Bash – eval used with piped command

bashpipe

I have file.txt with command stored in one line (this command is valid when running in console) and I want to execute it in one line with sh like

cat file.txt | eval

what is missing? any protips?

and what if I have file with many commands (one for each line) and I want to execute only one command (one whole line)?
my first idea is:

head -n5 | tail -n1 | eval

Best Answer

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)

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Bash – Text file look-up by column

OK, if the length of the columns is not known, I'd switch to a more powerful language than bash:

#!/usr/bin/perl
use warnings;
use strict;

my $string = shift;
open my $FH, '<', '1.txt' or die $!;
my $first_line = <$FH>;
my ($before, $name) = $first_line =~ /(.* )(NAME *)/;
my $column = length $before;
$string .= ' ' x (length($name) - length $string);     # adjust the length of $string
while (<$FH>) {
    if ($column == index $_, $string, $column) {
        /^\[([0-9]+)\]/ and print "$1\n";
    }
}

Stop Bash Shell PS1 Color at End of Command – Solution

You're basically wanting to reset the terminal color right before bash executes the command. This can be done with a trap.

For example:

trap '[[ -t 1 ]] && tput sgr0' DEBUG

Bash executes the DEBUG trap immediately before the command, so this will result in tput sgr0 (which resets formatting attributes) being run before each command.

The [[ -t 1 ]] is a safety check to make sure that STDOUT is actually a terminal. There might be some cases where bash's STDOUT isn't connected to a terminal (piping, remote ssh, etc), and so you don't want tput to send terminal escape codes.

Best Answer

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Bash – Text file look-up by column

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