Master Linux Redirection, Pipes, and Text‑Processing Commands

Redirection

Standard output (stdout) can be written to a file with command > file, overwriting existing content, while command >> file appends. Standard error (stderr) uses 2> file, and both streams can be combined with command > file 2>&1 or command >& file.

Pipes

The pipe operator commandA | commandB | commandC passes the stdout of the left command as stdin to the right command. Only commands that can read from stdin (e.g., less, more, head, tail) are suitable; commands like ls, cp, mv cannot be used directly.

tee Command

tee [OPTION]... [FILE]...

reads stdin, writes it to stdout, and optionally saves it to one or more files. Combined with a pipe, it allows simultaneous display and logging, e.g., ll /home | tee list_home.out or find /home -name .bashrc 2>&1 | tee find.out.

xargs Command

xargs [options] [command [initial-arguments]]

reads items from stdin, splits them by whitespace or newline, and builds command lines. Typical usage includes passing find results to other commands, such as find /usr/sbin -perm /7000 | xargs ls -lh or find /home -name "*.go" | xargs du -cb.

Text‑Processing Utilities

grep

Searches files for lines matching a pattern: grep pattern file. It can be combined with pipes, e.g., ps -ef | grep postgres.

cut

Extracts sections from each line of input. Use -d to set a delimiter and -f to select fields, e.g., echo $PATH | cut -d ':' -f 1, or export | cut -c 12- to trim the first 12 characters.

awk

Powerful pattern‑action language. Basic usage: awk '{print $1,$4}' file. The field separator can be changed with -F, e.g., awk -F, '{print $1,$4}' file.

sed

Stream editor for applying scripted edits to text. Common commands include a (append), c (change), d (delete), i (insert), p (print), and s (substitute). Example: sed -e '4a\naaaa' file adds a line after line 4; sed '2,5d' file deletes lines 2‑5.

sort

Sorts lines of text alphabetically or numerically: sort file.

wc

Counts lines, words, and bytes. Example: ps -ef | grep postgres | wc -l returns the number of matching processes.

uniq

Filters adjacent duplicate lines, typically after sort: uniq file.

tr

Translates or deletes characters. Example: cat file | tr a-z A-Z converts lowercase to uppercase.