shell - bash. Writing conditional tests and statements

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shell - bash. Writing conditional tests and statements - the options available

Every language that I teach has conditionals and loops, and the bash shell is no exception. From the three day bash course given last week - an example including lots of alternative tests (but by no means all varlients!

for tests in if

  if command ... ; then
The command is run and its return status is tested.

  if [ ... ] ; then
Shell tests such as if (is it a plain file) are run

  if test ... ; then
Shell tests such as if (is it a plain file) are run (same as test)

  if [[ ... ]] ; then
String comparison / expression tested

  if (( ... )) ; then
Numeric comparison / expression tested

bash also supports lazy and and or operators (&& and ||) which can be used for shortened conditional testng based on the return status of a command - which could be one of the above bracket expressions or a reguler command:

  WomanWithCat:bash grahamellis$ grep -q main demo.c || echo yes

  WomanWithCat:bash grahamellis$ grep -q main demo.c && echo yes

  yes

  WomanWithCat:bash grahamellis$ grep -q second demo.c && echo yes

  WomanWithCat:bash grahamellis$ grep -q second demo.c || echo yes

  yes

  WomanWithCat:bash grahamellis$ 

  You have new mail in /var/mail/grahamellis

  WomanWithCat:bash grahamellis$ [[ 4 == 5 ]] || echo one

  one

  WomanWithCat:bash grahamellis$ [[ 4 == 5 ]] && echo two

  WomanWithCat:bash grahamellis$

(grep -q is quiet mode - supress output and just return a status)
(written 2015-11-28)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles

A167 - Web Application Deployment - Shell Programming (bash)
  [63] Almost like old times - (2004-09-26)
  [64] Shell Script for CGI on the web - (2004-09-26)
  [749] Cottage industry or production line data handling methods - (2006-06-07)
  [827] No news is good news with Unix and Linux - (2006-08-10)
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  [1345] Perl and Shell coding standards / costs of an IT project - (2007-09-11)
  [1468] Lexical v Arithemetic testing, Bash and Perl - (2007-12-11)
  [1527] Selecting file names in a shell - one word or another - (2008-02-02)
  [1904] Ruby, Perl, Linux, MySQL - some training notes - (2008-11-23)
  [3791] The Kernel, Shells and Daemons. Greek Gods in computing - (2012-07-01)
  [4400] Commenting out an echo killed my bash backup script - (2015-01-19)
  [4487] Starting MySQL. ERROR! The server quit without updating PID file - how we fixed it. - (2015-05-06)
  [4584] Bash ... some new scripts to - handling user input - (2015-11-27)
  [4586] Extending your bash shell with aliases, functions and extra commands - (2015-11-28)

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Some other Articles

From single block to structure and object oriented programming
Progress on moving from Python 2 to Python 3 - training for both versions
Principles or a GUI and their practical application using wxPthon
What teach you in a week stays with you for a decade
shell - bash. Writing conditional tests and statements - the options available
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Back in the saddle again - excellent open source course from Well House Consultants
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