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time find ... | xargs ... | time find ... | xargs ... }}} and get the execution time of the entire pipeline, rather than just the simple command at the start of the pipe. (This is different from the behavior of the external command {{{time(1)}}}, for obvious reasons.) Because of this, people who want to redirect {{{time}}}'s output often encounter difficulty figuring out where all the file descriptors are going. It's not as hard as most people think, though -- the trick is to call {{{time}}} in a SubShell or block, and then capture stderr of the subshell or block (which will contain {{{time}}}'s results). If you need to redirect the actual command's stdout or stderr, you do that inside the subshell/block. For example: * File redirection: {{{ # Explicit, but inefficient. bash -c "time ls" 2>time.output # Slightly more efficient. ( time ls ) 2>time.output # Most efficient. { time ls; } 2>time.output # The general case: { time some command >stdout 2>stderr; } 2>time.output |
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and get the execution time of the entire pipeline, rather than just the simple command at the start of the pipe. (This is different from the behavior of the external command {{{time(1)}}}, for obvious reasons.) | * CommandSubstitution: {{{ foo=$( bash -c "time ls" 2>&1 ) # Captures *everything*. foo=$( { time ls; } 2>&1 ) # More efficient version. |
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Because of this, people who want to redirect {{{time}}}'s output often encounter difficulty figuring out where all the file descriptors are going. It's not as hard as most people think, though -- the trick is to call {{{time}}} in a different shell or block, and redirect stderr of ''that'' shell or block (which will contain {{{time}}}'s results). If you need to redirect the actual command's stdout or stderr, you do that inside the inner shell/block. For example: | # Keep stdout unmolested. # The shell's original FD 1 is saved in FD 3, which is inherited by the subshell. # Inside the innermost block, we send the time command's stdout to FD 3. exec 3>&1 foo=$( { time bar 1>&3; } 2>&1 ) # Captures stderr and time. exec 3>&- |
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* File Redirection {{{ bash -c "time ls" 2>time.output ( time ls ) 2>time.output { time ls; } 2>time.output # The general case: { time some command >stdout 2>stderr; } 2>time.output |
# Keep both stdout and stderr unmolested. exec 3>&1 4>&2 foo=$( { time bar 1>&3 2>&4; } 2>&1 ) # Captures time only. exec 3>&- 4>&- # same thing without exec { foo=$( { time bar 1>&3- 2>&4-; } 2>&1 ); } 3>&1 4>&2 }}} See FileDescriptor for full explanations of the redirection juggling. * Pipe: {{{ # Make time only output elapsed time in seconds TIMEFORMAT=%R # Keep stdout and stderr unmolested exec 3>&1 4>&2 { time foo 1>&3 2>&4; } 2>&1 | awk '{ printf "The task took %d hours, %d minutes and %.3f seconds\n", $1/3600, $1%3600/60, $1%60 }' exec 3>&- 4>&- |
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* Variable Redirection | A similar construct can be used to capture "core dump" messages, which are actually printed by the shell that launched a program, not by the program that just dumped core: |
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foo=$( bash -c "time ls" 2>&1 ) # Captures *everything*. | ./coredump >log 2>&1 # Fails to capture the message { ./coredump; } >log 2>&1 # Captures the message }}} |
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# Keep stdout unmolested. exec 3>&1 foo=$( { time bar 1>&3; } 2>&1 ) # Captures stderr and time. exec 3>&- |
The same applies to job control messages: |
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# Keep both stdout and stderr unmolested. exec 3>&1 4>&2 foo=$( { time bar 2>&4; } 2>&1 1>&3) # Captures time only. exec 3>&- 4>&- |
{{{ $ { sleep 1 & } >log 2>&1 $ cat log [1] 10316 [1]+ Done sleep 1 |
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Of course you may opt to redirect to `/dev/null` instead of a file. ---- CategoryShell CategoryExampleCode |
How can I redirect the output of 'time' to a variable or file?
Bash's time keyword uses special trickery, so that you can do things like
time find ... | xargs ...
and get the execution time of the entire pipeline, rather than just the simple command at the start of the pipe. (This is different from the behavior of the external command time(1), for obvious reasons.)
Because of this, people who want to redirect time's output often encounter difficulty figuring out where all the file descriptors are going. It's not as hard as most people think, though -- the trick is to call time in a SubShell or block, and then capture stderr of the subshell or block (which will contain time's results). If you need to redirect the actual command's stdout or stderr, you do that inside the subshell/block. For example:
- File redirection:
# Explicit, but inefficient. bash -c "time ls" 2>time.output # Slightly more efficient. ( time ls ) 2>time.output # Most efficient. { time ls; } 2>time.output # The general case: { time some command >stdout 2>stderr; } 2>time.output
foo=$( bash -c "time ls" 2>&1 ) # Captures *everything*. foo=$( { time ls; } 2>&1 ) # More efficient version. # Keep stdout unmolested. # The shell's original FD 1 is saved in FD 3, which is inherited by the subshell. # Inside the innermost block, we send the time command's stdout to FD 3. exec 3>&1 foo=$( { time bar 1>&3; } 2>&1 ) # Captures stderr and time. exec 3>&- # Keep both stdout and stderr unmolested. exec 3>&1 4>&2 foo=$( { time bar 1>&3 2>&4; } 2>&1 ) # Captures time only. exec 3>&- 4>&- # same thing without exec { foo=$( { time bar 1>&3- 2>&4-; } 2>&1 ); } 3>&1 4>&2
See FileDescriptor for full explanations of the redirection juggling.
- Pipe:
# Make time only output elapsed time in seconds TIMEFORMAT=%R # Keep stdout and stderr unmolested exec 3>&1 4>&2 { time foo 1>&3 2>&4; } 2>&1 | awk '{ printf "The task took %d hours, %d minutes and %.3f seconds\n", $1/3600, $1%3600/60, $1%60 }' exec 3>&- 4>&-
A similar construct can be used to capture "core dump" messages, which are actually printed by the shell that launched a program, not by the program that just dumped core:
./coredump >log 2>&1 # Fails to capture the message { ./coredump; } >log 2>&1 # Captures the message
The same applies to job control messages:
$ { sleep 1 & } >log 2>&1 $ cat log [1] 10316 [1]+ Done sleep 1
Of course you may opt to redirect to /dev/null instead of a file.