Bash

Bash Guide

Using Bash

Setting the default shell: http://www.unix.com/shell-programming-scripting/32664-how-change-default-shell-linux.html

Startup Process

The order for sourcing dot files in bash is as follows:

Once per login session:

1. source /etc/profile - this contains the system-wide profile settings; these are only executed once per login session, and should define any bash variables that need to be defined for everyone; examples include the ${PATH}, the ${MANPATH}, the machine name, and/or enabling things that are not bash-specific (remember the Csh shell will also source .profile).

2. source ~/.profile - this contains user-specific profile settings; these settings are the same as above, except they are variables that should only be set for one user. The user's ${PATH} and ${MANPATH} variables, in particular, should be set here.

See my dotprofile

3. source ~/.bash_profile - (?) This one I'm not so sure about, it's not a "standard" bash file to be sourced. Plus its role is a little vague. I use this to define settings for my bash history.

See my dotbash_profile

4. source /etc/bashrc - sources the system-wide bashrc file. This will define the default appearance of the command prompt, any aliases, or any settings related to the bash history that should apply to every user.

5. source ~/.bashrc - sources the user-specific bashrc file. This contains user-specific settings for the prompt, aliases, functions, etc.

See my dotbashrc

Bash Scripting

Looping

Looping in bash is really simple, and convenient - you can use other unix commands (most obviously "ls") to create lists, and then loop over each element of those lists. For example, if I want to print out the name of every file in my home directory (granted, not very useful, but this is just an example), I can do this:

for i in `/bin/ls -1 $HOME`; do
    echo $i
done

To do some serious bash looping kung-fu, check out Xargs.

Logical Operators

NOTE: the use of double brackets for logical condition checks is highly recommended.

There are several logical operators available for checking conditions in bash.

These can be tested using a simple "if" statement. In bash, if statements are of the form:

if [[ condition ]]; then
    cmd
else
    cmd2
fi

The spaces between the brackets and the condition are essential.

Integer Operators

Operator	Meaning	Example
-eq	returns true if arguments are equal	$ a=1; b=1; if [[ "$a" -eq "$b" ]]; then echo "true"; else echo "false"; fi true $ a=1; b=2; if [[ "$a" -eq "$b" ]]; then echo "true"; else echo "false"; fi false
-ne	returns true if arguments are not equal	$ a=1; b=1; if [[ "$a" -ne "$b" ]]; then echo "true"; else echo "false"; fi false $ a=1; b=2; if [[ "$a" -ne "$b" ]]; then echo "true"; else echo "false"; fi true
-gt	returns true if argument 1 is greater than argument 2	$ a=5; b=10; if [[ "$a" -gt "$b" ]]; then echo "true"; else echo "false"; fi false
-ge	returns true if argument 1 is greater than or equal to argument 2	$ a=15; b=10; if [[ "$a" -ge "$b" ]]; then echo "true"; else echo "false"; fi true $ a=10; b=10; if [[ "$a" -ge "$b" ]]; then echo "true"; else echo "false"; fi true
-lt	returns true if argument 1 less than argument 2	$ a=5; b=10; if [[ "$a" -lt "$b" ]]; then echo "true"; else echo "false"; fi true
-le	returns true if argument 1 less than or equal to argument 2	$ a=15; b=10; if [[ "$a" -le "$b" ]]; then echo "true"; else echo "false"; fi false $ a=10; b=10; if [[ "$a" -le "$b" ]]; then echo "true"; else echo "false"; fi true
\<	returns true if argument 1 less than argument 2 this needs to be escaped with a \ if it occurs inside single brackets; otherwise it can appear between double bracket or between double parentheses	$ a=10; b=10; if [ "$a" \< "$b" ]; then echo "true"; else echo "false"; fi false $ a=10; b=10; if [[ "$a" < "$b" ]]; then echo "true"; else echo "false"; fi false $ a=10; b=10; if (( "$a" < "$b" )); then echo "true"; else echo "false"; fi false
<=	returns true if argument 1 less than or equal to argument 2 this operator needs to go inside double parentheses	$ a=10; b=10; if (( "$a" <= "$b" )); then echo "true"; else echo "false"; fi true
>	returns true if argument 1 greater than argument 2 this needs to be escaped with a \ if it occurs inside single brackets; otherwise it can appear between double bracket or between double parentheses	$ z=10; zz=1000; $ if [ zz \> z ]; then echo "true"; else echo "false"; fi true $ if [[ zz > z ]]; then echo "true"; else echo "false"; fi true
>=	returns true if argument 1 greater than or equal to argument 2 this operator needs to go inside double parentheses	$ a=50; b=10; if (( $a >= $b )); then echo "true"; else echo "false"; fi true

