format...

```bash
$(date +"%Y-%m-%d") 
$(date +"%F") # equivalent to above
$(date +"$s") # seconds since epoch
$(date +"%Y-%m-%d-%H:%M:%S")
$(date +"%F-%X") # equivalent to above
```
## Make a directory for every file

Make a directory for every file

You can use [variable substitution](http://www.gnu.org/software/bash/manual/bash.html#Shell-Parameter-Expansion) to quickly make a directory for every file in a given location (from [here](https://unix.stackexchange.com/a/220026/39149)).

```bash
  cd path/to/directory
  FILE_EXT="txt"
  for x in *.txt; do
    mkdir "${x%\.txt}" && mv "$x" "${x%\.}"
  done
```

## Batch rename files

This is easy if you actually use ZSH rather than Bash courtesy of [zmv](https://blog.smittytone.net/2021/04/03/how-to-use-zmv-z-shell-super-smart-file-renamer/).

## Multi-character arguments

See [here](https://unix.stackexchange.com/a/353639/39149)

## xarg

* [Command-Line Finding and Filtering — Sympolymathesy, by Chris Krycho](https://v5.chriskrycho.com/journal/follow-up-on-command-line-finding-and-filtering/)

## Options

[Small getopts tutorial Bash Hackers Wiki](https://wiki.bash-hackers.org/howto/getopts_tutorial)

## My scripts

  * [[bash:scripts:aws_exports|aws_exports.sh]]
  * [[bash:scripts:aws_exports_parallel|aws_exports_parallel.sh]]


# Links 

  * [Bash-hackers.org](http://wiki.bash-hackers.org/)