Red Hat System Administration I
Abstract
| Goal | Create, view, and edit text files from command output or in a text editor. |
| Objectives |
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| Sections |
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| Lab |
Create, View, and Edit Text Files |
Save output or errors to a file with shell redirection, and process command output through multiple command-line programs with pipes.
A running program, or process, reads input and writes output. When you run a command from the shell prompt, it normally reads its input from the keyboard and sends its output to the terminal window.
A process uses numbered channels called file descriptors to get input and send output. All processes start with at least three file descriptors. Standard input (channel 0) reads input from the keyboard. Standard output (channel 1) sends normal output to the terminal. Standard error (channel 2) sends error messages to the terminal.
If a program opens separate connections to other files, then it might use higher-numbered file descriptors.
The next table summarizes the information about the file descriptors:
Table 5.1. Channels (File Descriptors)
| Number | Channel name | Description | Default connection | Usage |
|---|---|---|---|---|
| 0 |
stdin
| Standard input | Keyboard | Read only |
| 1 |
stdout
| Standard output | Terminal | Write only |
| 2 |
stderr
| Standard error | Terminal | Write only |
| 3+ |
| Other files | None | Read, write, or both |
The Input/Output (I/O) redirection changes how the process gets its input or output. Instead of getting input from the keyboard, or sending output and errors to the terminal, the process can read from or write to files. With redirection, you can save the messages to a file instead of displaying the output on the terminal. Alternatively, you can use redirection to discard output or errors, so they are not displayed on the terminal or saved.
You can redirect a process stdout to suppress the process output from appearing on the terminal.
If you redirect stdout to a file and the file does not exist, then the file is created.
If the file does exist and the redirection does not append to the file, then the redirection overwrites the file's contents.
To discard the output of a process, you can redirect to the empty /dev/null special file that discards channel output that is redirected to it.
As viewed in the following table, redirecting only
stdout does not suppress displaying stderr error messages on the terminal.
Table 5.2. Output Redirection Operators
| Usage | Explanation | Visual aid | |
|---|---|---|---|
| > file | Redirect stdout to overwrite a file. |
| |
| >> file | Redirect stdout to append to a file. |
| |
| 2> file | Redirect stderr to overwrite a file. |
| |
| 2> /dev/null | Discard stderr error messages by redirecting them to /dev/null. |
| |
| > file 2>&1 | Redirect stdout and stderr to overwrite the same file. |
| |
| &> file | |||
| >> file 2>&1 | Redirect stdout and stderr to append to the same file. |
| |
| &>> file |
Important
The order of redirection operations is important.
The following sequence redirects standard output to the output.log file and then redirects standard error messages to the same place as standard output (output.log).
> output.log 2>&1
The next sequence redirects in the opposite order.
This sequence redirects standard error messages to the default place for standard output (the terminal window, so no change) and then redirects only standard output to the output.log file.
2>&1 > output.log
For this reason, some people prefer to use the merging redirection operators:
&> output.loginstead of> output.log 2>&1&>> output.loginstead of>> output.log 2>&1(in Bash 4 or RHEL 6 and later)
However, system administrators and programmers prefer to avoid the newer merging redirection operators when using alternative shells to bash (known as Bourne-compatible shells) for scripting commands.
These new redirection operators are not standardized or implemented in those shells, and have other limitations.
Simplify many routine administration tasks by using redirection. Use the previous table to assist, and consider the following examples:
Save a time stamp in the /tmp/saved-timestamp file for later reference.
[user@host ~]$ date > /tmp/saved-timestampCopy the last 100 lines from the /var/log/secure file to the /tmp/last-100-log-secure file.
[user@host ~]$ tail -n 100 /var/log/secure > /tmp/last-100-log-secureConcatenate all four step files into one file in the tmp directory.
[user@host ~]$ cat step1.sh step2.log step3 step4 > /tmp/all-four-steps-in-oneList the home directory's hidden and regular file names, and save the output to the my-file-names file.
[user@host ~]$ ls -a > my-file-namesAppend a line to the existing /tmp/many-lines-of-information file.
[user@host ~]$ echo "new line of information" >> /tmp/many-lines-of-informationThe next few commands generate error messages because some system directories are inaccessible to normal users. Observe the error message redirection.
Redirect errors from the find command to the /tmp/errors file when viewing normal command output on the terminal.
[user@host ~]$ find /etc -name passwd 2> /tmp/errorsSave process output to the /tmp/output file and error messages to the /tmp/errors file.
[user@host ~]$ find /etc -name passwd > /tmp/output 2> /tmp/errorsSave process output to the /tmp/output file and discard error messages.
[user@host ~]$ find /etc -name passwd > /tmp/output 2> /dev/nullStore output and generated errors together to the /tmp/all-message-output file.
[user@host ~]$ find /etc -name passwd &> /tmp/all-message-outputAppend output and generated errors to the /tmp/all-message-output file.
[user@host ~]$ find /etc -name passwd >> /tmp/all-message-output 2>&1A pipeline is a sequence of one or more commands that are separated by the vertical bar character (|).
A pipeline connects the standard output of the first command to the standard input of the next command.
Use pipelines to manipulate and format the output of a process by other processes before it is output to the terminal. Imagine that data "flows" through the pipeline from one process to another, and is altered by each command in the pipeline that it flows through.
Note
Pipelines and I/O redirection both manipulate standard output and standard input. Pipelines send the standard output from one process to the standard input of another process. Redirection sends standard output to files, or gets standard input from files.
The following list shows some pipeline examples:
Redirect the output of the ls command to the less command to display it on the terminal one screen at a time.
[user@host ~]$ ls -l /usr/bin | lessRedirect the output of the ls command to the wc -l command, which counts the number of received lines from ls and prints that value to the terminal.
[user@host ~]$ ls | wc -lRedirect the output of the ls -t command to the head command to display the first 10 lines, with the final result redirected to the /tmp/first-ten-changed-files file.
[user@host ~]$ ls -t | head -n 10 > /tmp/first-ten-changed-filesWhen you combine redirection with a pipeline, the shell sets up the entire pipeline first, and then it redirects the input/output. If you use output redirection in the middle of a pipeline, then the output goes to the file and not to the next command in the pipeline.
In the next example, the output of the ls command goes to the /tmp/saved-output file, and the less command displays nothing on the terminal.
[user@host ~]$ ls > /tmp/saved-output | lessThe tee command overcomes this limitation.
In a pipeline, tee copies its standard input to its standard output and also redirects its standard output to the files that are given as arguments to the command.
If you imagine data as water that flows through a pipeline, then you can visualize tee as a "T" joint in the pipe that directs output in two directions.
The next example redirects the output of the ls command to the /tmp/saved-output file and passes it to the less command, so it is displayed on the terminal one screen at a time.
[user@host ~]$ ls -l | tee /tmp/saved-output | lessIf you use the tee command at the end of a pipeline, then the terminal shows the output of the commands in the pipeline and saves it to a file at the same time.
[user@host ~]$ ls -t | head -n 10 | tee /tmp/ten-last-changed-filesUse the tee command -a option to append the content to a file instead of overwriting it.
[user@host ~]$ ls -l | tee -a /tmp/append-filesImportant
You can redirect standard error through a pipeline, but you cannot use the merging redirection operators (&> and &>>).
The following example is the correct way to redirect both standard output and standard error through a pipeline:
[user@host ~]$ find / -name passwd 2>&1 | less
