120 lines
2.8 KiB
Markdown
120 lines
2.8 KiB
Markdown
---
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tags:
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- shell
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- regex
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---
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# Test values in Bash
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`test` is a built-in command that is used to compare values or determine whether
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something is the case.
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We can use the command `test` but it is more common to test a condition
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implicity by using square brackets. The square brackets are an alias for `test`.
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We use this alias when we use `IF` logic.
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When we run a test the result we get back is a return status of a `0` or a `1`.
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`0` indicates that the test was a success and `1` means failure. (Bear in mind
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this is in contrast to most all other programming languages.)
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If we run a test in the command line we won't get a `0` or a `1` or back, there
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will just be silence from the shell. We can explicitly invoke the return value
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with variable `$?`, e.g:
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```bash
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[ -d ~ ] # is the home directory a directory?
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echo $?
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0 # yes
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[ -d /bin/zsh ] # is this binary a directory?
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echo $
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1 # no
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```
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## Test structures
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Many tests can be run with flags as a shorthand like we saw above:
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### File operators
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```
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-a FILE True if file exists.
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-d FILE True if file is a directory.
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-e FILE True if file exists.
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-h FILE True if file is a symbolic link
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-s FILE True if file exists and is not empty.
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-N FILE True if the file has been modified since it was last read.
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```
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### String operators
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```
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-z STRING True if string is empty.
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-n STRING True if string is not empty.
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```
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## Differences between comparing numbers and strings
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- `=` is reserved for comparing strings
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- For numbers we use, e.g, `[ 4 -lt 5 ]`
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## Negation
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We can negate a test condition with `!`:
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```bash
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[ ! 4 -lt 3 ]; echo $?
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0
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```
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## Running process if test succeeds or fails
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We can use the following structure to run a process if a test condition obtains:
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```bash
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[[ -d ~ ]] && echo "/home is a directory"
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```
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## Extended test: `[[...]]`
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When we use **double brackets** we are using _extended_ `test`.
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The extended test supports the standard `test` comparisons and adds other
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features:
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- The use of Boolean operators:
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```
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[[ -d ~ || -a /bin/mash ]]; echo $?
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```
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### Using regular expressions
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Extended test also allows us to use regular expressions as part of our test
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conditions. In order to test against a regular expression we use `=~` as the
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comparison operator.
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```bash
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[[ "thomas" =~ t.* ]]; echo $?
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```
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Here the test succeeds because "thomas" begins with "t" followed by any other
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character.
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## Further examples
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**Test if a character exists in two strings**
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```bash
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string1="hello"
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string2="world"
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char="l"
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if [[ "$string1" == *"$char"* ]] && [[ "$string2" == *"$char"* ]]; then
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echo "Character '$char' exists in both strings."
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else
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echo "Character '$char' does not exist in both strings."
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fi
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```
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> Note: this syntax can also be used to test if a given element exists in an
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> [array](Lists_and_arrays_in_Bash.md).
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