Linux

Awk is a programming language designed for reading files line by line. It finds lines by a pattern and then runs an action on that line if the pattern matches. You can easily write a 1-line program on the command line and ask Awk to run that 1-line program on a file. Here is a program to count the number of "comment" lines in a script:

awk 'BEGIN{comment_count=0;} /^[[:space:]]*[#]/{comment_count++;} END{print(comment_count);}' file.sh

It is a good way to inspect the content of files, espcially log files or CSV files. But Awk can do some fairly complex file editing operations as well, like collating multiple files. It is a complete programming language.

Sed works similar to Awk, but it is much simplified, and designed mostly around CLI usage. The pattern language is similar to Awk, but the commands are usually just one or two letters representing actions like "print the line" or "copy the line to the in-memory buffer" or "dump the in-memory buffer to output."