CODEWITHSUNDEEP
bashio-redirection
mon
mon

Reputation: 22244

Bash : behaviors of the redirections

Question

Please confirm if my understandings are correct. Or correct if wrong and pointers to related technical article and documents would be appreciated.

These are doing the same.

  1. ls &>/tmp/01.log

  2. ls 2>&1 >/tmp/02.log

  3. ls > /tmp/03.log 2>&1

  4. ls 2>&1 1>/tmp/04.log

The result of 5 will be the same with 1 to 4 on a single core CPU where there will be no multiple execution threads in a process. If it could produce a different result, please help understand what is actually happening in each process and in the kernel.

For multi-core CPU environment, would it happen that while a thread on a core is writing to stdout, another thread on another core write to stderr and they could be interleaved?

  1. ls 1>/tmp/05.log 2>/tmp/05.log

Below could cause errors e.g. stderr tries to write but stdout has not output anything yet.

ls 1>/tmp/05.log 2>>/tmp/05.log

The same with 1 to 4.

  1. ls 1<&2 > /tmp/06.log

The result is the same with 1 to 4, although makes no sense of doing it.

  1. ls 2>&1 | cat<&0>&1 > /tmp/07.log

Researches

Upvotes: 0

Views: 41

Answers (1)

user1934428
user1934428

Reputation: 22225

Number 4 is different. Order matters: First FD 2 is redirected where FD 1 currently points to (by default the terminal, unless another redirection had been set up already in this process or by the parent process), and then FD 1 is redirected to the file.

Upvotes: 1

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