Linux: Conditional/Loop Commands

[last updated: 2022-12-28]
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Linux commands
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• On this page:
  • if:
  • for:
  • while:
  • break:

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• If:
  • if [ condition ]
    then
          statements
    else
          statements
    fi
  • Note: white space around the condition is required.
  • When condition compares numbers:
    if [ 45 -gt 40 ]
    if [ $num1 -lt $num2 ]
    Note: white space around arithmetic operator is required
    Operators available:
    -gt, -lt ... greater than, less than
    -eq, -ne ... equal, not equal
    -ge, -le ... greater than or equal, less than or equal
    you can also use the symbolic operators, <, >, <=, and >=
    however you must enclose them in double parentheses:
    if (($num1< $num2))
    Note: white space not needed anywhere
  • When condition compares strings:
    • if [ $str1 = $str2 ]
      -or- if [ $str1 == $str2 ]
      Note: white space required around variables and operator
    • However '==' acts differently inside double brackets:
      if [[ $str1 == z* ]] ... true if $str1 starts with the letter "z" .
      [Search online references to discover lots of complication using double brackets that is beyond my current understanding...]
    • if [ $str1 != $str2 ] ... not equal
    • if [[ $str1 < $str2 ]] ... less than ascii-alphabetically
      if [ $str1 \< $str2 ] ... must be escaped in single-brackets
• Compound Conditions:
  • -a , && ... logical and
    if [ "$num1" -eq "$num2" ] && [ "$num1" -eq "$num3" ]
    if [ "$num1" -eq "$num2" ] && [ "$str1" = "$str2" ]
    if [ "$str1" = "$str2" -a "$num1" = "$num2" ]
  • -o, || ... logical or work similarly

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