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Q&A Safe handling and display of untrusted data in the terminal

How about BSD vis? It's basically perfect for this. It translates all unprintable characters into escape sequences like \^C or (for characters above 128) \M-A. A corresponding unvis program can tr...

posted 3mo ago by forestbeasts‭ · edited 3mo ago by forestbeasts‭

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#2: Post edited by

forestbeasts‭ · 2026-06-09T11:19:52Z (3 months ago)
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How about BSD `vis`? It's basically perfect for this.
~~It translates all unprintable characters into escape sequences like `\^C` or (for characters above 128) `\M-A`.~~
Unfortunately we don't know of any good way to install it on our Debian box, there doesn't seem to be a package for it. So we wrote our own at some point. It's pretty simple.
```
// vis.c
// written by Topaz <[email protected]>
#include <stdio.h>
int main (int argc, char **argv) {
int c;
while ((c = getchar()) != EOF) {
int printable = 0;
if (c >= 32 && c < 127) {
printable = 1;
}
if (c == ' ' || c == '\t' || c == '\n') {
printable = 1;
}
if (c == '\\') {
printf("\\\\");
} else if (printable) {
putchar(c);
} else {
if (c & 0b10000000) {
// high bit set, M-x
c -= 128;
if (c < 32 || c == 127) {
printf("\\M^%c", (c + 64) % 128);
} else {
printf("\\M-%c", c);
}
} else {
// 0x7F should be \^?,
// a control character but +64 would be >128;
// so wrap it around
printf("\\^%c", (c + 64) % 128);
}
}
}
}
```

How about BSD `vis`? It's basically perfect for this.
It translates all unprintable characters into escape sequences like `\^C` or (for characters above 128) `\M-A`. A corresponding `unvis` program can translate it back (it's unambiguous because `vis` also doubles backslashes in the input).
Unfortunately we don't know of any good way to install it on our Debian box, there doesn't seem to be a package for it. So we wrote our own at some point. It's pretty simple.
```
// vis.c
// written by Topaz <[email protected]>
#include <stdio.h>
int main (int argc, char **argv) {
int c;
while ((c = getchar()) != EOF) {
int printable = 0;
if (c >= 32 && c < 127) {
printable = 1;
}
if (c == ' ' || c == '\t' || c == '\n') {
printable = 1;
}
if (c == '\\') {
printf("\\\\");
} else if (printable) {
putchar(c);
} else {
if (c & 0b10000000) {
// high bit set, M-x
c -= 128;
if (c < 32 || c == 127) {
printf("\\M^%c", (c + 64) % 128);
} else {
printf("\\M-%c", c);
}
} else {
// 0x7F should be \^?,
// a control character but +64 would be >128;
// so wrap it around
printf("\\^%c", (c + 64) % 128);
}
}
}
}
```

#1: Initial revision by

forestbeasts‭ · 2026-06-09T11:18:27Z (3 months ago)

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How about BSD `vis`? It's basically perfect for this.

It translates all unprintable characters into escape sequences like `\^C` or (for characters above 128) `\M-A`.

Unfortunately we don't know of any good way to install it on our Debian box, there doesn't seem to be a package for it. So we wrote our own at some point. It's pretty simple.

```
// vis.c
// written by Topaz <[email protected]>

#include <stdio.h>

int main (int argc, char **argv) {
	int c;
	while ((c = getchar()) != EOF) {
		int printable = 0;
		if (c >= 32 && c < 127) {
			printable = 1;
		}
		if (c == ' ' || c == '\t' || c == '\n') {
			printable = 1;
		}
		if (c == '\\') {
			printf("\\\\");
		} else if (printable) {
			putchar(c);
		} else {
			if (c & 0b10000000) {
				// high bit set, M-x
				c -= 128;
				if (c < 32 || c == 127) {
					printf("\\M^%c", (c + 64) % 128);
				} else {
					printf("\\M-%c", c);
				}
			} else {
				// 0x7F should be \^?,
				// a control character but +64 would be >128;
				// so wrap it around
				printf("\\^%c", (c + 64) % 128);
			}
		}
	}
}
```

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