☰

研究

tag-icon • tag-icon arguments macos
研究

這：

#!/bin/bash

# Run command ~100Kbytes long
/bin/echo $(perl -e 'print "x"x100000') | wc
# Run command ~54Kbytes long
# This line fails: line 7: /bin/echo: Argument list too long
/bin/echo $(perl -e 'print "x "x27000') | wc

# Same command, but run using xargs
# Run command ~100Kbytes long
perl -e 'print "x"x100000' | xargs -n 100000 /bin/echo | wc
# Run command ~54Kbytes long
# This line fails: xargs: /bin/echo: Argument list too long
perl -e 'print "x "x27000' | xargs -n 100000 /bin/echo | wc

在 GNU/Linux 上運作正常，但 2 54Kbytes 行在 MacOS X 上失敗。

ARG_MAX遠高於 100KBytes，請注意 100Kbytes 行確實不是失敗 - 54KBytes 線路失敗。

mac$ getconf ARG_MAX
262144
mac$ uname -a
Darwin macosx 11.4.2 Darwin Kernel Version 11.4.2: Thu Aug 23 16:26:45 PDT 2012; root:xnu-1699.32.7~1/RELEASE_I386 i386
# Kusalananda suggests it may be due to the size of the environment
mac$ env | wc
      27      32     956

為什麼 54Kbytes 指令失敗？

有沒有一種方法可以在不運行它的情況下預測參數列表對於 MacOS X 來說是否太長？

研究

這：

#!/bin/bash

runtest() {
    echo environment size:
    env | wc
    echo Run command ~100Kbytes long
    /bin/echo $(perl -e 'print "x"x100000') | wc
    echo Run command ~54Kbytes long
    # This line fails: line 7: /bin/echo: Argument list too long
    /bin/echo $(perl -e 'print "x "x27000') | wc
    
    # Same command, but run using xargs
    echo Run command ~100Kbytes long
    perl -e 'print "x"x100000' | xargs -n 100000 /bin/echo | wc
    echo Run command ~54Kbytes long
    # This line fails: xargs: /bin/echo: Argument list too long
    perl -e 'print "x "x27000' | xargs -n 100000 /bin/echo | wc
    echo
}

# Clean environment
runtest

# Make a huge environment
for a in `seq 5000`; do eval "a$a=1" ; done
for a in `seq 5000`; do eval "a$a() { 1; }" ; done
# This works as before
runtest

# Export environment
for a in `seq 5000`; do eval export a$a ; done
for a in `seq 5000`; do eval export -f a$a ; done
# Now the 100Kbytes commands fail, too
runtest

給出這個輸出：

environment size:
    6027    6032   47849
Run command ~100Kbytes long
       1       1  100001
Run command ~54Kbytes long
test: line 10: /bin/echo: Argument list too long
       0       0       0
Run command ~100Kbytes long
       1       1  100001
Run command ~54Kbytes long
xargs: /bin/echo: Argument list too long
       0       0       0

environment size:
    6027    6032   47849
Run command ~100Kbytes long
       1       1  100001
Run command ~54Kbytes long
test: line 10: /bin/echo: Argument list too long
       0       0       0
Run command ~100Kbytes long
       1       1  100001
Run command ~54Kbytes long
xargs: /bin/echo: Argument list too long
       0       0       0

environment size:
   16027   26032  126742
Run command ~100Kbytes long
test: line 7: /bin/echo: Argument list too long
       0       0       0
Run command ~54Kbytes long
test: line 10: /bin/echo: Argument list too long
       0       0       0
Run command ~100Kbytes long
xargs: insufficient space for argument
       0       0       0
Run command ~54Kbytes long
xargs: /bin/echo: Argument list too long
       0       0       0

所以，庫薩羅南達是對的出口環境可以產生影響。目前還不清楚計算這個的公式是什麼：也許純粹是大小？也許變數的數量也很重要？也許只是名字的長度？也許是這些的線性組合？

它仍然沒有解釋在給定環境中 100Kbytes 命令可以正常工作，但 54Kbytes 命令則不能。

就好像 MacOS 不僅對總大小有限制，而且對參數數量也有限制。

如果 MacOS 每個參數使用額外的 8 個位元組，這些數字也有意義：

# One big argument
100K * "x" = 100000+2 < 262144 # Works
# 27K small arguments
27K * "x " = 27K*(8+2) > 262144 # Fails
# 26K small arguments
26K * "x " = 26K*(8+2) < 262144 # Works

但 MacOS 能做到這一點嗎？

答案1

進一步研究顯示（MacOS 版本未知）：

Effective length = 
  length of arguments +
  5 * number of arguments +
  length of body/value of exported functions/variables +
  length of names of exported functions/variables +
  4 * number of exported functions/variables

如果有效長度 < 256 KB，則命令將運行。目前尚不清楚這是否適用於所有版本的 MacOS。

對於 MacOS El Capitan 10.11.4，這給出了悲觀命令列長度（假設您要執行的命令是/bin/echo x x x x ...）：

perl -e '                                                                             
  $envc=(keys %ENV);                                                                    
  $envn=length join"",(keys %ENV);                                                      
  $envv=length join"",(values %ENV);                                                    
  $maxlen=3+(262144 - $envn - $envv) / 5 - $envc*2;                                     
  print("Max len = $maxlen\n");
'

目前尚不清楚這是否適用於所有版本的 MacOS。

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