Exercise: Redirection Operators

A set of testing-relevant exercises to help testers use, and see the use of, redirection operators on the commandline.

To move information between commandline commands, use | to link inputs to outputs. Use < to take the input from a file, then use > and >> to put the output into a file. More info at Redirection Operators for Testers.

Use environments at https://redir.workroomprds.com

Password is password, please choose a name to go to a home page, from there pick code server, and you should be in vscode.

From there, open your home folder as a file browser, and open the terminal, and get the files needed with git clone https://github.com/workroomprds/for-redirection-operators.git

Exercise 1 – count the unique values in a table

We'll use items.csv

Use cut -d',' -f4 items.csv to see all the country codes.

Use cut -d',' -f4 items.csv | sort | uniq -c to pipe that through sort and through uniq -c . You'll see the count of unique country codes.

Try using an input redirect: cut -d',' -f4 < items.csv | sort | uniq -c .

• You can move that redirect to the front of the command: < items.csv cut -d',' -f4 items.csv | sort | uniq -c .
• You can use cat and a | to do (much) the same: cat items.csv | cut -d',' -f4 items.csv | sort | uniq -c .

By now, you'll be frustrated that one of those codes is not a country code? Use tail -n +2 to cut out the top line.

Let's use sort -nr and head -3 to see just the three countries with the greatest supply.

Use > to put that output somewhere permanent.

Exercise 2 – split up find's output

find has different outputs. It sends names of files found to stdout, and errors to stderr. Generally, stdout goes to the terminal (as a reply to your command), and so does stderr. If you're looking for one file on a disk, and you've got a heap of permission problems, you'll see mainly permission problems.

Let's use the following, without sudo

find / -name items.*

compare with

find / -name items.* 2>/dev/null

The 2> sends all the permission problems to dev/null, which is unix's bottomless pit, so you won't see permission problems at all.

Use > to send stdout to a file (leaving stderr visible)

Use &2> to send stderr to a file (leaving stdout visible)

Exercise 3 – reading a command

What does this do?

find /var/log -type f -print0 | xargs -0 file --mime-type | cut -d: -f2- | tr -d ' ' | sort | uniq -c | sort -nr

(sorry that it doesn't wrap...)

You can use it and try cutting bits out to see experimentally, or ask an LLM.

Exercise 4 – last 10 lines of recent logs

This uses redirections – and also xargs, which runs code on each line of input

• list files (with paths), sorted by recentness
• take the top 3
• show the most recent 10 line of use those files-with-paths

sudo ls -1td /var/log/* | head -3 | xargs -I {} sh -c ' sudo tail -10 {} '

do this to add a descriptive line between log files

sudo ls -1td /var/log/* | head -3 | xargs -I {} sh -c 'echo "=== {} ==="; sudo tail -10 {}'

Note the double sudo... one for the ls, and a different one for the tail inside the xargs block.

sprue from here...

Exercise 4 – streaming into tail

Exercise 5 – overwriting and appending