====== #!/bin/sh ======

  * [[bash]]
  * [[csh]]
  * [[ksh]]
  * [[tcsh]]
  * [[zsh]]

  * [[http://www.bo.infn.it/alice/alice-doc/mll-doc/impgde/node5.html|Bourne shell programming]]
  * [[http://www.mcsr.olemiss.edu/unixhelp/shell/oview1.1.html|Summary of shell facilities]]
  * [[http://mywiki.wooledge.org/SignalTrap|Capturing signal traps]]

''sh'' uses different syntax for operators than ''bash''.

Use ''-o'' for or in place of ''||'' and ''-a'' for and in place of ''&&''.

== Shell Symbols ==

  * **$1..$9** - arguments
  * **$0** - process name
  * **$#** - number of arguments
  * **$?** - exit status
  * **$$** - PID of this shell
  * **$!** - PID of the last background command
  * **$-** - option supplied at shell call (?)
  * **$*** - all arguments
  * **$@** - all arguments with quotes

== Conditional Statements ==

  * **-a** and (has higher precedence than -o)
  * **-o** or

== String Operators ==
 
  * **=** or **==** equal to
  * **-n** string is not null
  * **-z** string is null, that is, has zero length

== Numeric Operators ==

  * **-eq** number equals
  * **-ne** number does not equal
  * **-gt** greater than
  * **-ge** greater than or equal to
  * **-lt** less than
  * **-le** less than or equal to

== File Operators ==

  * **-e** file exists
  * **-f** file is not a directory or device file
  * **-d** file exists and is a directory
  * **-h** file exists and is a symlink
  * **-s** file exists and has a size greater than zero
  * **-w** file exists and is writeable
  * **-x** file exists and is executable
  * **-r** file exists and is readable
  * **-b** file is a block device
  * **-c** file is a character device
  * **-p** file is a pipe
  * **-S** file is a socket
  * **-O** you are owner of file
  * **-G** you are group owner of file

== Built-in commands ==

  * **:** - null command
  * **. file** - execute command in current shell
  * **read** - assign //var// from //stdin//
  * **readonly** - mark //var// as read only
  * **trap** - trap signals
  * **type** - inidicate how the shell interprets the command
  * **exec** - execute command in place of current shell without creating a new process
  * **hash** - rehash command table

== Redirect stderr ==

<code>
echo test 2> /dev/null
</code>

== Redirect stderr to stdout ==

<code>
echo test 2>&1
</code>

=== String Comparison ===

When checking for string length, include the string in quotes.

Both of these will return true:

<code>
[ -n string ]
[ -n ]
</code>

This will return false:

<code>
[ -n "" ]
</code>