/bin/sh：将标准错误捕获到变量中

Question

我将命令的 output 分配给变量A ：

A=$(some_command)

如何将标准错误“捕获”到变量B中？

我尝试了一些2>&1的变体并read ，但这不起作用：

A=$(some_command) 2>&1 | read B
echo $B

Answer 1

这是一个可能对您有所帮助的代码片段

#  capture stderr into a variable and print it
echo "capture stderr into a variable and print it"
var=$(lt -l /tmp 2>&1)
echo $var
capture stderr into a variable and print it
zsh: command not found: lt

#  capture stdout into a variable and print it
echo "capture stdout into a variable and print it"
var=$(ls -l /tmp)
echo $var

#  capture both stderr and stdout into a variable and print it
echo "capture both stderr and stdout into a variable and print it"
var=$(ls -l /tmp 2>&1)
echo $var




# more classic way of executing a command which I always follow is as follows. This way I am always in control of what is going on and can act accordingly

if somecommand ; then
    echo "command succeeded"
else
    echo "command failed"
fi

如果您必须在不同的变量中捕获 output 和 stderr，那么以下内容也可能会有所帮助

## create a file using file descriptor for stdout
exec 3> stdout.txt
# create a file using file descriptor for stderr
exec 4> stderr.txt

A=$($1 /tmp 2>&4 >&3);

## close file descriptor
exec 3>&-
exec 4>&-

## open file descriptor for reading
exec 3< stdout.txt
exec 4< stderr.txt

## read from file using file descriptor
read line <&3
read line2 <&4

## close file descriptor
exec 3<&-
exec 4<&-

## print line read from file
echo "stdout: $line"
echo "stderr: $line2"

## delete file

rm stdout.txt
rm stderr.txt

您可以尝试使用以下命令运行它

╰─ bash test.sh pwd
stdout: /tmp/somedir
stderr: 

╰─ bash test.sh pwdd
stdout: 
stderr: test.sh: line 8: pwdd: command not found

Answer 2

如评论中所述，您的用例可能会更好地使用其他脚本语言。 一个例子：在 Perl 你可以很简单地实现你想要的：

#!/usr/bin/env perl
use v5.26;                   # or earlier versions
use Capture::Tiny 'capture'; # library is not in core

my $cmd = 'date';
my @arg = ('-R', '-u');
 
my ($stdout, $stderr, $exit) = capture {
  system( $cmd, @arg );
};

say "STDOUT: $stdout";
say "STDERR: $stderr";
say "EXIT:   $exit";

我确信 python、ruby 和所有 rest 中都有类似的解决方案。

Answer 3

我再次尝试使用进程替换并想出了这个：

# command with no error
date +%b > >(read A; if [ "$A" = 'Sep' ]; then echo 'September'; fi ) 2> >(read B; if [ ! -z "$B" ]; then echo "$B"; fi >&2)
September

# command with error
date b > >(read A; if [ "$A" = 'Sep' ]; then echo 'September'; fi ) 2> >(read B; if [ ! -z "$B" ]; then echo "$B"; fi >&2)
date: invalid date “b“

# command with both at the same time should work too

我没有成功地将子进程中的变量“导出”回原始脚本。 不过也有可能。 我只是想不通。 但这至少使您可以访问stdout和stderr作为变量。 这意味着您可以对它们作为变量进行任何处理。 如果这对您有任何帮助，则取决于您的用例。 祝你好运：-）