in

View documentation for the for keyword

$ tldr for

There is no standard standalone Linux command named 'in', therefore no direct command synopsis applies. It operates as a keyword integrated into shell syntax for various control flow and iteration purposes.

The term 'in' is not a standalone executable command in standard Linux distributions. Instead, it functions as a crucial keyword primarily used within various shell scripting constructs and programming contexts. Its most common applications involve iterating through lists or ranges in 'for' loops (e.g., 'for item in list; do ... done') or defining patterns in 'case' statements (e.g., 'case $variable in pattern) ... esac').

Users often encounter or search for 'in' when conceptually trying to put something 'in' a directory, or install software 'in' a specific location, which can lead to confusion. It serves as a grammatical preposition or a logical operator within shell syntax, rather than an executable utility. Consequently, there is no manual page or executable file associated with a direct command named 'in'.

The primary caveat is that 'in' does not exist as an executable command on Linux systems. Attempting to run 'in' directly in a terminal will typically result in a 'command not found' error. Its sole function is as a syntactic element within shell scripts and programming languages, not as a utility that can be invoked independently.

The most prevalent use of 'in' is within shell 'for' loops to iterate over a list of items. For example:
'for file in *.txt; do echo "Processing $file"; done'

It is also commonly used in 'case' statements to match a value against a list of patterns:
'case $answer in yes|y) echo "Agreed.";;
no|n) echo "Disagreed.";;
*) echo "Invalid input.";;
esac'

Many users new to Linux or scripting might incorrectly assume 'in' is a command for actions like 'installing' or 'placing something into' a directory. This is not the case. Commands such as 'cp' (copy), 'mv' (move), 'install', or package managers like 'apt', 'yum', or 'dnf' are used for such operations. The term 'in' is exclusively used for specifying membership or iteration within a defined set or list.

sh(1), bash(1), for(1), case(1)