*Monday, 22nd June 2026 - 14:23 UTC+0100* ## Refresher - `grep` or `egrep` filters input based on regex pattern matching - `cat` concatenates file contents line by line - `tail` gives only the last `-n` lines, whereas `head` gives the first `-n` lines - `wc` does a word count, or line count (w/ flags `-w`, `-l`) - `sed` does pattern-matched string replacement. Regex is a vital skill for bash scripting as it will often be needed to filter files, data within files, match arguments, and a variety of other uses, so it is worth revising. ```bash user:linux:~$ grep a fruits.txt banana apple carrot ``` Square parentheses are a matching set, but adding a `^` inverses it. ```bash user@linux:~$ grep [pc] fruits.txt apple carrot ``` ## Extracting Scores with Shell `cat footy_scores.csv | grep 1959` outputs the line for 1959. Though if I were to use it on a different file, and if I wanted to count the number of lines in `two_cities.txt` that contained either the character *Sydney Carton* or *Charles Darnay*, I would use the following command instead: ```bash cat two_cities.txt | egrep 'Sydney Carton|Charles Darnay' | wc -l ``` > [!Note] > `egrep` is the same as `grep -E` ## A Simple Bash Script Concatenating files in Bash is one of the most basic scripts that can be done: ```bash user@linux:~$ cat file.ext laaaaaaaaa ``` ## Shell pipelines to Bash scripts But there are sime situations in which you may be given a `.csv` file and you'll need to do a shell piped command that will aggregate and show how many times `X` has done `Y`. In this example, how many times each team has won. ```bash cat footy_scores.csv | cut -d , -f 2 | tail -n +2 | sort | uniq -c ``` ## Extract and Edit using Bash Scripts Sometimes, you need to make multiple edits, and you need an easy way to do it. ```bash cat footy_scores.csv | sed 's/Real/Real Madrid Club de Fútbol/g' | sed 's/Atleti/Club Atlético de Madrid/g' > footy_scores_edited.csv ``` ## Arguments in Bash Scripts There are three streams in Bash scripting: - `STDIN` (standard input) - a stream of data into the program - `STDOUT` (standard output) - a stream of data out of the program - `STDERR` (standard error) - errors in the program Furthermore, `2> /dev/null` in script calls redirects `STDERR` to be deleted. Bash scripts can take arguments to be used inside by adding a space after the script execution call. - ARGV is the array of all the arguments given to the program - Each argument can be accessed via the ` notation. The first as `$1`, the second as `$2`, the third as `$3`, ad infinitum. - `$@` and `$*` give all the arguments in ARGV - `$#` gives the length (number) of arguments. An example of how it works, with `args.sh`: ```bash #!/usr/bash echo $1 echo $2 echo $@ echo "There are " $# "arguments" ``` `bash args.sh one two three four five` outputs: ``` one two one two three four five There are 5 arguments ``` --- [[Upskilling|← Back to Upskilling]]