Chapter 7Input/Output and Command-Line ProcessingI/O RedirectorsThe redirector <> is mainly meant for use with device files (in the /dev directory), i.e., files that correspond to hardware devices such as terminals and communication lines. Low-level systems programmers can use it to test device drivers; otherwise, it’s not very useful.
Here-documentsThe << label redirector essentially forces the input to a command to be the shell’s standard input, which is read until there is a line that contains only label. The input in between is called a here-document. Here-documents aren’t very interesting when used from the command prompt. In fact, it’s the same as the normal use of standard input except for the label. We could use a here-document to simulate the mail facility. When you send a message to someone with the mail utility, you end the message with a dot (.). The body of the message is saved in a file, msgfile: $ cat >> msgfile << . > this is the text of > our message. > . bashbug script MACHINE="i586"
OS="linux-gnu"
CC="gcc"
CFLAGS=" -DPROGRAM='bash' -DHOSTTYPE='i586' -DOSTYPE='linux-gnu' -DMACHTYPE='i586-pc-linux-gnu' -DSHELL -DHAVE_CONFIG_H
-I. -I. -I./lib -g -O2"
RELEASE="2.01"
PATCHLEVEL="0"
RELSTATUS="release"
MACHTYPE="i586-pc-linux-gnu"
TEMP=/tmp/bbug.$$
case "$RELSTATUS" in
alpha*|beta*)
BUGBASH=chet@po.cwru.edu ;;
*)
BUGBASH=bug-bash@prep.ai.mit.edu ;;
esac
BUGADDR="${1-$BUGBASH}"
UN=
if (uname) >/dev/null 2>&1; then
UN=`uname -a`
fi
cat > $TEMP <<EOF
From: ${USER}
To: ${BUGADDR}
Subject: [50 character or so descriptive subject here (for reference)]
Configuration Information [Automatically generated, do not change]:
Machine: $MACHINE
OS: $OS
Compiler: $CC
Compilation CFLAGS: $CFLAGS
name output: $UN
Machine Type: $MACHTYPE
bash Version: $RELEASE
Patch Level: $PATCHLEVEL
Release Status: $RELSTATUS
Description:
[Detailed description of the problem, suggestion, or complaint.]
Repeat-By:
[Describe the sequence of events that causes the problem
to occur.]
Fix:
[Description of how to fix the problem. If you don't know a
fix for the problem, don't include this section.]
EOF
vi $TEMP
mail $BUGADDR < $TEMP
File Descriptors
We’ll call this script start. The code is very terse: "$@" > logfile 2>&1 & This line executes whatever command and parameters follow start. (The command cannot contain pipes or output redirectors.) It sends the command’s standard output to logfile.Then, the redirector 2>&1 says, “send standard error (file descriptor 2) to the same place as standard output (file descriptor 1).” Since standard output is redirected to logfile, standard error will go there too. The final & puts the job in the background so that you get your shell prompt back. As a small variation on this theme, we can send both standard output and standard error into a pipe instead of a file: command 2>&1 | ... does this. (Make sure you understand why.) Here is a script that sends both standard output and standard error to the logfile (as above) and to the terminal:
"$@" 2>&1 | tee logfile &
These scripts have one shortcoming: you must remain logged in until the job completes. Although you can always type jobs (see Chapter 1) to check on progress, you can’t leave your terminal until the job finishes, unless you want to risk a breach of security. We’ll see how to solve this problem in the next chapter. Before we leave this topic, we should just note that 1> is the same as >, and 0< is the same as <. If you understand this, then you probably know all you need to know about file descriptors. String I/Oecho|printf|read
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