bash Control Structures

• Conditional expressions. Discussed at length in if-then-else construct. See also:
  • Shell Conditional operators
  • Arithmetic Expressions in BASH
• if-then-else
• loops including
  • while loops (there is no until loops but there is until keyword ;-) 
  • idiosyncratic "legacy" (Bourne-style) for loops
  • C-style for loops
  • Loops connected to a pipe
• case
• select

BASH supports the typical for other shells set of high level flow control constructs. Most of them were introduced by Born Shell:

• if/else Execute a list of statements if a certain condition is/is not true. Due to the age and compatibility reasons if statements has five different forms which is pretty confusing.
• for Execute a list of statements a fixed number of times
• while Execute a list of statements repeatedly while a certain condition holds true
• until  there is no until loop in shell. until keyword is just changes the test(execute statements repeatedly until a certain condition holds true) but test for loop termination remains at the top
• case Execute one of several lists of statements depending on the value of a variable
• select Allow the user to select one of a list of possibilities from a menu

Be warned: the syntax is not pretty.

One of the big improvements that modern versions of bash have when compared with the original Bourne shell is in the area of arithmetic. Bash re-implemented major advances of ksh93 and now have ((...)) constructs. Paradoxically early versions of the Unix shell had no built-in arithmetic; it had to be done by invoking a separate executable (expr), even just to add 1 to a variable.

All conditional statements in shell depends on the expressions and due to the age of the shell (Borne shell it's more then 30 years old) there are many historical layers with different capabilities.

Recommended Links

Advanced Bash-Scripting Guide

• The URL of the preferred download site for the pdf version of the ABS Guide is:
  
  http://personal.riverusers.com/~thegrendel/abs-guide.pdf
  
  This version is specially book-formatted for duplex printing and is usually more up-to-date than the version you can download from the LDP site. Note that it's a 2.6 MB download.
• http://personal.riverusers.com/~thegrendel/abs-guide-5.2.tar.bz2 The "bleeding edge" branch is the 'tar/BZ2' link. It will generally run anywhere from a few days to a couple of months ahead of the main release on the LDP site.

Last modified: April 04, 2008