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Computational Techniques for Life Sciences

IF/THEN/ELSE

Purpose

IF statements test if a condition is met.

Syntax

if [test condition]
then ACTIONS
elif [OTHER TESTS]
  OTHER_ACTIONS
else OTHER_ACTIONS
fi

OR

if [test condition]; then ACTIONS ; elif [OTHER TESTS]; OTHER ACTIONS; else OTHER_ACTIONS ; fi

prompt> a=apple
prompt> if  [[ $a == "pear" ]] ; then echo "MATCH" ; else echo "NO-MATCH"; fi
NO-MATCH
prompt> a=pear
prompt> if  [[ $a == "pear" ]] ; then echo "MATCH" ; else echo "NO-MATCH"; fi
MATCH

Common test conditions:

Strings and Arithmetic

Operator	Meaning
String	Comparison
=	Equal to
==	Equal to
!=	Not equal to
<	Less than (ASCII)
>	Greater than (ASCII)
-z	String is empty
-n	String is not empty

Arithmetic	Comparison
-eq	Equal to
-ne	Not equal to
-lt	Less than
-le	Less than or equal to
-gt	Greater than
-ge	Greater than or equal to

Arithmetic	Comparison within double parentheses (( … ))\
>	Greater than
>=	Greater than or equal to
<	Less than
<=	Less than or equal to

File tests

Operator	Tests Whether	Operator	Tests Whether
-e	File exists	-s	File is not zero size
-f	File is a regular file
-d	File is a directory	-r	File has read permission
-h	File is a symbolic link	-w	File has write permission
-L	File is a symbolic link	-x	File has execute permission
-b	File is a block device
-c	File is a character device	-g	sgid flag set
-p	File is a pipe	-u	suid flag set\
-S	File is a socket	-k	“sticky bit” set
-t	File is associated with a terminal

-N	File modified since it was last read	F1 -nt F2	File F1 is newer than F2
-O	You own the file	F1 -ot F2	File F1 is older than F2
-G	Group id of file same as yours	F1 -ef F2	Files F1 and F2 are hard links to the same file

! “NOT” (reverses sense of above tests)

Exercises

Write a script that reads the first two arguments from the command line, a filename and a number. If the file exists and the number is negative, change the permissions on the file to read-only (r--). If the file doesn’t exist or if the number is greater than 50, then create the file and change it’s permissions to rwx.

vi file_num.sh

#!/bin/bash

file=$1
value=$2
negcutoff=0
poscutoff=50

if [ -r $file ]
then
  if [ $value -lt $negcutoff ]
  then
    chmod 400 $file
  elif [ $value -gt $poscutoff ]
   then
     chmod 700 $file
  fi
else
  touch $file
  chmod 700 $file
fi

prompt> ./file_num.sh fake.txt 70; ls -l fake.txt ;
0 -rwx------  1 beckbw  staff  0 Jul 16 18:59 fake.txt