or Alt+.Ĭommands will often return output using STDOUT, errors through STDERR, and a Return Code to report errors in a more script-friendly manner. If you are in an interactive shell, you can also quickly get this value by typing Esc followed by.

$_ - Last argument from the last command.

A common pattern is to execute a command only for it to fail due to missing permissions you can quickly re-execute the command with sudo by doing sudo !!

!! - Entire last command, including arguments.

$$ - Process identification number (PID) for the current script.

$? - Return code of the previous command.

A more comprehensive list can be found here. Unlike other scripting languages, bash uses a variety of special variables to refer to arguments, error codes, and other relevant variables. Here $1 is the first argument to the script/function. Strings delimited with ' are literal strings and will not substitute variable values whereas ' delimited strings will. Strings in bash can be defined with ' and ' delimiters, but they are not equivalent. This behavior can be confusing to use at first, so always check for that. In general, in shell scripts the space character will perform argument splitting. Note that foo = bar will not work since it is interpreted as calling the foo program with arguments = and bar. To assign variables in bash, use the syntax foo=bar and access the value of the variable with $foo.

Thus, creating command pipelines, saving results into files, and reading from standard input are primitives in shell scripting, which makes it easier to use than general purpose scripting languages.įor this section we will focus on bash scripting since it is the most common. What makes shell scripting different from other scripting programming languages is that it is optimized for performing shell-related tasks.

Most shells have their own scripting language with variables, control flow and its own syntax. Shell scripts are the next step in complexity. However, in many scenarios you will want to perform a series of commands and make use of control flow expressions like conditionals or loops. So far we have seen how to execute commands in the shell and pipe them together. In this lecture, we will present some of the basics of using bash as a scripting language along with a number of shell tools that cover several of the most common tasks that you will be constantly performing in the command line.

Writing shell scripts