Command to escape a string in Bash

Question

I need a Bash command that will convert a string to something that is escaped. Here's an example:

echo "hello\world" | escape | someprog

Where the escape command makes "hello\world" into "hello\\\world". Then, someprog can use "hello\\world" as it expects. Of course, this is a simplified example of what I will really be doing.

Answer 1

In Bash:

printf "%q" "hello\world" | someprog

for example:

printf "%q" "hello\world"
hello\\world

This could be used through variables too:

printf -v var "%q\n" "hello\world"
echo "$var"
hello\\world

Answer 2

The Bash builtin ${…@Q} expansion has been added some time ago:

echo "hello\\world" | ( read -rsd '' x; echo ${x@Q} )
'hello\world'

The escaped output is in Bash format, so it might not be what you need.

See also: Which characters need to be escaped when using Bash?

Answer 3

Pure Bash, use parameter substitution:

string="Hello\ world"
echo ${string//\\/\\\\} | someprog

Answer 4

It may not be quite what you want, since it's not a standard command on anyone's systems, but since my program should work fine on POSIX systems (if compiled), I'll mention it anyway. If you have the ability to compile or add programs on the machine in question, it should work.

I've used it without issue for about a year now, but it could be that it won't handle some edge cases. Most specifically, I don't have any idea what it would do with newlines in strings; a case for \\n might need to be added. This list of characters is not authoritative, but I believe it covers everything else.

I wrote this specifically as a 'helper' program, so I could make a wrapper for things like scp commands.

It can likely be implemented as a shell function as well.

I therefore present escapify.c. I use it like so:

scp user@host:"$(escapify "/this/path/needs to be escaped/file.c")" destination_file.c

Please note: I made this program for my own personal use. It also will (probably wrongly) assume that if it is given more than one argument that it should just print an unescaped space and continue on. This means that it can be used to pass multiple escaped arguments correctly, but it could be seen as unwanted behavior by some.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
  char c='\0';
  int i=0;
  int j=1;
  /* Do not care if no arguments are passed; escaped nothing is still nothing. */
  if(argc < 2)
  {
    return 0;
  }
  while(j<argc)
  {
    while(i<strlen(argv[j]))
    {
      c=argv[j][i];
      /* This switch has no breaks on purpose. */
      switch(c)
      {
      case ';':
      case '\'':
      case ' ':
      case '!':
      case '"':
      case '#':
      case '$':
      case '&':
      case '(':
      case ')':
      case '|':
      case '*':
      case ',':
      case '<':
      case '>':
      case '[':
      case ']':
      case '\\':
      case '^':
      case '`':
      case '{':
      case '}':
        putchar('\\');
      default:
        putchar(c);
      }
      i++;
    }
    j++;
    if(j<argc) {
      putchar(' ');
    }
    i=0;
  }
  /* Newline at the end */
  putchar ('\n');
  return 0;
}

Answer 5

You can use Perl to replace various characters, for example:

echo "Hello\ world" | perl -pe 's/\\/\\\\/g'

Output:

Hello\\ world

Depending on the nature of your escape, you can chain multiple calls to escape the proper characters.

Answer 6

I could escape string with creating a function of bash.

function escape_str () {
  echo "$1" | sed 's/\\/\\\\/g' | sed 's/\"/\\"/g'
}

STR_JSON="{\"num\": 5}"

echo $STR_JSON
# shows {"num": 5}

ESCAPED_STR_JSON=$(escape_str "$STR_JSON")
echo $ESCAPED_STR_JSON
# shows {\"num\": 5}

DOUBLE_ESCAPED_STR_JSON=$(escape_str "$ESCAPED_STR_JSON")
echo $DOUBLE_ESCAPED_STR_JSON
# shows {\\\"num\\\": 5}