It’s again when I want to find out what packages I have manually installed (or by a script) from the output of apt list, with all output on one line.

It’s a pretty easy task if you’re familiar with Unix command utilities for text manipulation, and you may probably have come up with this solution:

apt list |
grep -F '[installed]' |
cut -d/ -f1 |
tr '\n' ' '
echo

    

The last echo is there because the output from tr doesn’t contain a newline at end, which would make your terminal look ugly. It’s also not well-Unix-styled because all output should end with a newline.

I was in the middle of the desire to learn AWK when I faced this task, so I did some search and wrote this AWK script:

#!/usr/bin/awk -f
BEGIN {
  FS = "/"
  ORS = " "
}
/[installed]/ {
  print $1
}
END {
  ORS = "\n"
  print ""
}

Save this file as whatever name you like, apply 755 permissions and run apt list | some.awk, and watch the magic go.

How’s it done?

For those absolutely new to AWK, this is a good example to start with.

The first thing is blocks. Each block begins with a “match condition” that means the block will be executed when the condition matches. A condition can be a statement (like $1 == "abcd" or NR % 2 == 0), a regular expression (like the example abov) or a special pattern (BEGIN and END).

If the condition is a statement, it is evaluated and tested for truthness. If it’s a regular expression, it’s matched against the whole record. Special patterns are intuitive: before processing the first record (BEGIN) or after processing every record (END).

The second thing is statements. In the example, there are only two kinds of statements: print and variable assignments. print is plain (so far) and it prints whatever’s after it. Here $1 means the first field of the record.

The two assigned variables are special. FS stands for Field Separator and it’s set to a slash, so the first field is everything before the first slash. ORS stands for Output Record Separator, and it’s what print adds at the end of each print statement, much like the keyword variable end in Python 3’s print() function.

More complex statements like conditions, loops and arithmetics are also possible, but I’m avoiding them here because they don’t appear in this script.

Finally, it comes to the execution of the script. By default, awk takes the first non-option command-line argument as the AWK program to execute. To specify that the program is written in a file, you need the -f option, whence the shebang line.