String Operators

Operator	Meaning	Example
=	returns true if two strings are equal	$ a=foo; b=foo; if [[ "$a" = "$b" ]]; then echo "true"; else echo "false"; fi true $ a=foo; b=bar; if [[ "$a" = "$b" ]]; then echo "true"; else echo "false"; fi false
==	returns true if two strings are equal; equivalent to =	$ a=foo; b=foo; if [[ "$a" == "$b" ]]; then echo "true"; else echo "false"; fi true $ a=foo; b=bar; if [[ "$a" == "$b" ]]; then echo "true"; else echo "false"; fi false Note the difference between quoting a string and not quoting a string: [[ $a == b* ]] # returns true if $a starts with b (pattern matching) [[ $a == "b*" ]] # returns true if $a is literally equal to b* (literal matching)
!=	returns true if strings are not equal	$ a=foo; b=bar; if [[ "$a" != "$b" ]]; then echo "true"; else echo "false"; fi true
<	returns true if argument 1 is less than (alphabetically) argument 2; capitals come before non-capitals	$ a=foo; b=bar; if [[ "$a" < "$b" ]]; then echo "true"; else echo "false"; fi false $ a=Foo; b=bar; if [[ "$a" < "$b" ]]; then echo "true"; else echo "false"; fi true $ a=bar; b=foo; if [[ "$a" < "$b" ]]; then echo "true"; else echo "false"; fi true
>	returns true if argument 1 is greater than (alphabetically) argument 2; capitals come before non-capitals	$ a=foo; b=bar; if [[ "$a" > "$b" ]]; then echo "true"; else echo "false"; fi true $ a=Foo; b=bar; if [[ "$a" > "$b" ]]; then echo "true"; else echo "false"; fi false
-z	returns true if the string is null	$ a=""; b="foobar"; if [ -z "$a" ]; then echo "true"; else echo "false"; fi true $ a=""; b="foobar"; if [ -z "$b" ]; then echo "true"; else echo "false"; fi false
-n	returns true if the string is NOT null	$ a=""; b="foobar"; if [ -n "$a" ]; then echo "true"; else echo "false"; fi false $ a=""; b="foobar"; if [ -n "$b" ]; then echo "true"; else echo "false"; fi true

Compound Operators

You can

      ! ~             logical and bitwise negation
      <>       left and right bitwise shifts
      =
                       comparison
      == !=       equality and inequality
      &               bitwise AND
      ^               bitwise exclusive OR
      |                bitwise OR
      &&             logical AND
      ||              logical OR

      expr?expr:expr
                       conditional operator
      = *= /= %= += -= <>= &= ^= |=
                       assignment
      expr1 , expr2
                       comma

Basic Math Operators

You can do some basic integer math using bash as follows:

Addition

$ sum=$((2+2))  &&   echo $sum
4

$ sum=$((sum+10))  &&   echo $sum
14

Subtraction

$ diff=$((100-50))   &&   echo $diff
50

$ diff=$((diff-25))   &&   echo $diff
25

Multiplication

$ prod=$((2*8))    &&   echo $prod
16

$ a=9; b=3; prod=$((a*b))    &&   echo $prod
27

Division

$ a=14; b=2; div=$((a/b))   &&   echo $div
7

Remainder

$ a=10; b=4; z=$((a%b))   &&   echo $z
2

Exponentiation

$ z=5   &&   echo $z
5

$ zz=$((z**2))   &&   echo $zz
25

NOTE: Bash does integer math only! To do fraction math, use the program bc or awk.

Complex Math Operators

You can use the increment and decrement operators ++ and -- as follow:

Increment++

The increment operator can be used as either a prefix or postfix operator, i.e. as ++i or i++. Example:

$ z=1   &&   echo $z
1

$ ((++z))   &&   echo $z
2

$ ((++z))   &&   echo $z
3

$ ((++z))   &&   echo $z
4

This can be used in bash for loops as follows:

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

Decrement--

Like the increment operator, this can be a prefix or postfix operator, i.e. --i or i--:

$ z=1   &&   echo $z
1

$ ((--z))   &&   echo $z

$ ((--z))   &&   echo $z
-1

$ ((--z))   &&   echo $z
-2

$ ((--z))   &&   echo $z
-3

Notice that echo $z doesn't print anything if z=0.

Bash Keyboard Shortcuts

Editing Shortcuts

The following are keyboard shortcuts for easy navigation/editing of commands that are on the Bash command line.

Movement

• C-a - Move to beginning of line
• C-e - Move to end of line

• M-f - Move forward 1 word
• M-b - Move backward 1 word

• C-f - Move forward 1 character
• C-b - Move backward 1 character

Cutting and Pasting

• C-k - kill (cut) from cursor to end of line
• C-w - kill (cut) from cursor to previous whitespace
• C-y - yank (paste) previously killed text at cursor

Uncategorized

• C-L - clear screen, reprint current line at top
• C-u - undo last edit
• C-d - delete character under cursor

References

Floating point math in the shell:

http://www.novell.com/coolsolutions/tools/17043.html

Fast bash math:

http://www.bytemycode.com/snippets/snippet/350/

Bash script iterate through array of values

http://www.tech-recipes.com/rx/636/bash-shell-script-iterate-through-array-values/