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Every process that the shell starts has three default streams: standard input (stdin), standard output (stdout), and standard error (stderr). The previous chapters covered variables, loops, and functions; this chapter covers the mechanism that connects those streams to files, pipes, and other descriptors. The shell does not pass the terminal directly to the process. It…
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mv is the command that moves files and directories from one place to another — and also the command that renames them. In Unix, those are the same operation: renaming a file is moving it to a new name in the same directory. That duality is the key to understanding mv. It’s fast because it…
The default gateway is the route of last resort. When the kernel examines a packet’s destination and finds no matching entry in the routing table, it forwards the packet to the default gateway. This single route is what connects a host or a local network to everything beyond its directly attached subnets. Without it, the…
grep is the command-line equivalent of the find function in a text editor: it searches input for lines that match a pattern and prints the matching lines to standard output. It is one of the most frequently used commands in Linux administration, and the LFCS exam tests it as part of the “Search files and…
The shell remembers the commands you’ve typed — this is called command history. It’s one of the most productivity-boosting features of the command line, letting you recall, search, and rerun past commands instantly. How History Works Every command you enter is saved to a history list: Default behavior: Viewing History Command Description history Show all…
A pipe connects the standard output of one command to the standard input of another. The shell creates the connection in the kernel, and the two commands run concurrently: the writer produces data, and the reader consumes it as it arrives. The previous chapter covered redirection, which connects a command’s streams to files; this chapter…
Deleting is the most dangerous thing you do on a Linux system. There’s no Trash, no Recycle Bin, no undo — rm unlinks a file and the space is gone. Two commands handle deletion: rm removes files and (with the right flags) directories, and rmdir removes empty directories only. They’re simple to type and catastrophic…
Classic routing makes a decision based solely on the destination address. A packet arrives, the kernel searches its routing table for the longest matching prefix, and forwards the packet accordingly. This model assumes that every packet destined for the same network should follow the same path. Policy routing breaks that assumption. It allows the administrator…
sed is the stream editor. It reads input line by line, applies a set of editing commands to each line, and writes the result to standard output. Unlike an interactive editor, sed is not meant for hand-editing a file. It is a filter: it takes a stream of text, transforms it according to a script,…