Redirection
Ninereasons (Talk | contribs) m (correct example) |
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*::: <span style=color:blue >stderr stuff</span> | *::: <span style=color:blue >stderr stuff</span> | ||
*::: <span style=color:green >stdout stuff</span> | *::: <span style=color:green >stdout stuff</span> | ||
+ | *: ''Note:'' To close a file descriptor, say <code>n<&-</code> | ||
+ | *:: $ mycmd >out.txt 1<&-; | ||
+ | *:: ''-sh: fails without messages'' | ||
+ | *:: <span style=color:blue >-bash: mycmd: write error: Bad file descriptor</span> | ||
=== A little more about ''fd n'' in <code>sh</code> === | === A little more about ''fd n'' in <code>sh</code> === |
Revision as of 16:10, 27 June 2006
Redirection is sending the output of a program to somewhere other than where it would otherwise go - for example you can redirect the output of an ls command to a text file for later processing or to the grep command for filtering. Common operands include: >, >>, <, <<, and the ever popular |.
> sends output to a file (may include special files such as /dev/null) >> appends output to a file (without overwriting it) < read file tostdin
<< read to stdin from<<delimiter
todelimiter
(a HERE doc). | sends output to a program (frequently, a system command like grep)
If you're using the bash or bourne shells, you also have some special options available to you: you can redirect standard input, standard output and standard error messages with far greater flexibility and reliability. Other shells such as csh are notably limited in redirection capability, making them better suited to interactive use than to shell programming or other complex uses.
Shell pipes and redirection
There are three standard input/output file descriptors (fd) that are preconnected to the shell process running on your FreeBSD machine. Most commands that you would run from the command line expect these file descriptors to be open and accessible. The first (fd 0) provides a stream of input to your program, the second (fd 1) provides a stream of output, and the third (fd 2) provides a stream of diagnostic messages (usually to your terminal).
When you open a terminal, before running the shell the terminal device is opened three times to preconnect these file descriptors. The shell then inherits the file descriptors, and passes them on to each process run from the shell.
On FreeBSD systems it looks like this:
file descriptor | Stream | file descriptor file | device path |
---|---|---|---|
0 | Standard input | /dev/stdin |
/dev/fd/0 |
1 | Standard output | /dev/stdout |
/dev/fd/1 |
2 | Standard error | /dev/stderr |
/dev/fd/2 |
The example below demonstrates that by default, all of these input/output streams are directed to your terminal (color is added).
$ for i in stdin stdout stderr; do echo $i stuff > /dev/$i; done stdin stuff stdout stuff stderr stuff
Redirection means that, the file descriptor is temporarily reassigned to somewhere other than the terminal device (a file, a pipe, another file descriptor). The newly assigned process can reassign the open fd in turn, so that the stream of data is passed along, for example, by writing to stdout
, which has been temporarily reassigned to the stdin
for the next process by means of a pipe (|). This handing off of open file descriptors by the shell, from one sub-process to another, is called a "shell pipeline".
There are fd n (3-9) descriptors potentially available, but only 0-2 are preconnected to the shell; the others must be created from the shell. In some shells, a standard file descriptor can be detached and reassigned to another file descriptor for as long as the terminal device is open, using the exec command (examples appear below). This capability is just one case among many of why sh
is useful for scripting, and tcsh
(which lacks it) is not (see the FAQ).
Redirection in sh
compared to tcsh
The c-shells (tcsh
or csh
) and the Bourne shells (sh
or bash
) do not handle redirection or piping in quite the same way.
- tcsh and sh
- Write output to a file
- % mycmd > out.txt
- stderr stuff
- % mycmd > out.txt
- Append output to a file
- % mycmd >> out.txt
- stderr stuff
- % mycmd >> out.txt
- Redirect the output of a remote command to local.txt.
- % localcmd "remotecmd" > local.txt
- stderr stuff
- % localcmd "remotecmd" > local.txt
- Same command as above, showing only the changes compared to local.txt.
