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https://github.com/fish-shell/fish-shell
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changes according to feedback
Changes according to the feedback have been made: - What is a shell section has been moved before Installation and Start section - Content changes have been made as suggested in both of the above sections.
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Sabine Maennel
ridiculousfish
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1 changed files with 122 additions and 124 deletions
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@ -34,6 +34,123 @@ So shells serve as user-interface between applications and the operating system.
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<a href="#introduction"><-</a><hr>
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\section shell What is a Shell
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This section is about the basics of shells. *fish* is a shell and shells have a lot in common:
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- <a href="#shell-standards">Shell Standards</a>: Why shells adjust to standards
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- <a href="#man-page">Manual Pages</a>: Commands usually come with a standardized manual page
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- <a href="#shell-syntax">Command Syntax</a>: Shell commands have a standard syntax
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- <a href="#shell-programming">Commands versus Programs</a>: Commands differ from normal programs
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- <a href="#shebang-line">Shebang Line</a>: How the shell knows the language of a script
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\subsection shell-standards Shell Standards
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A shell is an interface to the operating system that reads from the commandline of a terminal. A shell's task is to identify and interpret commands. The commands can come from different applications and can be written in different programming languages.
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This can only work smoothly if shells adapt to some common standards. For shells there is the POSIX standard, see <a href="https://en.wikipedia.org/wiki/Command-line_interface#Command-line_interpreter">Command-line interpreters</a>. `fish` tries to satisfy the POSIX standard wherever it does not get into the way of its own design principles, see <a href="design.html">`fish` Design</a>.
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\subsection man-page Manual Pages
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There is a common standard on how to receive help on shell commands: applications provide a manual page to their commands that can be opened with the `man` command:
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\fish
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>man COMMAND
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\endfish
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This convention helps to make sure help can be found on commands no matter where they originate from. *fish*'s internal commands all come with a manual page.
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\subsection shell-syntax Command Syntax
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Shells also support some common syntax for executing commands. That way a command can be started in the same way, regardless of the application, where it comes from, and the shell, where it is executed in.
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The pattern below is a basic pattern:
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\fish
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COMMAND [OPTIONS] ARGUMENTS
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\endfish
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- <b>`COMMAND`</b>: the name of the executeable
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- <b>`[OPTIONS]`</b>: options can change the behaviour of a command
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- <b>`ARGUMENTS`</b>: input to the command
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For external <b>commands</b> the executable is usually stored on file and the file path must be included in the variable `$PATH`, so that the file can be found: see <a href="#variables-special">Special Variables</a>.
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<b>Options</b> come in two forms: a short name, that is a hyphen with a single letter; or a long name, consisting of two hyphens with words connected by hyphens. Example: `-h` and `--help` are often used to print a help message. Options are a fixed set, described in the manual pages of the command, see <a href="#man-page">Manual Pages</a>
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<b>Arguments</b> are the arbitrary input part of a command: often it is a file or directory name, sometimes it is a string or a list.
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Example:
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\fish
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>echo -s Hallo World!
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HalloWorld!
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\endfish
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- both `Hallo` and `World!` are arguments to the echo command
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- `-s` is an option that suppresses spaces in the output of the command
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\subsection shell-programming Commands versus Programs
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<b>Programs</b> in other languages can often be regarded as black boxes: they get complex input and return complex output. Sometimes they produce side effects such as writing to a file or reporting an error, but the emphasis is on: arguments in and return values out:
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Arguments → |Program| → Return Values
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<b>Shell commands</b> are different:
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- the side effects take the center of the stage: they transform an <b>input stream of data</b> into an <b>output stream of data</b>. Both of these streams are usually the terminal, but they can be redirected.
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- the arguments become options or switches paired with data: the switches influence the behaviour of the command
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- the return value shrinks to an <b>exit code</b>: this exit code is 0 when the command executes normally and between 1 and 255 otherwise.
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<table with=100%>
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<tr><td>Input Stream ⇒<td rowspan="2">|Shell Command|<td>⇒ Output Stream
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<tr><td>switch and data as arguments →<td>→ exit code
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</table>
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This leads to another way of programming and especially of combining commands:
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There are two ways to combine shell commands:
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- Commands can pass on their streams to each other: one command takes the output stream of another command as its input stream and the two commands execute in parallel. This is called piping, see <a href="#piping">Piping</a>.
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Example:
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\fish
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# Every line of the `ls` command is immediatelly passed on to the `grep` command
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>ls -l | grep "my topic"
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\endfish
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- Commands can pass on all their output as a chunk</b>: the output stream of one command is bundled and taken as data argument for the second command. This is called command substitution, see <a href="#expand-command-substitution">Command Substitution</a>.
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Example:
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\fish
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# the output of the inner `ls` command is taken as the input argument for the outer `echo` command
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>echo (ls a*)
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\endfish
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\subsection shebang-line Shebang Line
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Since script for shell commands can be written in many different languages, they need to carry information about what interpreter is needed to execute them: For this they are expected to have a first line, the shebang line, which names an executable for this purpose:
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Example:
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A scripts written in `bash` it would need a first line like this:
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```
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#!/bin/bash
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```
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- this line tells the shell to execute the file with the *bash* interpreter, that is located at the path `/bin/bash`.
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For a script, written in another language, just replace the interpreter `/bin/bash` with the language interpreter of that other language (for example `/bin/python` for a `python` script)
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This line is only needed when scripts are executed by another interpreter, so for *fish* internal commands, that are executed by *fish* the shebang line is not necessary.
