Evaluate arguments as expressions and print the result. Strings can be compared and searched. 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. Exit status for expr is 0 (expression is nonzero and nonnull), 1 (expression is 0 or null), or 2 (expression is invalid).
Use these to produce mathematical expressions whose results are printed.
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).
Use these to compare two arguments. Arguments can also be words, in which case comparisons assume a < z and A < Z . If the comparison statement is true, the result is 1; if false, the result is 0. Symbols > and < must be escaped.
Use these to compare two arguments. Depending on the values,
the result can be
Division happens first; result is 10:
Addition happens first; result is 7 (truncated from 7.5):
Add 1 to variable i ; this is how variables are incremented in shell scripts:
Print 1 (true) if variable a is the string "hello":
Print 1 (true) if b plus 5 equals 10 or more:
In the examples below, variable p is the string "version.100". This command prints the number of characters in p :
Match all characters and print them:
Print the number of lowercase letters at the beginning of p :
Match the lowercase letters at the beginning of p :
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.)
In a shell script, rename files to their first five letters:
(To avoid overwriting files with similar names, use mv -i.)