Bash – How to ignore interrupts with piped commands

bashinterruptpipe

In the following chain of piped commands, when an interrupt is sent with Ctrl-C, ping is able to print its summary statistics before exiting, as long as tee has the -i (ignore interrupts) flag:

ping -D localhost 2>&1 | tee -a -i ping.log

However, with another command in the chain, ping's summary does not get printed:

ping -D localhost 2>&1 | sed -u 's/^\[$[0-9]*\.[0-9]*$\]$.*$$/echo "[`date -d @\1 +"%Y-%m-%d %H:%M:%S"`] \2"/e' | tee -a -i ping.log

How can the above be made to print the summary?

Does sed have an option to ignore interrupts? In general how can interrupts be handled gracefully with piped commands?

Best Answer

ping -D localhost 2>&1 | (trap '' INT; exec sed -u 's/^\[$[0-9]*\.[0-9]*$\]$.*$$/echo "[`date -d @\1 +"%Y-%m-%d %H:%M:%S"`] \2"/e') | tee -a -i ping.log

Calling trap '' INT tells the shell to ignore SIGINT. The exec is optional but nice to have, since the subshell process is no longer necessary after the trap.

Related Solutions

Linux – ‘fast interrupts’ in Linux

As of today, you can mostly forget about the SA_INTERRUPT flag.
In between 2.6.18 and 2.6.24 it was just a migration helper for the new IRQF_DISABLED flag.
2.6.24 removed all SA_* flags and replaced them with IRQF_* flags.
2.6.35 marked this "new" flag as deprecated.

If you have a kernel before 2.6.18, you'll probably won't use it (see Justin's answer).

Today's usage of IRQF_DISABLE differs among the architectures. x86 still only uses it for time critical functions (time.c, hpet.c) and some xen stuff.

Concerning the difference; a normal interrupt can be interrupted by an other interrupt (preemption), a "fast" one on the other hand, can not.

Bash – eval used with piped command

eval does not read its command string from stdin.

eval "$(cat file.txt)"
# or easier, in ksh/bash/zsh
eval "$(<file.txt)"
# usually you cannot be sure that a command ends at the end of line 5
eval "$(head -n 5 file.txt)"

Instead of eval you can use standard . or bash/zsh/ksh source if the commands are in a file anyway:

source ./file

(note that it's important to add that ./. Otherwise source looks for file in $PATH before considering the file in the current directory. If in POSIX mode, bash would not even consider the file in the current directory, even if not found in $PATH).

That does not work with choosing a part of the file, of course. That could be done by:

head -n 5 file.txt >commands.tmp
source ./commands.tmp

Or (with ksh93, zsh, bash):

source <(head -n 5 file.txt)

Best Answer

Related Solutions

Linux – ‘fast interrupts’ in Linux

Bash – eval used with piped command

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