Redirections

Redirections control where command input and output go. They let you save output to files, read input from files, or otherwise manipulate how commands perform I/O operations.

What are file descriptors?

A file descriptor is a non-negative integer that identifies an open file or I/O channel in a process. When a process opens a file, the operating system assigns it a file descriptor, which the process uses to read from or write to that file.

The first three file descriptors have standard meanings:

• 0: Standard input – the source of input data
• 1: Standard output – the destination for command results
• 2: Standard error – the destination for error messages and diagnostics

By default, these are connected to the terminal, but they can be redirected to files or other destinations.

Redirection syntax

A redirection consists of a special operator followed by a target (such as a file or file descriptor). Redirections can appear anywhere in a simple command, or after the body of a compound command.

For example, the > operator redirects standard output to a file:

$ mkdir $$ && cd $$ || exit
$ echo "Hello, World!" > output.txt
$ cat output.txt
Hello, World!

The < operator redirects standard input from a file:

$ mkdir $$ && cd $$ || exit
$ printf 'One\nTwo\nThree\n' > input.txt
$ while read -r line; do
>     echo "Read: $line"
> done < input.txt
Read: One
Read: Two
Read: Three

Redirection operators

Yash-rs supports these redirection operators:

• < (file): Redirects standard input from a file.

• > (file): Redirects standard output to a file.

  • If the clobber shell option is set (default), > behaves like >|.
  • If clobber is not set, > fails if the file exists and is a regular file or a symlink to a non-existent file. Otherwise, it creates a new file or opens the existing one. This is useful for preventing accidental overwriting of files.

  $ mkdir $$ && cd $$ || exit
  $ set -o noclobber
  $ echo "Hello, World!" > file.txt
  $ cat file.txt
  Hello, World!
  $ echo "Another redirection" > file.txt
  error: cannot open the file
   --> <stdin>:5:30
    |
  5 | echo "Another redirection" > file.txt
    |                              ^^^^^^^^ file.txt: File exists (os error 17)
    |
  

• >| (file): Redirects standard output to a file, overwriting it if it exists.

  • Always overwrites existing files, regardless of the clobber option.
  • Truncates the file if it exists, or creates it if not.

  $ mkdir $$ && cd $$ || exit
  $ echo "Hello, World!" >| file.txt
  $ cat file.txt
  Hello, World!
  $ echo "Another redirection" >| file.txt
  $ cat file.txt
  Another redirection
  

• >> (file): Redirects standard output to a file, appending to it if it exists.

  • Appends to the file if it exists, or creates it if not.

  $ mkdir $$ && cd $$ || exit
  $ echo "Hello, World!" >> file.txt
  $ cat file.txt
  Hello, World!
  $ echo "Another line" >> file.txt
  $ cat file.txt
  Hello, World!
  Another line
  

• <> (file): Opens a file for both reading and writing.

  • Opens the file if it exists, or creates it if not.

• <&: Duplicates or closes standard input, depending on the target word:

  • If the word is a file descriptor number, standard input becomes a copy of that descriptor (which must be open for reading).
  • If the word is -, standard input is closed. No error is reported if it is already closed.
    • If standard input is closed, commands that read from it will fail. To provide empty input, use < /dev/null instead.

• >&: Duplicates or closes standard output, depending on the target word:

  • If the word is a file descriptor number, standard output becomes a copy of that descriptor (which must be open for writing).

  • If the word is -, standard output is closed. No error is reported if it is already closed.

    • If standard output is closed, commands that write to it will fail. To discard output, use > /dev/null instead.

  • For example, >&2 redirects standard output to standard error:

    $ echo "error: please specify a user" >&2
    error: please specify a user
    

• << (delimiter): Opens a here-document.

• <<- (delimiter): Opens a here-document with automatic removal of leading tabs.

Specifying file descriptors

Redirection operators starting with < default to standard input; those starting with > default to standard output. To redirect a different descriptor, prefix the operator with its number (no space):

For example, to redirect standard error to a file:

$ grep "pattern" input.txt 2> error.log

If you insert a space, the number is treated as a command argument, not a file descriptor:

$ mkdir $$ && cd $$ || exit
$ echo 2 > output.txt
$ cat output.txt
2

Some shells allow using a variable name in braces {} instead of a file descriptor. For file-opening redirections, the shell allocates a new descriptor and assigns it to the variable. For descriptor-copying redirections, the shell uses the descriptor stored in the variable.

This is not yet implemented in yash-rs, but would look like:

$ exec {fd}> output.txt
$ echo "Hello, World!" >&$fd
$ cat output.txt
Hello, World!

Target word expansion

Except for here-documents, the word after a redirection operator is expanded before use. The following expansions are performed:

• Tilde expansion
• Parameter expansion
• Command substitution
• Arithmetic expansion
• Quote removal

Pathname expansion may be supported in the future.

$ mkdir $$ && cd $$ || exit
$ file="output.txt"
$ echo "Hello, World!" > "$file"
$ cat "$file"
Hello, World!

Persistent redirections

By default, redirections only apply to the command they are attached to. To make a redirection persist across multiple commands, use the exec built-in without arguments:

$ mkdir $$ && cd $$ || exit
$ exec > output.txt
$ echo "Hello, World!"
$ echo "More greetings!"
$ cat output.txt >&2
Hello, World!
More greetings!

You can use the >& operator to save a file descriptor before redirecting it, and restore it later:

$ mkdir $$ && cd $$ || exit
$ exec 3>&1 # Save current standard output to file descriptor 3
$ exec > output.txt # Redirect standard output to a file
$ echo "Hello, World!"
$ exec >&3 # Restore standard output from file descriptor 3
$ exec 3>&- # Close file descriptor 3
$ echo "More greetings!"
More greetings!
$ cat output.txt
Hello, World!

Semantic details

Applying a redirection to a compound command is different from applying it to a simple command inside the compound command. Each use of < opens a new file descriptor at the start of the file. If the redirection is inside a loop, the file descriptor is reset to the beginning on each iteration:

$ printf 'One\nTwo\nThree\n' > input.txt
$ while read -r line < input.txt; do
>     echo "Read: $line"
> done
Read: One
Read: One
Read: One
Read: One
Read: One
(The loop never ends…)

If a command has multiple redirections, they are applied in order. If several affect the same file descriptor, the last one takes effect:

$ mkdir $$ && cd $$ || exit
$ echo "Hello, World!" > dummy.txt > output.txt 2> error.txt
$ cat dummy.txt
$ cat output.txt
Hello, World!
$ cat error.txt

Note the difference between > /dev/null 2>&1 and 2>&1 > /dev/null:

• > /dev/null 2>&1 redirects both standard output and standard error to /dev/null, discarding both.
• 2>&1 > /dev/null redirects standard error to standard output, and then redirects standard output to /dev/null. This means standard error is still printed to the terminal (or wherever standard output was originally directed).

$ cat /nonexistent/file > /dev/null 2>&1
$ cat /nonexistent/file 2>&1 > /dev/null
cat: /nonexistent/file: No such file or directory