Common commands

  • awk - pattern scanning and processing language
  • sed - stream editor
  • tac - concatenate and print files in reverse


sed is a stream editor. It’s useful for manipulating text in files, or as part of a pipeline.

Replace a string in a file

sed -i 's/old/new/g' file.txt


Deduplicating lines in a file which have the same column value

Remove duplicate lines in a file, where values in one column are the same, whilst preserving order. For example, removing duplicate properties in a key/value property file (e.g. a Java .properties file). Either keep the first occurrence of the duplicate column line:

awk -F'=' '!seen[$1]++'

…or keep the last occurrence of the duplicate column line:

tac | awk -F'=' '!seen[$1]++' | tac

Looping over a CSV and extracting some values

If you have a CSV and you want to use the fields in each line as arguments for a command, you can use the IFS (internal field separator) variable to split the line into fields:

cat > users.csv <<EOF

cat users.csv | while IFS=, read -r username password ; do echo $username has a password of $password; done

There’s an example of this in the Google Cloud page.

Checking if the first part of an email address is longer than N

cat > emails.txt <<EOF

awk -F'@' '{ if (length($1) > 10) { print $1 } }' emails.txt
awk -F'@' '(length($1) > 10)' emails.txt

Looping over a username:password list and creating a command for each

Useful for performing the same action repeatedly, e.g. perform an HTTP request, test an app, etc.

If you have a list of usernames/passwords like this:


You can use read and a while loop to generate some commands like this - this example runs a Playwright test for each user:

while IFS=":" read -r username password
    APPLICATION_URL="https://$" USERNAME="$username" PASSWORD="$password" npx playwright test my-app-test.spec.js
done <<< "$LOGINS"