The bash shell#

These are programming notes for programming in the bash shell.

---

bash startup#

~/.bashrc	executed when a new shell is started
~/.profile	executed when the user logs in
/etc/profile.d/*	executed when any user logs in

Programs such as Netbeans are not able to see environment variables set in .bashrc as it is run after the environment Netbeans is started in has been constructed. Environment variables set in .profile can be seen by programs started after login.

You must logout and back in in order for changes to .profile or /etc/profile.d/*to take affect.

---

Redirecting IO#

2>file		send stderr to file

2>&1		send stderr to stdout

---

Accessing shell arguments#

$0	command name

$1	first argument

$@	all arguments (except $0)

$#	number of arguments

shift	shift arguments

"$@"    passing all arguments to another shell or program
        Use with double quotes.  If you do not do it this way,
        arguments with spaces will not be handled correctly.

$?      return code of last executed command

---

Quotes#

$ variables do not get evaluated within single quotes.

$ variables get evaluated within double quotes.

---

List iteration#

(In this example, through all the arguments)

for var in $@; do
    echo $var
done

for file in *.c; do
    echo $file
done

---

While loop with math#

m=10
i=0
while [ $i -lt $m ]; do
	echo $i
	let i+=1
done

---

Case statement#

case $1 in
	xxx)
		echo 1
		echo 2
		;;
	yyy)
		echo 3
		echo 4
		;;
	*) 
		echo Unknown argument $1
		;;
esac

---

Variable assignment#

var=val      no space around =

---

For loops#

for ((i=1 ; i < 5 ; i++)); do
    echo $i
done

---

Arrays#

Start from 0

a[3]=val	assignment

${a[3]}		get value

a=(val1 val2 val3)	make array

${#a[@]}	number of elements

${a[@]}		all elements as a list

---

Conditionals#

if [ condition ]; then
   ....
elif [ condition ]; then
   ....
else
   ....
fi

---

Conditions#

-f file		if file exists
-d dir		if dir exists
a = b		string equal
a != b		string not equal
-z x		string is null
x		string is not null
x -eq y		x numerically equal to y
x -ne y		x numerically not equal to y
x -gt y		x greater than y
x -lt y		x less than y
x -ge y		x greater than or equal to y
x -le y		x less than or equal to y
! cond		not condition

if [ cond ] &&  [ cond ]; then
   ...
fi
if [ cond ] ||  [ cond ]; then
   ...
fi

---

Here documents - inline input#

cmd << label
..
..
..
label

all lines up to label are input into command

---

Getting user input#

echo This program .....
while true; do
    read -p "Is that what you with to do?  " yn
    case $yn in
        [Yy]* ) break;;
        [Nn]* ) exit;;
        * ) echo "Please answer yes or no.";;
    esac
done

---

Setting command line editing keyboard configuration#

set -o emacs
set -o vi