This directory of Linux commands is from Linux in a Nutshell, 5th Edition.
Click on any of the 687 commands below to get a description and list of available options. All links in the command summaries point to the online version of the book on Safari Bookshelf.
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expr
expr arg1 operator arg2 [ operator arg3 ... ]
Evaluate arguments as expressions and print the results. Arguments and operators must be separated by spaces. In most cases, an argument is an integer, typed literally or represented by a shell variable. There are three types of operators: arithmetic, relational, and logical, as well as keyword expressions. Exit status for expr is 0 (expression is nonzero and nonnull), 1 (expression is 0 or null), or 2 (expression is invalid).
Arithmetic operators
Use these to produce mathematical expressions whose results are printed:
Add arg2 to arg1.
Subtract arg2 from arg1.
Multiply the arguments.
Divide arg1 by arg2.
Take the remainder when arg1 is divided by arg2.
Addition and subtraction are evaluated last, unless they are grouped inside parentheses. The symbols *, (, and ) have meaning to the shell, so they must be escaped (preceded by a backslash or enclosed in single quotes).
Relational operators
Use these to compare two arguments. Arguments can also be words, in which case comparisons are defined by the locale. If the comparison statement is true, the result is 1; if false, the result is 0. Symbols > and < must be escaped.
Are the arguments equal?
Are the arguments different?
Is arg1 greater than arg2?
Is arg1 greater than or equal to arg2?
Is arg1 less than arg2?
Is arg1 less than or equal to arg2?
Logical operators
Use these to compare two arguments. Depending on the values, the result can be arg1 (or some portion of it), arg2, or 0. Symbols | and & must be escaped.
Logical OR; if arg1 has a nonzero (and nonnull) value, the result is arg1; otherwise, the result is arg2.
Logical AND; if both arg1 and arg2 have a nonzero (and nonnull) value, the result is arg1; otherwise, the result is 0.
Like grep; arg2 is a pattern to search for in arg1. arg2 must be a regular expression. If part of the arg2 pattern is enclosed in \( \) (escaped parentheses), the result is the portion of arg1 that matches; otherwise, the result is simply the number of characters that match. By default, a pattern match always applies to the beginning of the first argument (the search string implicitly begins with a ^). Start the search string with .* to match other parts of the string.
Keywords
Return the first position in string that matches the first possible character listed in character-list. Continue through character-list until a match is found, or return 0.
Return the length of string.
Same as string : regex.
Treat token as a string, even if it would normally be a keyword or an operator.
Return a section of string, beginning with start, with a maximum length of length characters. Return null when given a negative or nonnumeric start or length.
Examples
Division happens first; result is 10:
expr 5 + 10 / 2
Addition happens first; result is 7 (truncated from 7.5):
expr \( 5 + 10 \) / 2
Add 1 to variable i. This is how variables are incremented in shell scripts:
i=`expr $i + 1`
Print 1 (true) if variable a is the string "hello":
expr $a = hello
Print 1 (true) if b plus 5 equals 10 or more:
expr $b + 5 \>= 10
Find the 5th, 6th, and 7th letters of the word character:
expr substr character 5 3
In the examples that follow, variable p is the string "version.100". This command prints the number of characters in p:
expr $p : '.*'
Match all characters and print them:
expr $p : '\(.*\)'
Print the number of lowercase letters at the beginning of p:
expr $p : '[a-z] *'
Match the lowercase letters at the beginning of p:
expr $p : '\([a-z] *\)'
Truncate $x if it contains five or more characters; if not, just print $x. (Logical OR uses the second argument when the first one is 0 or null, i.e., when the match fails.)
expr $x : '\(.....\)' \| $x
In a shell script, rename files to their first five letters:
mv $x `expr $x : '\(.....\)' \| $x`
(To avoid overwriting files with similar names, use mv -i.)
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