Bash

Bash Quick Reference: Bash/Quick

Bash Quick Prototype: Bash/Prototype

Bash Guide

Using Bash

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

You can set the default shell in Mac's Terminal by going to Terminal > Preferences and specifying the login shell.

Startup Process

Main article: Dot files

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.

Bash Loop One-Liner: Seq and Destroy

Sometimes, you want to do a bash loop, but you want to do it on the fly, so you don't have to fire up your text editor and make a bash script.

Seq to the rescue!

The seq command is a way of creating very simple numerical sequences:

$ seq 1 10
1
2
3
4
5
6
7
8
9
10

But seq has some other features that make it very handy (see the Seq page for a full list of examples).

Seq can be very easily combined with bash to create one-line for loops.

Let's pose a simple scenario: say you've imported a bunch of photos from your camera, and their naming convention looks like this:got a lit of photos like this:

IMG_0000.jpg IMG_0001.jpg IMG_0002.jpg IMG_0003.jpg IMG_0004.jpg IMG_0005.jpg IMG_0006.jpg IMG_0007.jpg IMG_0008.jpg IMG_0009.jpg IMG_0010.jpg IMG_0011.jpg IMG_0012.jpg IMG_0013.jpg IMG_0014.jpg IMG_0015.jpg IMG_0016.jpg IMG_0017.jpg IMG_0018.jpg IMG_0019.jpg IMG_0020.jpg IMG_0021.jpg IMG_0022.jpg IMG_0023.jpg IMG_0024.jpg IMG_0025.jpg IMG_0026.jpg IMG_0027.jpg IMG_0028.jpg IMG_0029.jpg IMG_0030.jpg IMG_0031.jpg IMG_0032.jpg IMG_0033.jpg IMG_0034.jpg IMG_0035.jpg IMG_0036.jpg IMG_0037.jpg IMG_0038.jpg IMG_0039.jpg IMG_0040.jpg IMG_0041.jpg IMG_0042.jpg IMG_0043.jpg IMG_0044.jpg IMG_0045.jpg IMG_0046.jpg IMG_0047.jpg IMG_0048.jpg IMG_0049.jpg IMG_0050.jpg

Now, let's say we want to move some photos to another folder, but we only want to move the odd-numbered photographs starting at 10 and ending at 40.

We can whip up a quick seq command that'll give us even numbers from 10 to 40:

$ seq 10 2 40

But we need the whole thing to be formatted into a comma-separated list, so we can feed it to a bash command, like mv {this,that} the_other/.. So let's use the -s flag:

$ seq -s, 10 2 40
10,12,14,16,18,20,22,24,26,28,30,32,34,36,38,40,

Now we can use bash's backticks to plop that command in the middle of another mv command:

$ mv directory1/IMG_00{`seq -s, 10 2 40`}.jpg directory2/.

which unfolds to:

mv directory1/IMG_0010.jpg directory2/. mv directory1/IMG_0012.jpg directory2/. mv directory1/IMG_0014.jpg directory2/. mv directory1/IMG_0016.jpg directory2/. mv directory1/IMG_0018.jpg directory2/. mv directory1/IMG_0020.jpg directory2/. mv directory1/IMG_0022.jpg directory2/. mv directory1/IMG_0024.jpg directory2/. mv directory1/IMG_0026.jpg directory2/. mv directory1/IMG_0028.jpg directory2/. mv directory1/IMG_0030.jpg directory2/. mv directory1/IMG_0032.jpg directory2/. mv directory1/IMG_0034.jpg directory2/. mv directory1/IMG_0036.jpg directory2/. mv directory1/IMG_0038.jpg directory2/. mv directory1/IMG_0040.jpg directory2/.

Voila!

But what if the range of file numbers strides 100? If you wanted to go from 80 to 120, you would end up with mismatching zero-padding. To get around this, you could either force seq to output all numbers with a fixed width (using the -w flag), or you could get more fancy by specifying a printf (print format) string for seq to use when printing each number (via the -f flag in seq).

Let's repeat the example, but this time using the printf approach. First, we want to print a 4-digit number that is zero-padded, meaning our printf string is going to be %04g. Now the seq command looks like:

$ seq -s, -f'%04g' 80 2 120
0080,0082,0084,0086,0088,0090,0092,0094,0096,0098,0100,0102,0104,0106,0108,0110,0112,0114,0116,0118,0120,

(Note we could have also used the printf string 'IMG_%04g.jpg' to make things a little more compact.)

Now we can feed that directly to the mv command:

$ mv directory1/IMG_{`seq -s, -f'%04g' 80 2 120`}.jpg directory2/.

which unfolds into the set of bash commands:

mv directory1/IMG_0080.jpg directory2/. mv directory1/IMG_0082.jpg directory2/. mv directory1/IMG_0084.jpg directory2/. mv directory1/IMG_0086.jpg directory2/. mv directory1/IMG_0088.jpg directory2/. mv directory1/IMG_0090.jpg directory2/. mv directory1/IMG_0092.jpg directory2/. mv directory1/IMG_0094.jpg directory2/. mv directory1/IMG_0096.jpg directory2/. mv directory1/IMG_0098.jpg directory2/. mv directory1/IMG_0100.jpg directory2/. mv directory1/IMG_0102.jpg directory2/. mv directory1/IMG_0104.jpg directory2/. mv directory1/IMG_0106.jpg directory2/. mv directory1/IMG_0108.jpg directory2/. mv directory1/IMG_0110.jpg directory2/. mv directory1/IMG_0112.jpg directory2/. mv directory1/IMG_0114.jpg directory2/. mv directory1/IMG_0116.jpg directory2/. mv directory1/IMG_0118.jpg directory2/. mv directory1/IMG_0120.jpg directory2/.

Voila!

You can find more information about seq, and how to couple it with other Unix utilities, at the Seq page on my wiki.

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.

A comprehensive list of logical condition checks is here: http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_01.html

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 combine logical operators using and/or operators:

Operator	Meaning	Example
-a	Logical and (both conditions must be true for condition to be true)	if [ "$expr1" -a "$expr2" ] then echo "Both expr1 and expr2 are true." else echo "Either expr1 or expr2 is false." fi
-o	Logical or (either condition must be true for condition to be true)	if [ "$expr1" -o "$expr2" ] then echo "Either expr1 or expr2 is true." else echo "Both expr1 and expr2 are false." fi

Basic Math

Main article: Bash Math#Basic Math

Complex Math

Main article: Bash Math#Complex Math

Math Operators

Main article: Bash Math#Operators

Bash Scripts for Heavy Bash Users

See https://github.com/alexanderepstein/Bash-Snippets

• weather
• geoip info
• currency
• encryption/decryption
• movies
• cheat - finding command options and code pieces
• taste - "find similar things to X"

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/