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<a href="#shell"><-</a><hr>
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\section install Installation and Start
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This section is on how to install, uninstall, start and exit a *fish* shell and on how to make *fish* the default shell:
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@ -80,139 +197,20 @@ Once *fish* has been installed, open a terminal. If *fish* is not the default sh
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\subsection bash Executing Bash
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If *fish* is your default shell you can still execute *bash* commands in case you need to:
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If *fish* is your default shell and you want to copy commands from the internet, that are written in a different shell language, *bash* for example, you can proceed in the following way:
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- a single command can be executed with:
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Consider, that `bash` is also a command. With `man bash` you can see that there are two ways to do this:
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- `bash` has a switch `-c` to read from a string:
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\fish
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>bash -c SomeBashCommand
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\endfish
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- or a *bash* shell can be opened with:
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\fish
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>/bin/bash
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\endfish
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This might be useful, when copying complicated *bash* commands from a website. Translating them into *fish* syntax can be avoided that way.
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- or `bash` without a switch, opens a *bash* shell that you can use and `exit` afterwards.
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<a href="#install"><-</a><hr>
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\section shell What is a Shell
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This section is about the basics of shells. *fish* is a shell and shells have a lot in common:
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- <a href="#shell-standards">Shell Standards</a>: Why shells adjust to standards
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- <a href="#man-page">Manual Pages</a>: Commands usually come with a standardized manual page
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- <a href="#shell-syntax">Command Syntax</a>: Shell commands have a standard syntax
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- <a href="#shell-programming">Commands versus Programs</a>: Commands differ from normal programs
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- <a href="#shebang-line">Shebang Line</a>: How the shell knows the language of a script
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\subsection shell-standards Shell Standards
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A shell is an interface to the operating system that reads from the commandline of a terminal. A shell's task is to identify and interpret commands. The commands can come from different applications and can be written in different programming languages.
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This can only work smoothly if shells adapt to some common standards. For shells there is the POSIX standard, see <a href="https://en.wikipedia.org/wiki/Command-line_interface#Command-line_interpreter">Command-line interpreters</a>. `fish` tries to satisfy the POSIX standard wherever it does not get into the way of its own design principles, see <a href="design.html">`fish` Design</a>.
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\subsection man-page Manual Pages
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There is a common standard on how to receive help on shell commands: applications provide a manual page to their commands that can be opened with the `man` command:
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\fish
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>man COMMAND
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\endfish
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This convention helps to make sure help can be found on commands no matter where they originate from. *fish*'s internal commands all come with a manual page.
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\subsection shell-syntax Command Syntax
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Shell commands also have some syntax in common, no matter where they originate from:
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Commands consist of three parts:
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\fish
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COMMAND [OPTIONS] ARGUMENTS
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\endfish
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- <b>`COMMAND`</b>: the name of the executeable
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- <b>`[OPTIONS]`</b>: options can change the behaviour of a command
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- <b>`ARGUMENTS`</b>: input to the command
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For external <b>commands</b> the executable is usually stored on file and the file path must be included in the variable `$PATH`, so that the file can be found: see <a href="#variables-special">Special Variables</a>.
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<b>Options</b> come in two forms: a short name, that is a hyphen with a single letter; or a long name, consisting of two hyphens with words connected by hyphens. Example: `-h` and `--help` are common options for opening the manual page of a command. Options are a fixed set, described in the manual pages of the command, see <a href="#man-page">Manual Pages</a>
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<b>Arguments</b> are the arbitrary input part of a command: often it is a file or directory name, sometimes it is a string or a list.
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Example:
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\fish
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>echo -s Hallo World!
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HalloWorld!
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\endfish
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- both `Hallo` and `World!` are arguments to the echo command
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- `-s` is an option that suppresses spaces in the output of the command
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\subsection shell-programming Commands versus Programs
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Shell commands differ from other programs in the following way:
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- they cannot return variables
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Instead:
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- They transform an <b>input stream</b> into an <b>output stream</b>. Both of these streams are usually the terminal, but they can be redirected.
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- They return an <b>exit code</b>: this exit code is 0 when the command executes normally and between 1 and 255 otherwise.
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This leads to another way of programming for shell commands: they can directly return variables, but they can pass on content to other commands in following ways:
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<b>Commands can pass on content as a stream</b>: one command takes the output stream of another command as its input stream:
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Example:
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\fish
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>ls -l | grep "my topic"
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\endfish
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- every line of the `ls` command is immediatelly passed on to the `grep` command and treated separately.
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In this case the two commands execute in parallel. This is called piping, see <a href="#piping">Piping</a>.
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<b>Commands can pass on all of their output stream as a chunk</b>:
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Example:
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\fish
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>echo (ls a*)
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\endfish
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- the `echo` command takes the output of the `ls` command as a chunk and prints it to the terminal. The `echo` command waits for the `ls` command to finish, before it executes.
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In this case one command waits for the other command to finish, before it executes. This is called command substitution, see <a href="#expand-command-substitution">Command Substitution</a>.
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\subsection shebang-line Shebang Line
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Since script for shell commands can be written in many different languages, they need to carry information about what interpreter is needed to execute them: For this they are expected to have a first line, the shebang line, which names an executable for this purpose:
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Example:
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A scripts written in `bash` it would need a first line like this:
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```
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#!/bin/bash
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```
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- this line tells the shell to execute the file with the *bash* interpreter, that is located at the path `/bin/bash`.
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For a script, written in another language, just replace the interpreter `/bin/bash` with the language interpreter of that other language (for example `/bin/python` for a `python` script)
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This line is only needed when scripts are executed by another interpreter, so for *fish* internal commands, that are executed by *fish* the shebang line is not necessary.
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<a href="#shell"><-</a><hr>
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\section syntax Syntax overview
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