Working with Files through the Command Line

#webdev #terminal #shell #linux

`cat`

The cat command con*cat*enates and prints the contents of files.
cat <filename> will read the contents of a file and print them out. For example, cat instructions.txt will read in from the instructions.txt file and then print the contents out to the screen.
If we provide cat with multiple files, it will concatenate their contents and output them.
cat peanutbutter.js jelly.css will output peanutbutter.js first and immediately after print the contents of jelly.css
```
cat <filename>
cat <file1> <file2>
```

The less command displays the contents of a file, one page at a time. We can navigate forwards and backwards through the file, which is especially useful with very large files.
less somefile.txt will display the contents of somefile.txt using less.
When viewing a file using less :
- press space to go to the next page of the file
- press b to go back to the previous page
- press Enter or Down arrow to scroll by one line
- to search, type forward slash / followed by a pattern
- press q to quit
```
less <filename>
```

tac (cat spelled backwards) will concatenate and print files in reverse. It prints each line of a file, starting with the last line. You can think of it as printing in reverse "vertically"
```
tac file.txt
```

The rev command prints the contents of a file, reversing the order of each line. Think of it as a "horizontal" reverse, whereas tac is a "vertical" reverse.
```
rev file.txt
```

The head command prints a portion of a file, starting from the beginning of the file. By default, it prints the first 10 lines of a file.
head warAndPeace.txt would print the first 10 lines of the warAndPeace.txt file
We can also specify a number of lines for head to print using the -n option (or --lines) followed by an integer.
head -n 21 warAndPeace.txt would print the first 21 lines of the warAndPeace.txt file
We can also use an even shorter syntax to specify a number of lines: head -3 filename.txt will print the first 3 lines of the file.
```
head filename.txt
head -n 13 filename.txt
```

The tail command works similarly to the head command, except it prints from the END of a file. By default, it prints the last 10 lines of a file.
tail warAndPeace.txt would print the last 10 lines of the warAndPeace.txt file
The same -n option with head also works with the tail command.
```
tail filename.txt
```

The word count command can tell us the number of words, lines, or bytes in files. By default, it prints out three numbers: the lines, words, and bytes in a file.
We can use the -l option to limit the output to the number of lines.
The -w option limits the output to the number of words in the file.
```
wc -l students.txt
```

The sort command outputs the sorted contents of a file (it does not change the file itself). By default, it will sort the lines of a file alphabetically.
sort names.txt would print each line from names.txt, sorted in alphabetical order
The -r option tells the sort command to sort in reverse order.
sort names.txt -r would print each line from names.txt, sorted in REVERSE alphabetical order.
```
sort filename.txt
sort -r filename.txt
```
Sorting Numerically
- The -n option tells the sort command to sort using numerical order.
- sort -n prices.txt would print each line from names.txt, sorted in numerical order.
- We could also reverse it with sort -nr prices.txt
- The -u option tells the sort command to ignore duplicates and instead only sort unique values
```
sort -n prices.txt
sort -u prices.txt
```