commit 7ce6e72ddfa408f6128cf95702b5b761d1ba8518
parent ec88cf30fc7c1b450821b9eaf9c177bc985d4c17
Author: Sebastiano Tronto <sebastiano@tronto.net>
Date: Wed, 27 Mar 2024 17:50:07 +0100
Added tomorrow's blog post
Diffstat:
3 files changed, 142 insertions(+), 0 deletions(-)
diff --git a/src/blog/2024-03-27-rev/rev.md b/src/blog/2024-03-27-rev/rev.md
@@ -18,3 +18,5 @@ Since [text is complicated](https://www.youtube.com/watch?v=gd5uJ7Nlvvo),
constitutes a character.
And that's it. See you soon for another (longer) post in this series.
+
+*Next in the series: [cut](../2024-03-28-cut)*
diff --git a/src/blog/2024-03-28-cut/cut.md b/src/blog/2024-03-28-cut/cut.md
@@ -0,0 +1,138 @@
+# UNIX text filters part 2.4 of 3: cut
+
+*This post is part of a [series](../../series)*
+
+Have you ever had to extract a bunch of data from a
+[CSV](https://en.wikipedia.org/wiki/Comma-separated_values) file?
+CSV is a common file format where multiple values are stored
+in each line of a plain text file, separated by a comma or some
+other separator.
+In most cases it is quite a simple file format to deal with, unless
+you want to write a generic parser that has to take into account
+all the special cases. But let's say you just want to write a quick
+and dirty shell script to read some values out of a single file.
+With `cut` you can get the job done pretty quickly!
+
+## cut
+
+Getting straight to the point, if you want to print columns 1, 3
+and 4 of each line of `myfile.csv` you can use:
+
+```
+$ cut -f 1,3,4 -d , myfile.csv
+```
+
+Let's break this down.
+
+## Fields, characters and bytes
+
+The `-f` option tells `cut` that you want to read lines field-by-field,
+where fields are are separated by the argument to the `-d` option.
+In our example the separator is a comma, but you can use any
+character. If unspecified, the separator defaults to a TAB.
+
+Instead of `-f` you could use `-c` (character) or `-b` (byte). If
+you pick one of these, the separator is not to be specified, and
+instead of field-by-field the rows are read character-by-character
+or byte-by-byte. The difference between a byte and a character
+depends on your
+[locale](https://en.wikipedia.org/wiki/Locale_(computer_software)),
+more specifically on the value of the environment variable `LC_CTYPE`.
+
+## Picking columns
+
+Columns are 1-based, hence the argument `1,3,4` gets, surprise
+surprise, the first, third and fourth columns of each line. The
+order you write the column indices does not matter: if you write
+`3,4,1` you still get the columns in the order they appear in the
+original file. If you repeat some indices, e.g. `1,3,4,1`, the
+repeated column is printed only once.
+
+You can also use ranges: for example `1,2,5-10` will print the first
+column, the second, and all the ones from the fifth to the tenth;
+as another example, `-3` will print the first 3 columns - unbounded
+ranges are interpreted as "from the start" and "until the end".
+
+## Examples
+
+Let see some examples!
+
+### Simple csv parsing
+
+Let's say `myfile.csv` is the following:
+
+```
+2024-01-13,-,4.50,out
+2024-02-04,groceries,52.42,out
+2024-02-20,reimbursement,89.99,in
+2024-03-10,stuff,1.01,out
+```
+
+Then running the following command command:
+
+```
+$ cut -f 3,4 -d , myfile.csv
+```
+
+will result in:
+
+```
+4.50,out
+52.42,out
+89.99,in
+1.01,out
+```
+
+### Fixed-width table
+
+Say you have a table like this in `table.txt`:
+
+```
+| WCA ID | Type | Result | Days |
+---------------------------------------
+| 1982THAI01 | Single | 22.95 | 7749 |
+| 2014CZAP01 | Single | 0.49 | 2443 |
+| 2011TRON02 | Single | 16 | 1747 |
+| 2015GORN01 | Single | 0.91 | 1673 |
+| 2015DUYU01 | Single | 3.47 | 1660 |
+| 2009ZEMD01 | Single | 6.88 | 1617 |
+```
+
+and you want to print out only the first and last columns. These
+columns are from character 2 to 13 and 33 to 38 respectively, or
+1-14 and 32-29 if you include the borders. So you can select them
+with the `-b` or `-c` option (they are equivalent in this case)
+like this:
+
+```
+$ cut -c 1-13,32-39 table.txt
+```
+
+and you will get:
+
+```
+| WCA ID | Days |
+---------------------
+| 1982THAI01 | 7749 |
+| 2014CZAP01 | 2443 |
+| 2011TRON02 | 1747 |
+| 2015GORN01 | 1673 |
+| 2015DUYU01 | 1660 |
+| 2009ZEMD01 | 1617 |
+```
+
+Since the ranges start at 1 and end at the last index, the following
+command would produce the same result:
+
+```
+$ cut -c -13,32- table.txt
+```
+
+## Conclusion
+
+I have not used `cut` much until today, the main reason being that
+the rare times I needed to parse a csv file I usually had to do
+something more complicated with the data than just printing it out.
+For this reason I have always relied on more complete languages,
+like C or Python, rather than shell scripting. But `cut` is definitely
+a convenient tool to be familiar with, given how simple it is!
diff --git a/src/series/series.md b/src/series/series.md
@@ -31,6 +31,8 @@ of complexity: `grep`, `sed` and `awk`. Work in progress.
* Part 2: [sed](../blog/2023-12-03-sed)
* Part 2.1: [tr](../blog/2024-01-13-tr)
* Part 2.2: [head and tail](../blog/2024-02-20-head-and-tail)
+* Part 2.3: [rev](../blog/2024-03-27-rev)
+* Part 2.4: [cut](../blog/2024-03-28-cut)
* Part 3: awk (coming "soon")
## The UNIX shell as an IDE
