move dir fundamentals to basics

basics/bash.md

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+# STIN, STOUT, STERR
+
+Input is 0, output is 1, error is 2.
+
+Pipe standard output to log.txt while also outputting it.
+
+> cat file.txt |& tee -a log.txt
+
+Copy file and *if* that's successful, delete it where it stands.
+
+> scp archive.tar.gz pi@192.168.0.31:/home/pi && rm archive.tar.gz
+
+A double pipe will try one, and do the other if that fails.
+
+> cp -r ~/Archive ~/Backup || tar czf Archive.tar.gz *
+
+# REGEX
+Regular expression characters include:
+
+\\ ^ $ . | ? * + () [] {}
+
+As a result, grep cannot read these characters as literal characters unless they are escaped.  E.g.
+
+> grep wtf\? log.txt
+
+... will search the string 'wtf?' in the file log.txt.  Another version is egrep (now used with 'grep -e') which uses more characters as special characters, or fgrep, which treats all characters as literal strings.
+
+# Environmental Variables
+PWD, USER, PATH
+
+To display all environmental (but not local) variables, use
+
+> env
+
+Set a variable with
+
+> colour=red
+
+Display your variable with
+
+> echo $colour
+
+Export this to the entire system using:
+
+> export colour=blue
+
+# Search commands
+
+> apropos cat
+
+# Working with Text
+
+Convert every tab to ten spaces.
+
+> expand -t 10 file.txt
+
+Or the reverse, with 3 spaces converting to a tab.
+
+> unexpand -t 3 file.txt
+
+Format a file by cutting text after 60 characters.
+
+> fmt -w 60 file.txt
+
+Indent all but the first line of a paragraph.
+
+> fmt -t file.txt
+
+Look at the new lines of a file only:
+
+> tail -f /var/log/syslog
+
+The sort function arranges lines alphabetically.  Use -r to reverse and -n to sort by number.
+
+# Sed
+
+> sed -i s/hey/hoi/g greetings.txt
+
+Edit all examples of hey to hoi in greetings and print that to the file.
+
+# Measurement
+Measure how long a script takes for super-autism powers.
+
+> time [bash script]
+
+# Functions
+
+> function my_funct(){ do_thing $1; }
+
+Remove a function with
+
+> unset my_function
+
+# Paths
+Every shell has various paths from where it can execute binary files.  Find out your current one with:
+
+> echo $PATH
+
+To add a directory to a path, e.g. /usr/share/bin, you can declare it in addition to the old path with:
+
+> PATH=$PATH:/usr/share/bin
+
+And then check it by echoing the path again.
+
+Before this, probably best to check the path exists with:
+
+> if [ -e /usr/share/bin ]; then
+
+> 	echo yes
+
+> fi
+ 
+# Pipes, Pedantry and Brackets
+
+Things that [[ ]] statements can do which [ ] statements cannot:
+
+- Intuitive and easy 'and' statements.
+	- [[ -z $var && -d ~/LK ]]
+- Intuitive and easy 'or' statements.
+	- [[ -d LK || -f ghost-backup.zip ]]
+- Simple expression comparisons
+	- [[ $v1 > $v2 ]]
+- Simple expression comparisons with clumsy strings
+	- [[ item-1 > item-2 ]]
+- Vague comparisons
+	- [[ $answer =~ ^y(es)?$ ]]
+
+# exec
+
+exec will start a process running as just that process.  In a bash script, the line:
+
+> unison rat
+
+... will startup `unison` as a sub-process of bash. But:
+
+> exec unison rat
+
+... starts unison as its own process.
+
+# Brace expansion
+
+> mv picture{,-1}.jpg
+
+This expands to 
+
+> mv picture.jpg picture-1.jpg
+
+# `for` Statements
+
+for f in *tiff;do
+	convert "$f" "${f/.tiff/.png}"
+done