Bash – How to Use Output Redirection Dynamically

bashio-redirectionshell-script

I'm trying to add a debug option to a script of mine. Normally I want to hide any output, like warnings etc, so I put >/dev/null 2>&1 behind a lot of commands.

Now, when I want to debug my script, I would have to manually remove those, or put every command inside an if ... fi testing for a $DEBUG Variable.

I thought putting >/dev/null 2>&1 inside a Variable ($REDIR) and writing command arg1 $REDIR would do the trick. If I want to debug, I would only need to leave $REDIR empty.

But a short test on my shell showed me, that it won't work that way:

~$ echo "bla" >/dev/null 2>&1
~$ REDIR=>/dev/null 2>&1
~$ echo "bla" $REDIR
bla
~$

Using " or ' around >/dev/null 2>&1 didn't work either for obvious reasons.

So why does my idea not work here? Did I misunderstand something about putting commands etc. into Variables and calling them?

Best Answer

For such a purpose I usually define a function like run. This can correctly handle args with spaces and others in most cases.

#!/bin/bash

run() {
        if $DEBUG; then
                v=$(exec 2>&1 && set -x && set -- "$@")
                echo "#${v#*--}"
                "$@"
        else
                "$@" >/dev/null 2>&1
        fi
}

DEBUG=false
run echo "bla"

DEBUG=true
run echo "bla"
run printf "%s . %s . %s\n" bla "more bla" bla

Output:

$ bash debug.sh 
# echo bla
bla
# printf '%s . %s . %s\n' bla 'more bla' bla
bla . more bla . bla

Related Solutions

Shell – error handling in expect

Expect's model of interaction does not include a separation of stdout and stderr streams. When you spawn a process, you create a new pseudo-TTY which behaves almost identically to what a user would see in their terminal, and a user cannot typically distinguish between the two. Try it:

$ echo "This is stdout"
This is stdout
$ echo "This is stderr" 1>&2
This is stderr

Without doing something to label the difference, Expect cannot distinguish them (just like a user on the command line). So Expect may be the wrong tool for what you need to do.

Now, without knowing exactly why you need this, it's hard to come up with an alternative method for you. For what you need, it looks like you can accomplish this with a single ssh command:

ssh -q -o StrictHostKeyChecking=no $username@$hostname "ls -lrt; df -h" 1>>successlog 2>>errorlog

For another scripting language with built-in support for spawning a child process and getting its stdout and stderr separately, try Python and its subprocess module.

import shlex
from subprocess import Popen, PIPE
cmd = Popen(shlex.split('ssh -q -o StrictHostKeyChecking=no $username@$hostname "ls -lrt; df -h"'), stdout=PIPE, stderr=PIPE)
cmd.wait() # Wait for the command to complete
success_output = cmd.stdout
error_output = cmd.stderr

But if you really, really want to do this in Expect, I think the best approach may be to check the exit value of the last process and to write to your error log file when it is non-zero. Here's an example of something like that:

spawn ssh -q -o StrictHostKeyChecking=no $username@$hostname
# Receive password prompt and send the password here...
expect {
    -ex $prompt { send "ls -lrt\r" }
    timeout {
        send_user "Everything is terrible forever.\n"
        exit 1
    }
}
expect {
    -ex $prompt {
        set output $expect_out(buffer)
        send {echo RETVAL::$?::}
        send "\r"
    }
    timeout {
        send_user "Everything is terrible forever.\n"
        exit 1
    }
}
expect {
    -ex {RETVAL::0::} {
        set fp [open successlog a]
        puts $fp $output
        close $fp
    }
    -re {RETVAL::[1-9][0-9]*::} {
        set fp [open errorlog a]
        puts $fp $output
        close $fp
    }
    timeout {
        send_user "Everything is terrible forever.\n"
        exit 1
    }
}

Bash – How to use a variable instead of a file to output this wc command

You can use a small hack with redirections:

byteCount=$( exec 3>&1 ; 
     dd if=$file  bs=1  skip=$skippedBytes | tee -a >(wc -c >&3) $file.output |\
     $($exportCommandString $file) > /dev/null ;  3>&1   )

It redirects all output to 3, that you've created with exec, and then returns it back to one at the end.

You also need to redirect all output from $exportCommandString to /dev/null, otherwise it will be mixed with wc output.

All stderr will work as usual, there is no any changes.

p.s.: you can use tee -a file instead of tee >(cat - >> file)).

p.p.s.: You can't export variables from subshell, which is always created while using pipe | in bash or $(). So there is no way to make something like

tee -a >(VAR=$(wc -c)) $file.output

Best Answer

Related Solutions

Shell – error handling in expect

Bash – How to use a variable instead of a file to output this wc command

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