Conditions

General

• A shell script can test the exit code of any command that can be invoked from the cli
  • That's why you should include an exit command at the end of any scripts you write
• test and [ ] are synonymous
  • don't name programs test or it will get jacked up
• Here is the first line of an if statement written each way
  • if test -f blah.py
  • if [ -f blah.py ]
    • The spaces after the first brace and before the second one are required
    • Mnemonic: [ ] means test, and you wouldn't do test-f
• Conditions fall into 3 categories
  • string comparison
  • arithmetic comparison
  • file conditionals

string comparison

• string1 = string2
• string1 != string2
• -n string => true if the string is not null
• -z string => true if the string is null (an empty string)

arithmetic comparison

• expression1 -eq expression2
• expression1 -ne expression2
• expression1 -gt expression2
• expression1 -ge expression2
• expression1 -lt expression2
• expression1 -le expression2
• ! expression = true if expression is false, and vice versa

file conditionals

• -d file => true if the file is a directory
• -e file => true if the file exists
• -f file => true if it's a regular file, more portable than -e
• -g file => true if sgid is set on the file
• -r file => true if file is readable
• -s file => true if file is not zero in sie
• -u file => tru if suid is set on file
• -w file => true if file is writeable
• -x file => true if file is executable