Unix bash string comparison and numeric comparison - unix1998/technical_notes GitHub Wiki

In bash scripting, gt and lt are used for numeric comparisons, while > and < are used for string comparisons or file redirection. They are not the same and are used in different contexts:

Numeric Comparisons

• -gt: Greater than
• -lt: Less than

These are used within double square brackets [ ... ](/unix1998/technical_notes/wiki/-...-) or single square brackets [ ... ] for comparing integers.

Example:

#!/bin/bash

a=5
b=10

if [ $a -gt $b ]; then
  echo "$a is greater than $b"
else
  echo "$a is not greater than $b"
fi

if [ $a -lt $b ]; then
  echo "$a is less than $b"
else
  echo "$a is not less than $b"
fi

String Comparisons and Redirection

• >: Greater than (for string comparison) or output redirection
• <: Less than (for string comparison) or input redirection

When used for string comparison within double square brackets [ ... ](/unix1998/technical_notes/wiki/-...-):

Example:

#!/bin/bash

a="apple"
b="banana"

if [ $a > $b ](/unix1998/technical_notes/wiki/-$a->-$b-); then
  echo "$a is greater than $b"
else
  echo "$a is not greater than $b"
fi

if [ $a < $b ](/unix1998/technical_notes/wiki/-$a-<-$b-); then
  echo "$a is less than $b"
else
  echo "$a is not less than $b"
fi

When used for file redirection:

Example:

#!/bin/bash

echo "This will be written to a file" > output.txt
cat < input.txt

Summary

• Use -gt and -lt for numeric comparisons.
• Use > and < for string comparisons or file redirection.

They are not interchangeable and should be used appropriately based on the context.

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In bash, the symbols <> are not used for inequality comparisons. Instead, you can use != for both numeric and string comparisons. Here's how you can perform inequality checks for both numeric and string values:

Numeric Comparisons

For numeric inequality, you can use -ne (not equal to).

Example:

#!/bin/bash

a=5
b=10

if [ $a -ne $b ]; then
  echo "$a is not equal to $b"
else
  echo "$a is equal to $b"
fi

String Comparisons

For string inequality, you can use !=.

Example:

#!/bin/bash

a="apple"
b="banana"

if [ "$a" != "$b" ]; then
  echo "$a is not equal to $b"
else
  echo "$a is equal to $b"
fi

Using [ ... ](/unix1998/technical_notes/wiki/-...-) for More Complex Conditions

You can also use [ ... ](/unix1998/technical_notes/wiki/-...-) for more complex string comparisons or conditions involving logical operators.

Example:

#!/bin/bash

a="apple"
b="banana"

if [ "$a" != "$b" ](/unix1998/technical_notes/wiki/-"$a"-!=-"$b"-); then
  echo "$a is not equal to $b"
else
  echo "$a is equal to $b"
fi

# Example with logical NOT
if [ ! "$a" == "$b" ](/unix1998/technical_notes/wiki/-!-"$a"-==-"$b"-); then
  echo "$a is not equal to $b (using logical NOT)"
else
  echo "$a is equal to $b (using logical NOT)"
fi

Summary

• For numeric inequality, use -ne.
• For string inequality, use !=.
• <> is not used for inequality in bash.
• You can use logical NOT (!) in combination with [ ... ](/unix1998/technical_notes/wiki/-...-) for complex conditions.

This approach ensures correct operation in bash scripts for both numeric and string comparisons.