Test values in Bash

test is a built-in command that is used to compare values or determine whether something is the case.

We can use the command test but it is more common to test a condition implicity by using square brackets. The square brackets are an alias for test. We use this alias when we use IF logic.

When we run a test the result we get back is a return status of a 0 or a 1. 0 indicates that the test was a success and 1 means failure. (Bear in mind this is in contrast to most all other programming languages.)

If we run a test in the command line we won't get a 0 or a 1 or back, there will just be silence from the shell. We can explicitly invoke the return value with variable $?, e.g:

[ -d ~ ] # is the home directory a directory?
echo $?
0   # yes

[ -d /bin/zsh ] # is this binary a directory?
echo $
01  # no

Test structures

Many tests can be run with flags as a shorthand like we saw above:

File operators

-a FILE        True if file exists.
-d FILE        True if file is a directory.
-e FILE        True if file exists.
-h FILE        True if file is a symbolic link.z
-s FILE        True if file exists and is not empty.
-N FILE        True if the file has been modified since it was last read.

String operators

-z STRING      True if string is empty.
-n STRING      True if string is not empty.

Differences between comparing numbers and strings

= is reserved for comparing strings
For numbers we use, e.g, [ 4 -lt 5 ]

Negation

We can negate a test condition with !:

[ ! 4 -lt 3 ]; echo $?
0

Extended test: `[[...]]`

When we use double brackets we are using extended test.

The extended test supports the standard test comparisons and adds other features:

The use of Boolean operators:
```
[[ -d ~ || -a /bin/mash ]]; echo $?
```

Further examples

Test if a character exists in two strings

string1="hello"
string2="world"
char="l"

if [[ "$string1" == *"$char"* ]] && [[ "$string2" == *"$char"* ]]; then
    echo "Character '$char' exists in both strings."
else
    echo "Character '$char' does not exist in both strings."
fi

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