10.1 GNU and Unix Commands (Topic 1.3) 10.1.1 Objective 1: Work Effectively on the Unix Command Line 10.1.1.1 The interactive shell and shell variables -
A shell provides the command prompt and interprets commands. -
A shell variable holds a value that is accessible to shell programs. -
PATH is a shell variable that contains a listing of directories that hold executable programs. -
Commands must be bash built-ins, found in the PATH, or explicitly defined in order to succeed. -
When shell variables are exported, they become part of the environment. 10.1.1.2 Entering commands -
Commands are comprised of a valid command, with or without one or more options and arguments, followed by a carriage return. -
Interactive commands can include looping structures more often used in shell scripts. 10.1.1.3 Command history, editing, and substitution -
Shell sessions can be viewed as a conversation. History, expansion, and editing make that dialogue more productive. -
Commands can be reissued, modified, and edited. Examples are shown in Table 10-1. -
Command substitution allows the result of a command to be placed into a shell variable. Table 10-1. Shell Expansion, Editing, and Substitution Examples | Expansion | !! !n ^string1^string2 | | Editing | Ctrl-P, previous line Ctrl-K, kill to end of line Ctrl-Y, paste (yank) text | | Substitution | VAR=$(command) | 10.1.1.4 Recursive execution -
Many commands contain either a -r or -R option for recursive execution through a directory hierarchy. -
The find command is inherently recursive, and is intended to descend through directories looking for files with certain attributes or executing commands. 10.1.2 Objective 2: Process Text Streams Using Text Processing Filters The following programs modify or manipulate text from files and standard input: - cut [files]
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Cut out selected columns or fields from one or more files. - expand files
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Convert tabs to spaces in files. - fmt [files]
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Format text in files to a specified width by filling lines and removing newline characters. - head [files]
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Print the first few lines of files. - join file1 file2
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Print a line for each pair of input lines, one each from file1 and file2, that have identical join fields. - nl [files]
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Number the lines of files, which are concatenated in the output. - od [files]
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Dump files in octal, hexadecimal, ASCII, and other formats. - paste files
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Paste together corresponding lines of one or more files into vertical columns. - pr [file]
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Convert a text file into a paginated, columnar version, with headers and page fills. - split [infile] [outfile]
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Split infile into a specified number of line groups; the output will go into a succession of files of outfileaa, outfileab, and so on. - tac [file]
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Print file to standard output in reverse line order. - tail [files]
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Print the last few lines of one or more files. - tr [string1 [string2]]
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Translate characters by mapping from string1 to the corresponding character in string2. - wc [files]
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Print counts of characters, words, and lines for files. 10.1.2.1 The stream editor, sed sed is a popular text-filtering program found on nearly every Unix system; it has the following syntax: - sed command [files]
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- sed -e command1 [-e command2] [files]
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- sed -f script [files]
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Execute sed commands, or those found in script, on standard input or files. 10.1.3 Objective 3: Perform Basic File Management -
Filesystem creation is called formatting, which prepares a disk partition for use. Linux usually uses the native ext2 (second extended) filesystem. -
The Linux filesystem is arranged into a hierarchical structure anchored at the root directory, or /. Beneath this is a tree of directories and files. -
Identification information for a filesystem object is stored in its inode, which holds location, modification, and security information. Filesystems are created with a finite number of inodes. 10.1.3.1 File and directory management commands The following commands are essential for the management of files and directories: - cp file1 file2
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- cp files directory
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Copy file1 to file2, or copy files to directory. - mkdir directories
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Create one or more directories. - mv source target
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Move or rename files and directories. - rm files
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Delete one or more files from the filesystem. When used recursively (with the -r option), rm also removes directories. - rmdir directories
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Delete directories, which must be empty. - touch files
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Change the access and/or modification times of files by default to the present time. 10.1.3.2 File-naming wildcards Wildcards (also called file globs) allow the specification of many files at once. A list of commonly used wildcards can be found in Table 10-2. Table 10-2. File-Naming Wildcards * | Match zero or more characters. | ? | Match exactly one character. | [characters] | Match any single character from among characters listed between brackets | [!characters] | Match any single character other than characters listed between brackets. | [a-z] | Match any single character from among the range of characters listed between brackets. | [!a-z] | Match any single character from among the characters not in the range listed between brackets. | {frag1, frag2, frag3, ...} | Brace expansion: create strings frag1, frag2, and frag3, etc., such that {file_one,two,three} yields file_one, file_two, and file_three. | 10.1.4 Objective 4: Use Unix Streams, Pipes, and Redirects -