Note: Many programs recognize '-' as a shortcut for '/dev/stdin'. These two commands are equivalent.- % localcmd "remotecmd" | diff /dev/stdin local.txt
- % localcmd "remotecmd" | diff - local.txt
- stderr stuff
- Direct
stdout
+stderr
to file- % localcmd >& out.txt
- Sort lines of jumble.txt into sorted.txt
Note: the sequence in which redirection appears is not important. All of the following are exactly equivalent.- % <jumble.txt sort >sorted.txt
- % >sorted.txt sort <jumble.txt
- % sort < jumble.txt >sorted.txt
- stdout stuff
- Sort unique lines of jumble.txt into sorted.txt
- % <jumble.txt sort | uniq >sorted.txt
- stderr stuff
- % <jumble.txt sort | uniq >sorted.txt
- Sort HERE doc delimited by "lines"
- % <<lines sort
- ? a second line
- ? a first line
- lines
a first line
a second line
stderr stuff
- ? a second line
- % <<lines sort
- Write output to a file
- tcsh only
- Discard errors, watch output (probably evil)
Note: There is no reliable way to do this in tcsh. Here we exploit the fact that terminal reads fromstdin
.- % ( mycmd > /dev/stdin ) > & /dev/null
- stdout stuff
- % ( mycmd > /dev/tty ) > & /dev/null
- stdout stuff
- % ( mycmd > /dev/stdin ) > & /dev/null
- Append output to out.txt; discard messages
- % (mycmd >> out.txt) >& /dev/null
- Write output to out.txt; store and watch errors
Note: this happens to be easier in tcsh - a rare event. Compare the same task in sh.- % ( mycmd > out.txt ) | & tee err.txt
- stderr stuff
- % ( mycmd > out.txt ) | & tee err.txt
- Discard errors, watch output (probably evil)
- sh only
- Discard errors, watch output
- $ mycmd 2> /dev/null
- stdout stuff
- $ mycmd 2> /dev/null
- Append output to out.txt; discard messages
- $ mycmd 2> /dev/null >> out.txt
- Write output to out.txt; store and watch errors
Note: this happens to be harder in sh - a rare event. Compare the same task in tcsh.- $ exec 3>&1 ; mycmd 2>&1 >&3 1>out.txt | tee err.txt ; exec 1<&3 3<&-;
- stderr stuff
- $ exec 3>&1 ; mycmd 2>&1 >&3 1>out.txt | tee err.txt ; exec 1<&3 3<&-;
- Write messages to err.txt; write output to out.txt and copy output to terminal
- $ mycmd 2> err.txt | tee out.txt
- stdout stuff
- $ mycmd 2> err.txt | tee out.txt
- Assign a variable from stored.txt
- $ <stored.txt read var; mycmd $var
- stdout stuff
- $ <stored.txt read var; mycmd $var
- Assign first three lines of stored.txt to three different variables
- $ exec 3<&0; exec <test; read v1; read v2; read v3; exec 0<&3 3<&-; echo $v1 $v2 $v3
- stderr stuff
- stdout stuff
- $ exec 3<&0; exec <test; read v1; read v2; read v3; exec 0<&3 3<&-; echo $v1 $v2 $v3
- Use all unique lines in stored.txt as variable input, appending to result.txt
- $ exec 3<&0; exec <test; sort | uniq | while read line; do mycmd $line >> result.txt ; done; exec 0<&3 3<&-;
- stderr stuff
- $ exec 3<&0; exec <test; sort | uniq | while read line; do mycmd $line >> result.txt ; done; exec 0<&3 3<&-;
- Note: By default, redirection pointed right represents
stdout
, so that these two commands are exactly equivalent:- $ mycmd 1> out.txt
- $ mycmd > out.txt
- stderr stuff
- Note: By default, redirection pointed left represents
stdin
, so that these two commands are exactly equivalent:- $ mycmd 0< in.txt
- $ mycmd < in.txt
- stderr stuff
- stdout stuff
- Note: To close a file descriptor, say
n<&-
- $ mycmd >out.txt 1<&-;
- -sh: fails without messages
- -bash: mycmd: write error: Bad file descriptor
- Discard errors, watch output
A little more about fd n in sh
Let's say you want to send output to your screen and errors to a file. You can't just do
samizdata# myprogram 1>&2 2>&1 > errors.txt
because when you do the first switch, it's done right away and when the second >& comes around, it's getting the switched data. This is where the other, normally unused, file descriptors 3-9 come in. You can use them as place-holders, such as:
samizdata# myprogram 3>&2 2>&1 1>&3 | command
will make the output of myprogram do this: 3 point to the same place as 2, 2 point to 1, and finally, 1 point to 3 and then pipe all of it to command