Write strace output to a file

Question

I'm trying to write the output of strace ls to a file. I know that I need to use > in order to forward output of a command to a file, but it doesn't work. It creates a file but the command prints the output of strace ls to stdout but writes the file name into the file.

$ strace ls > ls_sys.txt
...
strace output
...
$ cat ls_sys.txt
ls_sys.txt

Solution

By default strace outputs to stderr. By simply typing man strace, you will have the full documentation of strace. In the manual page, it states that the -o option can be used to output it to a file instead of stderr. You can type man [insert command here] for the vast majority of programs, and have all the documentation you will need to effectively use them.

Comments

[smw]

Related: piping strace to grep however it will likely be simpler to use the strace -o option rather than shell redirection

[fpmurphy]

Have you tried strace -o ls_sys.txt ls?

[yeled zevel]

By default strace outputs to stderr, which is not accessible via grep or file redirection. Good to know! And I looked at the manual page of strace but I didn't see the -o option, my fault. Thanks!

[john doe]

no problem, I actually just figured out this strace option myself yesterday. When you are in a man page, you can press / and it will bring a dialog that you can search. In my case, I simply searched for output and found the part of the man page I was looking for.

[JdeBP]

What on Earth makes you think that you cannot redirect standard error or arrange for it to be sent through a pipe to grep?

[Stephen Kitt]

stderr can be redirected, using 2>. See this question.

[Stéphane Chazelas]

Note that it redirects both the strace tracing and the ./elf_file errors to strace_output.txt. For that reason, it's better to use strace -o strace_output.txt ./elf_file or strace -o strace_output.txt ./elf_file 1> stdout.txt 2> stderr.txt if you want traces, stdout and stderr in different files.

[alkino]

A bash-ism I guess is command |& command it merges and pipes both stdout and stderr to the next command.