Skip to content

Latest commit

 

History

History
266 lines (178 loc) · 6.94 KB

input-and-output.md

File metadata and controls

266 lines (178 loc) · 6.94 KB
Raw

Unix Programming and Regular Expressions Workshop - Input and Output

Sections:

Redirections

Quick Review of Redirection Operators we have seen so far in the workshop.

Opening files in Unix is manipulated using integer file descriptors:

  • 0 corresponds to standard input (stdin)
  • 1 corresponds to standard output (stdout)
  • 2 corresponds to standard error (stderr)
File Handle Name Description
0 stdin Standard Input
1 stdout Standard Output
2 stderr Standard Error

Here are the following redirection operators:

  • < redirect input
  • > redirect output
  • >> append instead of truncate
  • | will pipe stdout of an action as stdin for the corresponding pipeline
  • <<, <<- corresponds to Here Documents
  • >&2 will redirect output to standard error (stderr)
  • do an action 2>&1 | tee myoutput

New Redirection Operators:

  • set -o | grep noclobber
  • set -o noclobber this works in bash and zsh
    • > will not truncate a file instead will throw an error
  • >| this redirection will override noclobber option
  • <> will open a file for reading and writing
    • somecommand <> somefile

Moving Descriptors

You can use exec with shell redirections and no arguments to manage your shell's file descriptors

Any open files are inherited by child programs

exec myaction >& anotherAction

This copies output of myAction into anotherAction

exec someAction >& -

This closes the file descriptor

You need to move file descriptors by copying them and then closing them

Using exec with arguments has different behavior:

  • Exec will retain the same process ID number
  • The arguments will be passed into a new program
  • Any open file descriptors will remain open

Run the script capture.sh

./scripts/input-and-output/capture.sh > something

This will redirect stdout to a file and the exec command will redirect error to myerr files

echo some file > text

Create a file called text

set -C

enable noclobber option

echo another thing > text

The shell will now throw error and not perform truncation

echo another thing >| text

Now this command will work because of >| operator which will forcibly truncate

Read Command Description

read: read [-ers] [-a array] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]

  • Read a line from the standard input and split it into fields.

  • Reads a single line from the standard input, or from file descriptor FD if the -u option is supplied.

  • The line is split into fields as with word splitting, and the first word is assigned to the first NAME, the second word to the second NAME, and so on, with any leftover words assigned to the last NAME.

  • Only the characters found in $IFS are recognized as word delimiters.

The Read Command does the following:

  • Changes the field separator
  • Reads a loop
  • Reads exactly one line
  • The read command has bash extensions

The read command can read one line of text and split into words, assign to variables

  • IFS can control word splitting
  • Exit status of 1 upon end of file

If there are any remaining words they are assigned to the last variable and you can use one variable to get a whole line

The Read command is often run in a subshell if you use a pipeline or a loop

Also the variables defined in a loop won't be available outside the loop body

Read Command Options

Read Command Options (Bash):

  • -a array assign the words read to sequential indices of the array variable ARRAY, starting at zero

  • -d delim continue until the first character of DELIM is read, rather than newline

  • -euse Readline to obtain the line in an interactive shell

  • -i text use TEXT as the initial text for Readline

  • -n nchars return after reading NCHARS characters rather than waiting for a newline, but honor a delimiter if fewer than NCHARS characters are read before the delimiter

  • -N nchars return only after reading exactly NCHARS characters, unless EOF is encountered or read times out, ignoring any delimiter

  • -p prompt output the string PROMPT without a trailing newline before attempting to read

  • -r do not allow backslashes to escape any characters

  • -s do not echo input coming from a terminal -t timeout time out and return failure if a complete line of input is not read within TIMEOUT seconds.

    • The value of the TMOUT variable is the default timeout.
    • TIMEOUT may be avfractional number.
    • If TIMEOUT is 0, read returns immediately, without trying to read any data, returning success only if input is available on the specified file descriptor.
    • The exit status is greater than 128 if the timeout is exceeded

  • -u fd read from file descriptor FD instead of the standard input

Read Command Usage

IFS Docs

You can set IFS=' ... ' to read var1, var2, ... to change field separators while the read command is executing

The read command can be piped into a loop

some_processing |
  while read somevar1 somevar2 
  do
    some loop body
  done

The read command can also be used with input redirection

while read somevar1 somevar2
do
  some loop body
done < someFile

It is necessary to put the redirection after done

The read command will continue reading input if you use the \

If you use the -r option though the read command will only read one line

Read Command Examples

printf "Enter your name: " ; read name

Here the ; is necessary to get read input

echo "Your name is $name"
printf "Enter your first_name and last_name: " ; read firstName lastName
echo $firstName $lastName
read sentence

Use the \ to make multiple lines else when you hit enter after your first line the input is complete

echo $sentence
FOODS="pizz cookies bread shrimp"
echo $FOODS | sed 's;pizz;pizza;' | while read foods
do
echo $foods
done

This is a contrived example showing how you can use pipe with while loop and use that to pipe into read

echo $?

The exit status of 0 means success

bash

Enter a bash subshell if in zsh

read -p "Enter your name: " -t 4 name

This will prompt for you to enter your name and timeout after 4 seconds if you haven't entered anything

If you check the exit status it is greater than 128 upon a timeout

echo $?

Bread Crumb Navigation

Previous Next
← Looping Command Process Substitution →