---
title: Eboard 13  A Unix Miscellany
number: 13
section: eboards
held: 2017-05-04
---
CSC 282.01, Class 13:  A Unix Miscellany
========================================

_Overview_

* Preliminaries
    * Questions
* A problem
* Refresher: Selecting files
* Backticks
* Refresher: Shell variables
* The joy of sed
* Regular expressions
* Refresher: Loops
* Command-line history

### Good things to do

* Today's CS Extra!  (Science 2022)

A problem
---------

Used `CamelCase` rather than `snake_style` for field names.

Problem: Want to take every appropriate file (except db, which needs
migration) and change `TranslationLanguage` to `translation_language`.

* Subproblem 1: How do I identify groups of files, not all of which
  are in the same directory?
* Subproblem 2: How do I transmit that list of files to another program
  to work with?
* Subproblem 3: How do I do the editing/modification efficiently?

Refresher: Selecting files
--------------------------

* `*.txt` means all the files that end with txt
* `grep -l PATTERN *.txt` means all the files with PATTERN in their bodies
* `find . -name "*.txt" -print`

Backticks
---------

```
vi `find . -name "*.txt" -print`
```

Captures the output of one command, lets you use it as the command-line
parameters to another command.

Refresher: Shell variables
--------------------------

```
name="Sam"
var="Hello $name"
echo $var
```

Most shell variables are only available to the shell, and not to subprocesses.

```
export var
export var=VALUE
```

makes the variable available to subprocesses.

We can use backticks with shell variables.

```
name=`command`
```

Detour: Substitution expressions in `vi`
----------------------------------------

* First instance on each line:  `:1,$s/Hello/Hi/`
* Instance at the left margin on each line:  `:1,$s/^Hello/Hi/`
* All instances on each line:  `:1,$s/Hello/Hi/g`

The joy of sed
--------------

`sed` - Stream editor

```
sed -e "s/Hello/Hi" file > newfile
```

Note that he default behavior of `sed` is to keep the original file
unchanged and send the modified version to stdout.

You can add `-i` for in-place, at least in GNU sed.

Regular expressions
-------------------

This is the `sed` syntax, or parts thereof

* Most characters (except special characters, like \) match themselves
    * `a` matches the letter a
    * `3` matches the character 3
* `^` matches the start of line
* `$` matches the end of the line
* `\^` matches the caret character
* `\\` matches the backslash character
* `.` matches any one character
* `[abc]` matches any one character in the group within the brackets
* `[a-z]` matches any lowercase letter.  (ASCII ranges)
* `*` matches zero or more copies of the previous regular expression
    * `a*` is zero or more a's
    * `aa*` is one or more a's
* `\(` and `\)` serve the same role as parentheses in arithmetic
     * `\(aa\)*` matches even numbers of a's
* In the replacement, `\1` is the first parenthesized expression, `\2` is
  the second, and so on and so forth

To duplicate any single letter that happens after `H`

```
s/H\(.\)/H\1\1/g
```

Refresher: Loops
----------------

```
for VAR in LIST; do
  COMMAND
done
```

```
while (TEST) do
  COMMAND
done
```

To rename all of my files that contain the word "Sam" to end with `.sam`
rather than `.txt`.

```
for file in `grep -l Sam *.txt`; do
  newfile=`echo $file | sed -e 's/txt$/sam/'`
  mv $file $newfile
done
```

Sam's favorite use of `while`: Track the status of downloads

```
while (1) do
  date
  ls -l FILE
  sleep 60
done
```

Command-line history
--------------------