Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
if [ condition−true ]
then
command 1
command 2
...
else
# Optional (may be left out if not needed).
# Adds default code block executing if original condition tests false.
command 3
command 4
...
fi
When if and then are on same line in a condition test, a semicolon must terminate the if statement. Both
if and then are keywords. Keywords (or commands) begin statements, and before a new statement on the
same line begins, the old one must terminate.
if [ −x "$filename" ]; then
Else if and elif
elif
elif is a contraction for else if. The effect is to nest an inner if/then construct within an outer one.
if [ condition1 ]
then
command1
command2
command3
elif [ condition2 ]
# Same as else if
then
command4
command5
else
default−command
fi
The if test condition−true construct is the exact equivalent of if [ condition−true ]. As
it happens, the left bracket, [ , is a token which invokes the test command. The closing right bracket, ] , in an
if/test should not therefore be strictly necessary, however newer versions of Bash require it.
The test command is a Bash builtin which tests file types and compares strings. Therefore, in a Bash
script, test does not call the external /usr/bin/test binary, which is part of the sh−utils package.
Likewise, [ does not call /usr/bin/[, which is linked to /usr/bin/test.
bash$ type test
test is a shell builtin
bash$ type '['
[ is a shell builtin
bash$ type '[['
[[ is a shell keyword
bash$ type ']]'
]] is a shell keyword
bash$ type ']'
bash: type: ]: not found
Chapter 7. Tests 43