/* Code Comments */testing with bash
2020-08-22
|~6 min read
|1171 words
Recently I decided to rewrite my alias for git checkout to handle partial branch names as well as remotes. In the process, I learned a bit about testing with Bash.
There are two ways to write a test, i.e. an evaluation in bash:
test <expression>
[ <expression> ]There are, however, a number of flags/options available.
For example, in the case of my git checkout, I was defining a variable as the output of a grep. I wanted to test whether anything was assigned to the variable. The idiomatic way to do that seems to be checking if the string has a length. This is what the -z flag does.
if test -z $SOME_VAR
then
echo \$SOME_VAR has a length of 0
else
echo \$SOME_VAR has a length of 0Alternatively,
if [ -z $SOME_VAR ]
then
echo \$SOME_VAR has a length of 0
else
echo \$SOME_VAR has a length of 0It took me a while to actually track down what those brackets meant or how the -z worked. The point though is that there’s so much more! When I found the manual page for test my mind came alive with possibilities! Excited to continue digging into bash scripting!
Test\
TEST(1) BSD General Commands Manual TEST(1)
NAME
test, [ -- condition evaluation utility
SYNOPSIS
test expression
[ expression ]
DESCRIPTION
The test utility evaluates the expression and, if it evaluates to true,
returns a zero (true) exit status; otherwise it returns 1 (false). If
there is no expression, test also returns 1 (false).
All operators and flags are separate arguments to the test utility.
The following primaries are used to construct expression:
-b file True if file exists and is a block special file.
-c file True if file exists and is a character special file.
-d file True if file exists and is a directory.
-e file True if file exists (regardless of type).
-f file True if file exists and is a regular file.
-g file True if file exists and its set group ID flag is set.
-h file True if file exists and is a symbolic link. This operator
is retained for compatibility with previous versions of
this program. Do not rely on its existence; use -L in-
stead.
-k file True if file exists and its sticky bit is set.
-n string True if the length of string is nonzero.
-p file True if file is a named pipe (FIFO).
-r file True if file exists and is readable.
-s file True if file exists and has a size greater than zero.
-t file_descriptor
True if the file whose file descriptor number is
file_descriptor is open and is associated with a terminal.
-u file True if file exists and its set user ID flag is set.
-w file True if file exists and is writable. True indicates only
that the write flag is on. The file is not writable on a
read-only file system even if this test indicates true.
-x file True if file exists and is executable. True indicates only
that the execute flag is on. If file is a directory, true
indicates that file can be searched.
-z string True if the length of string is zero.
-L file True if file exists and is a symbolic link.
-O file True if file exists and its owner matches the effective
user id of this process.
-G file True if file exists and its group matches the effective
group id of this process.
-S file True if file exists and is a socket.
file1 -nt file2
True if file1 exists and is newer than file2.
file1 -ot file2
True if file1 exists and is older than file2.
file1 -ef file2
True if file1 and file2 exist and refer to the same file.
string True if string is not the null string.
s1 = s2 True if the strings s1 and s2 are identical.
s1 != s2 True if the strings s1 and s2 are not identical.
s1 < s2 True if string s1 comes before s2 based on the binary value
of their characters.
s1 > s2 True if string s1 comes after s2 based on the binary value
of their characters.
n1 -eq n2 True if the integers n1 and n2 are algebraically equal.
n1 -ne n2 True if the integers n1 and n2 are not algebraically equal.
n1 -gt n2 True if the integer n1 is algebraically greater than the
integer n2.
n1 -ge n2 True if the integer n1 is algebraically greater than or
equal to the integer n2.
n1 -lt n2 True if the integer n1 is algebraically less than the inte-
ger n2.
n1 -le n2 True if the integer n1 is algebraically less than or equal
to the integer n2.
If file is a symbolic link, test will fully dereference it and then eval-
uate the expression against the file referenced, except for the -h and -L
primaries.
These primaries can be combined with the following operators:
! expression True if expression is false.
expression1 -a expression2
True if both expression1 and expression2 are true.
expression1 -o expression2
True if either expression1 or expression2 are true.
( expression )
True if expression is true.
The -a operator has higher precedence than the -o operator.
Some shells may provide a builtin test command which is similar or iden-
tical to this utility. Consult the builtin(1) manual page.
GRAMMAR AMBIGUITY
The test grammar is inherently ambiguous. In order to assure a degree of
consistency, the cases described in the IEEE Std 1003.2 ("POSIX.2"), sec-
tion D11.2/4.62.4, standard are evaluated consistently according to the
rules specified in the standards document. All other cases are subject
to the ambiguity in the command semantics.
In particular, only expressions containing -a, -o, ( or ) can be ambigu-
ous.
EXIT STATUS
The test utility exits with one of the following values:
0 expression evaluated to true.
1 expression evaluated to false or expression was missing.
>1 An error occurred.
EXAMPLES
Implement test FILE1 -nt FILE2 using only POSIX functionality:
test -n "$(find -L -- FILE1 -prune -newer FILE2 2>/dev/null)"
This can be modified using non-standard find(1) primaries like -newerca
to compare other timestamps.
COMPATIBILITY
For compatibility with some other implementations, the = primary can be
substituted with == with the same meaning.
SEE ALSO
builtin(1), expr(1), find(1), sh(1), stat(1), symlink(7)
STANDARDS
The test utility implements a superset of the IEEE Std 1003.2 ("POSIX.2")
specification. The primaries <, ==, >, -ef, -nt, -ot, -G, and -O are ex-
tensions.
HISTORY
A test utility appeared in Version 7 AT&T UNIX.
BUGS
Both sides are always evaluated in -a and -o. For instance, the writable
status of file will be tested by the following command even though the
former expression indicated false, which results in a gratuitous access
to the file system:
[ -z abc -a -w file ]
To avoid this, write
[ -z abc ] && [ -w file ]
BSD October 5, 2016 BSDRelated Posts
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