73 lines
1.9 KiB
Markdown
73 lines
1.9 KiB
Markdown
---
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categories:
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- Programming Languages
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tags:
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- shell
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---
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# Test in bash
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`test` is a built-in command that is used to compare values or determine whether something is the case.
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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.
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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.)
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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 will just be silence from the shell. We can explicitly invoke the return value 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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01 # 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.z
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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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## 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 features:
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- The use of Boolean operators:
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```bash
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[[ -d ~ || -a /bin/mash ]]; echo $?
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```
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