bash alias xargs – How to Have xargs Use Alias Instead of Binary

aliasbashxargs

Bash 4.2 on CentOS 6.5:

In my ~/.bash_profile I have a bunch of aliases, including:

alias grep='grep -n --color=always'

so that I can get color highlighting and print line numbers automatically when running grep. If I run the following, highlighting works as expected:

$ grep -Re 'regex_here' *.py

However, when I ran this recently:

$ find . -name '*.py' | xargs grep -E 'regex_here'

the results were not highlighted and line numbers weren't printed, forcing me to go back and explicitly add -n --color=always to the grep command.

Does xargs not read aliases in the environment?
If not, is there a way to make it do that?

Best Answer

An alias is internal to the shell where it is defined. It is not visible to other processes. The same goes for shell functions. xargs is a separate application, which is not a shell, so doesn't have a concept of aliases or functions.

You can make xargs invoke a shell instead of invoking grep directly. However just invoking a shell isn't enough, you have to define the alias in that shell as well. If the alias is defined in your .bashrc, you can source that file; however this may not work your .bashrc performs other tasks that don't make sense in a non-interactive shell.

find . -name '*.py' | xargs bash -c '. ~/.bashrc; grep -E regex_here "$@"' _

Beware of the intricacies of nested quoting when typing the regexp. You can simplify your life by passing the regexp as a parameter to the shell.

find . -name '*.py' | xargs bash -c '. ~/.bashrc; grep -E "$0" "$@"' regex_here

You can perform the alias lookup explicitly. Then xargs will see grep -n --color=always.

find . -name '*.py' | xargs "${BASH_ALIASES[grep]}" regex_here

In zsh:

find . -name '*.py' | xargs $aliases[grep] regex_here

By the way, note that find … | xargs … breaks on filenames containing spaces (among others). You can fix this by changing to null-delimited records:

find . -name '*.py' -print0 | xargs -0 "${BASH_ALIASES[grep]}" regex_here

or by using -exec:

find . -name '*.py' -exec "${BASH_ALIASES[grep]}" regex_here {} +

Instead of calling find, you can do everything entirely inside the shell. The glob pattern **/ traverses directories recursively. In bash, you need to run shopt -s globstar to enable this glob pattern first.

grep regex_here **/*.py

This has a few limitations:

If a lot of files match (or if they have long paths), the command may fail because it exceeds the maximum command line length.
In bash ≤4.2 (but not in more recent versions, nor in ksh or zsh), **/ recurses into symbolic links to directories.

Another approach is to use process substitution, as suggested by MariusMatutiae.

grep regex_here <(find . -name '*.py')

This is useful when **/ isn't applicable: for complex find expressions, or in bash ≤4.2 when you don't want to recurse under symbolic links. Note that this breaks on file names containing spaces; a workaround is to set IFS and disable globbing, but it's starting to get a bit complex:

(IFS=$'\n'; set -f; grep regex_here <(find . -name '*.py') )

Related Solutions

Bash – Multicolored Grep

Here's a different approach. I have a little Perl script which I have already posted in another answer that will highlight the user provided patterns in different colors. A slightly modified version of the script will act like grep:

#!/usr/bin/env perl
use Getopt::Std;
use strict;
use Term::ANSIColor; 

my %opts;
getopts('hic:l:',\%opts);
    if ($opts{h}){
      print<<EoF; 
Use -l to specify the pattern(s) to highlight. To specify more than one 
pattern use commas. 

-l : A Perl regular expression to be colored. Multiple expressions can be
     passed as comma separated values: -l foo,bar,baz
-i : makes the search case sensitive
-c : comma separated list of colors;

EoF
      exit(0);
    }

my $case_sensitive=$opts{i}||undef;
my @color=('bold red','bold blue', 'bold yellow', 'bold green', 
       'bold magenta', 'bold cyan', 'yellow on_blue', 
       'bright_white on_yellow', 'bright_yellow on_red', 'white on_black');
if ($opts{c}) {
   @color=split(/,/,$opts{c});
}
my @patterns;
if($opts{l}){
     @patterns=split(/,/,$opts{l});
}
else{
    $patterns[0]='\*';
}

# Setting $| to non-zero forces a flush right away and after 
# every write or print on the currently selected output channel. 
$|=1;

while (my $line=<>) 
{ 
    my $want=0;
    for (my $c=0; $c<=$#patterns; $c++){
    if($case_sensitive){
        if($line=~/$patterns[$c]/){
           $line=~s/($patterns[$c])/color("$color[$c]").$1.color("reset")/ge;
           $want++;
        }
    }
    else{
        if($line=~/$patterns[$c]/i){
          $line=~s/($patterns[$c])/color("$color[$c]").$1.color("reset")/ige;
          $want++;
        }
      }
    }
print STDOUT $line if $want>0;
}

If you save that script as cgrep somewhere in your PATH and make it executable, you can specify up to 10 different patterns, each of which will be printed in a different color:

enter image description here

$ cgrep -h
Use -l to specify the pattern(s) to highlight. To specify more than one 
pattern use commas. 

-l : A Perl regular expression to be colored. Multiple expressions can be
     passed as comma separated values: -l foo,bar,baz
-i : makes the search case sensitive
-c : comma separated list of colors;

Bash – Non-printing escape sequence: when

The "non-printing escape sequence" is needed when using non-printing characters in $PS1 because bash needs to know the position of the cursor so that the screen can be updated correctly when you edit the command line. Bash does that by counting the number of characters in the $PS1 prompt and then that's the column number the cursor is in.

However, if you place non-printing sequences in $PS1, then that count is wrong and the line can be messed up if you edit the command line. Hence the introduction of the \[ and \] markers to indicate that the enclosed bytes should not be counted.

Best Answer

Related Solutions

Bash – Multicolored Grep

Bash – Non-printing escape sequence: when

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