Diff for "BashFAQ/047"

Differences between revisions 6 and 14 (spanning 8 versions)

How can I redirect stderr to a pipe?

A pipe can only carry standard output (stdout) of a program. To pipe standard error (stderr) through it, you need to redirect stderr to the same destination as stdout. Optionally you can close stdout or redirect it to /dev/null to only get stderr. Some sample code:

# Bourne
# Assume 'myprog' is a program that writes to both stdout and stderr.

# version 1: redirect stderr to the pipe while stdout survives (both come
# mixed)
myprog 2>&1 | grep ...

# version 2: redirect stderr to the pipe without getting stdout (it's
# redirected to /dev/null)
myprog 2>&1 >/dev/null | grep ...

Another simple example of redirection stdout and stderr:

# Bourne
{ command | stdout_reader; } 2>&1 | stderr_reader

For further explanation of how redirections and pipes interact, see FAQ #55.

This has an obvious application with programs like dialog, which draws (using ncurses) windows onto the screen (stdout), and returns results on stderr. One way to deal with this would be to redirect stderr to a temporary file. But this is not necessary -- see FAQ #40 for examples of using dialog specifically!

In the examples above (as well as FAQ #40), we either discarded stdout altogether, or sent it to a known device (/dev/tty for the user's terminal). One may also pipe stderr only but keep stdout intact (without a priori knowledge of where the script's output is going). This is a bit trickier.

# Bourne
# Redirect stderr to a pipe, keeping stdout unaffected.

exec 3>&1                       # Save current "value" of stdout.
myprog 2>&1 >&3 | grep ...      # Send stdout to FD 3.
exec 3>&-                       # Now close it for the remainder of the script.

# Thanks to http://www.tldp.org/LDP/abs/html/io-redirection.html

The same can be done without exec:

# POSIX
$ myfunc () { echo "I'm stdout"; echo "I'm stderr" >&2; }
$ { myfunc 2>&1 1>&3- | cat  > stderr.file 3>&-; } 3>&1
I'm stdout
$ cat stderr.file
I'm stderr

The fd 3 is closed (1>&3- and 3>&-) so that the commands do not inherit it. You can check the difference on linux trying the following:

# Bash
{ bash <<< 'lsof -a -p $$ -d1,2,3'   ;} 3>&1
{ bash <<< 'lsof -a -p $$ -d1,2,3' 3>&-  ;} 3>&1

To show a dialog one-liner:

# Bourne
exec 3>&1
dialog --menu Title 0 0 0 FirstItem FirstDescription 2>&1 >&3 | sed 's/First/Only/'
exec 3>&-

This will have the dialog window working properly, yet it will be the output of dialog (returned to stderr) being altered by the sed.

A similar effect can be achieved with ProcessSubstitution:

# Bash
perl -e 'print "stdout\n"; warn "stderr\n"' 2> >(tr '[:lower:]' '[:upper:]')

This will pipe standard error through the tr command.

See this redirection tutorial (with an example that redirects stdout to one pipe and stderr to another pipe).

CategoryShell

-  ⇤ ← Revision 6 as of 2007-11-30 03:03:46 → 
  Size: 3163
  Editor: GreyCat
  Comment: change internal links
+   ← Revision 14 as of 2011-06-25 00:21:24 → ⇥
  Size: 3188
  Editor: pool-72-81-225-126
  Comment:
-Deletions are marked like this.
+Additions are marked like this.
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-[[Anchor(faq47)]]
+<<Anchor(faq47)>>
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-                                                                                 A pipe can only carry stdout of a program. To pipe stderr through it, you
need to redirect stderr to the same destination as stdout. Optionally
you can close stdout or redirect it to {{{/dev/null}}} to only get stderr. Some
sample code:
+A pipe can only carry standard output (stdout) of a program. To pipe standard error (stderr) through it, you need to redirect stderr to the same destination as stdout. Optionally you can close stdout or redirect it to {{{/dev/null}}} to only get stderr. Some sample code:
-Line 10:
+Line 6:
-# Assume 'myprog' is a program that outputs both, stdout and stderr.
+# Bourne
# Assume 'myprog' is a program that writes to both stdout and stderr.
-Line 12:
+Line 9:
-# version 1: redirect stderr towards the pipe while stdout survives (both come
+# version 1: redirect stderr to the pipe while stdout survives (both come
-Line 16:
+Line 13:
-# version 2: redirect stderr towards the pipe without getting stdout (it's
+# version 2: redirect stderr to the pipe without getting stdout (it's
-Line 19:
+Line 16:
-# version 3: redirect stderr towards the pipe while the "original" stdout gets
# closed
myprog 2>&1 >&- | grep ...
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+Line 18:
-For further explanation of how redirections and pipes interact, see [:BashFAQ#faq55:FAQ #55].
+Another simple example of redirection stdout and stderr:
{{{
# Bourne
{ command | stdout_reader; } 2>&1 | stderr_reader
}}}
-Line 27:
+Line 24:
-This has an obvious application with programs like {{{dialog}}}, which draws (using ncurses) windows onto the screen (stdout), and returns results on stderr.  One way to deal with this would be to redirect stderr to a temporary file.  But this is not necessary -- see [:BashFAQ#faq40:FAQ #40] for examples of using dialog specifically!
+For further explanation of how redirections and pipes interact, see [[BashFAQ/055|FAQ #55]].
-Line 29:
+Line 26:
-In the examples above (as well as [:BashFAQ#faq40:FAQ #40]), we either discarded stdout altogether, or sent it to a known device ({{{/dev/tty}}} for the user's terminal).  One may also pipe stderr only but keep stdout intact (without ''a priori'' knowledge of where the script's output is going).  This is a bit trickier.
+This has an obvious application with programs like {{{dialog}}}, which draws (using ncurses) windows onto the screen (stdout), and returns results on stderr.  One way to deal with this would be to redirect stderr to a temporary file.  But this is not necessary -- see [[BashFAQ/040|FAQ #40]] for examples of using dialog specifically!

In the examples above (as well as [[BashFAQ/040|FAQ #40]]), we either discarded stdout altogether, or sent it to a known device ({{{/dev/tty}}} for the user's terminal).  One may also pipe stderr only but keep stdout intact (without ''a priori'' knowledge of where the script's output is going).  This is a bit trickier.
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+Line 31:
+# Bourne
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-$ myfunc () { echo "I'm stdout"; echo "I'm stderr" >&2;}
$ { myfunc 2>&1 1>&3- | cat  > stderr.file 3>&- ; } 3>&1
+# POSIX
$ myfunc () { echo "I'm stdout"; echo "I'm stderr" >&2; }
$ { myfunc 2>&1 1>&3- | cat  > stderr.file 3>&-; } 3>&1
-Line 53:
+Line 54:
+# Bash
-Line 59:
+Line 61:
+# Bourne
-Line 66:
+Line 69:
-A similar effect can be achieved with process substitution:
+A similar effect can be achieved with ProcessSubstitution:
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+Line 71:
-# Bash only.
perl -e 'print "stdout\n"; warn "stderr\n"' 2> >(tr a-z A-Z)
+# Bash
perl -e 'print "stdout\n"; warn "stderr\n"' 2> >(tr '[:lower:]' '[:upper:]')
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+Line 77:
-A redirection tutorial (with an example that redirects stdout to a pipe and stderr to another pipe):
 * http://bash-hackers.org/wiki/doku.php?id=howto:redirection_tutorial
+See this [[http://wiki.bash-hackers.org/howto/redirection_tutorial|redirection tutorial]] (with an example that redirects stdout to one pipe and stderr to another pipe).

----
CategoryShell