exec not found (because of the file descriptor)

Question

here is my shell script:

#!/bin/sh
exec 100>&1
exec 1>hello.txt
echo "hello exec"
echo "hello world"
exec 1>&100 100>&-

but when I run it, the system prompts that:

exec: 2: 100: not found

And when I use the command lsof to check the info of its file descriptor, it prompts that:

 lsof -p 31931 -d 100
COMMAND     PID   USER   FD   TYPE     DEVICE SIZE/OFF    NODE NAME
dbus-daem  1230 kaiwii  100u  unix 0x00000000      0t0 3753938 socket
bash      31931 kaiwii  cwd    DIR        8,7     4096 6316395 /home/kaiwii/test
bash      31931 kaiwii  rtd    DIR        8,7     4096       2 /
bash      31931 kaiwii  txt    REG        8,7   822420 8208388 /bin/bash
bash      31931 kaiwii  mem    REG        8,7  1434180 1688385 /lib/i386-linux-gnu/libc-2.13.so
bash      31931 kaiwii  mem    REG        8,7    38500 1688435 /lib/i386-linux-gnu/libnss_nis-2.13.so
bash      31931 kaiwii  mem    REG        8,7    79672 1688425 /lib/i386-linux-gnu/libnsl-2.13.so
bash      31931 kaiwii  mem    REG        8,7    26400 1688427 /lib/i386-linux-gnu/libnss_compat-2.13.so
bash      31931 kaiwii  mem    REG        8,7   117960 1688372 /lib/i386-linux-gnu/ld-2.13.so
bash      31931 kaiwii  mem    REG        8,7     9736 1688395 /lib/i386-linux-gnu/libdl-2.13.so
bash      31931 kaiwii  mem    REG        8,7   223468 1687611 /lib/libncurses.so.5.7
bash      31931 kaiwii  mem    REG        8,7    42580 1688431 /lib/i386-linux-gnu/libnss_files-2.13.so
bash      31931 kaiwii  mem    REG        8,7   123384 5685786 /usr/share/locale-langpack/zh_CN/LC_MESSAGES/bash.mo
bash      31931 kaiwii  mem    REG        8,7  8322432  319489 /usr/lib/locale/locale-archive
bash      31931 kaiwii    0r   CHR      136,0      0t0       3 /dev/pts/0
bash      31931 kaiwii    1u   CHR      136,0      0t0       3 /dev/pts/0
bash      31931 kaiwii    2u   CHR      136,0      0t0       3 /dev/pts/0
bash      31931 kaiwii  255u   CHR      136,0      0t0       3 /dev/pts/0

For the sake that I am not familiar to the command exec especially about how to revise the file descriptor, I have no idea what happened! By the way, when I change the file descriptor 100 to 8, it runs okay!

Answer 1

This is because you're running the script with /bin/sh which probably links to dash or other minimal shell interpreter. XCU 2.7 Redirection says:

Open files are represented by decimal numbers starting with zero. The largest possible value is implementation-defined; however, all implementations shall support at least 0 to 9, inclusive, for use by the application.

So some of these implementations support maximum 9 fd.

If you want to use more, use a full featured shell like bash , change /bin/sh with /bin/bash

Answer 2

From the GNU bash manual at http://www.gnu.org/software/bash/manual/bashref.html#Redirections :

Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally.

---

In general, only the first 3 descriptors are assigned by the system:

• 0 = stdin , usually input from a terminal
• 1 = stdout , usually output to a terminal
• 2 = stderr , also usually output to a terminal, but reserved for error messages so they don't get sent down a pipe accidentally.

You're free to use the higher numbers.