187 lines
3.5 KiB
Markdown
Executable File
187 lines
3.5 KiB
Markdown
Executable File
# Bash Conditional Expressions
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In computer science, conditional statements, conditional expressions, and conditional constructs are features of a programming language, which perform different computations or actions depending on whether a programmer-specified boolean condition evaluates to true or false.
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In Bash, conditional expressions are used by the `[[` compound command and the `[`built-in commands to test file attributes and perform string and arithmetic comparisons.
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Here is a list of the most popular Bash conditional expressions. You do not have to memorize them by heart. You can simply refer back to this list whenever you need it!
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## File expressions
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* True if file exists.
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```bash
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[[ -a ${file} ]]
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```
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* True if file exists and is a block special file.
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```bash
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[[ -b ${file} ]]
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```
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* True if file exists and is a character special file.
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```bash
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[[ -c ${file} ]]
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```
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* True if file exists and is a directory.
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```bash
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[[ -d ${file} ]]
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```
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* True if file exists.
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```bash
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[[ -e ${file} ]]
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```
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* True if file exists and is a regular file.
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```bash
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[[ -f ${file} ]]
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```
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* True if file exists and is a symbolic link.
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```bash
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[[ -h ${file} ]]
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```
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* True if file exists and is readable.
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```bash
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[[ -r ${file} ]]
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```
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* True if file exists and has a size greater than zero.
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```bash
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[[ -s ${file} ]]
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```
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* True if file exists and is writable.
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```bash
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[[ -w ${file} ]]
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```
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* True if file exists and is executable.
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```bash
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[[ -x ${file} ]]
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```
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* True if file exists and is a symbolic link.
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```bash
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[[ -L ${file} ]]
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```
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## String expressions
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* True if the shell variable varname is set (has been assigned a value).
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```bash
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[[ -v ${varname} ]]
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```
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True if the length of the string is zero.
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```bash
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[[ -z ${string} ]]
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```
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True if the length of the string is non-zero.
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```bash
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[[ -n ${string} ]]
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```
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* True if the strings are equal. `=` should be used with the test command for POSIX conformance. When used with the `[[` command, this performs pattern matching as described above (Compound Commands).
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```bash
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[[ ${string1} == ${string2} ]]
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```
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* True if the strings are not equal.
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```bash
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[[ ${string1} != ${string2} ]]
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```
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* True if string1 sorts before string2 lexicographically.
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```bash
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[[ ${string1} < ${string2} ]]
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```
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* True if string1 sorts after string2 lexicographically.
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```bash
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[[ ${string1} > ${string2} ]]
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```
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## Arithmetic operators
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* Returns true if the numbers are **equal**
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```bash
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[[ ${arg1} -eq ${arg2} ]]
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```
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* Returns true if the numbers are **not equal**
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```bash
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[[ ${arg1} -ne ${arg2} ]]
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```
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* Returns true if arg1 is **less than** arg2
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```bash
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[[ ${arg1} -lt ${arg2} ]]
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```
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* Returns true if arg1 is **less than or equal** arg2
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```bash
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[[ ${arg1} -le ${arg2} ]]
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```
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* Returns true if arg1 is **greater than** arg2
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```bash
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[[ ${arg1} -gt ${arg2} ]]
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```
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* Returns true if arg1 is **greater than or equal** arg2
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```bash
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[[ ${arg1} -ge ${arg2} ]]
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```
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As a side note, arg1 and arg2 may be positive or negative integers.
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As with other programming languages you can use `AND` & `OR` conditions:
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```bash
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[[ test_case_1 ]] && [[ test_case_2 ]] # And
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[[ test_case_1 ]] || [[ test_case_2 ]] # Or
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```
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## Exit status operators
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* returns true if the command was successful without any errors
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```bash
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[[ $? -eq 0 ]]
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```
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* returns true if the command was not successful or had errors
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```bash
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[[ $? -gt 0 ]]
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```
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