A central concept for Linux and Unix systems is that everything is a file. -
Many system devices are represented in the filesystem using a device file, such as /dev/ttyS0 for a serial port. 10.1.4.1 Standard I/O -
The shell provides the standard I/O capability, offering three default file descriptors to running programs. -
Standard input (stdin) is a text input stream, by default attached to the keyboard. -
Standard output (stdout) is an output stream for normal program output. -
Standard error (stderr) is an additional output stream meant for error messages. 10.1.4.2 Pipes and redirection -
It is possible to tie the output of one program to the input of another. This is known as a pipe and is created by joining commands using the pipe symbol (|). -
Pipes are a special form of redirection, which allows you to manage the origin of input streams and the destination of output streams. Redirection syntax for various shells differs slightly. See Table 10-3 for examples of common redirection operators. Table 10-3. Common Redirection Operators | Send stdout to file. | $ cmd > file $ cmd 1> file | | Send stderr to file. | $ cmd 2> file | | Send both stdout and stderr to file. | $ cmd > file 2>&1 | | Receive stdin from fil.e | $ cmd < file | | Append stdout to fil.e | $ cmd >> file $ cmd 1>> file | | Append stderr to file. | $ cmd 2>> file | | Append both stdout and stderr to file. | $ cmd >> file 2>&1 | | Pipe stdout from cmd1 to cmd2. | $ cmd1 | cmd2 | | Pipe stdout and stderr from cmd1 to cmd2. | $ cmd1 2>&1 | cmd2 | | Pipe stdout from cmd1 to cmd2 while simultaneously writing it to file1 using tee. | $ cmd1 tee file1 | cmd2 | 10.1.5 Objective 5: Create, Monitor, and Kill Processes 10.1.5.1 Monitoring commands - ps
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Generate a one-time snapshot of the current processes on standard output. - pstree
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Display a hierarchical list of processes in a tree format. - top
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Generate a continuous, formatted, real-time process activity display on a terminal or in a terminal window. 10.1.5.2 Signaling processes kill -sigspec [pids] -
Send sigspec to pids. Table 10-4. Common Signals HUP | 1 | Hangup, reread configuration. | INT | 2 | Interrupt, stop running. | KILL | 9 | Stop immediately. | TERM | 15 | Terminate nicely. | TSTP | 18 | Stop executing, ready to continue. | 10.1.5.3 Shell job control Shells can run processes in the background, where they execute on their own, or in the foreground, attached to a terminal. Each process handled in this way is known as a job. Jobs are manipulated using job control commands: - bg [jobspec]
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Place jobspec in the background as if it had been started with &. - fg [jobspec]
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Place jobspec in the foreground, making it the current job. - jobs [jobspecs]
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List jobspecs on standard output. 10.1.6 Objective 6: Modify Process Execution Priorities -
A process' execution priority is managed by the kernel. -
You can bias the execution priority by specifying a nice number in the range of -20 to +19 (default is 0). -
Positive nice numbers reduce priority; negative nice numbers increase priority and are reserved for the superuser. - nice -adjustment [command]
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Apply nice number adjustment to the process created to run command. - renice [+|-]nicenumber targets
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Alter the nicenumber, and thus the scheduling priority, of one or more running target processes. 10.1.7 Objective 7: Perform Searches of Text Files Making Use of Regular Expressions -
Regular expressions are used to match text. The term is used to describe the loosely defined text-matching language as well as the patterns themselves. A regular expression is often called a regex or a regexp. -
Regular expressions are made up of metacharacters (with special meaning) and literals (everything that is not a metacharacter). 10.1.7.1 Position anchors These operators match line position: ^ -
Match the beginning of a line. $ -
Match the end of a line. 10.1.7.2 Character sets These operators match text: - [abc]
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- [a-z]
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Match any single character from among listed characters or from among the characters comprising a range. - [^abc]
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- [^a-z]
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Match any single character not among listed characters or ranges. - \<word\>
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Match words bounded by whitespace. - . (A single period, or dot)
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Match any single character except a newline. - \
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Turn off (escape) the special meaning of a metacharacter that follows. 10.1.7.3 Modifiers These operators modify the way other operators are interpreted: - *
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Match zero or more of the character that precedes it. - ?
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Match zero or one instance of the preceding regex. - +
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Match one or more instances of the preceding regex. - \{ n,m\}
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Match a range of occurrences of the single character or regex that precedes this construct. - |
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Match the character or expression to the left or right of the vertical bar. |
