How do I test existence of a text file in bash running under Unix like operating systems?
You need to use the test command to check file types and compare values. The same command can be used to see if a file exist of not. The syntax is as follows:
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A file with the suid flag set shows an s in its permissions.rwsr-xr-t 1 root 178236 Oct 2 2000 /usr/sbin/pppd. Check if a file exists: if -f /some-file ; then Echo the file exists fi. Related macOS commands: macOS How To. File type tests. These options test for particular types of files. All cases will only return True (0) if the file exists.-b file True if file is a Block special device. -b demofile -c file True if file is a Character special device. -c demofile -d file True if file is a Directory. -d demofile -e file True if file Exists. Returns true if file is readable by the user running test.-s file: Returns true if file exists, and is not empty.-S file: Returns true if file is a socket.-t fd: Returns true if file descriptor fd is opened on a terminal.-u file: Returns true if file has the setuid bit set.-w file: Returns true if the user running test has write permission to. Simple Bash command-line test that an Os X application exists if open -Ra '$'; then # Control will enter here if $.app exists. Else # Control will enter here if $.app doesn't exist.
The following command will tell if a text file called /etc/hosts exists or not using bash conditional execution :
Sample outputs:
The same code can be converted to use with if.else.fi which allows to make choice based on the success or failure of a test command:
File test operators
The following operators returns true if file exists:
The syntax is same (see File operators (attributes) comparisons for more info):
On Unix-like operating systems, test is a builtin command of the Bash shell that can test file attributes, and perform string and arithmetic comparisons.
Description
test provides no output, but returns an exit status of 0 for 'true' (test successful) and 1 for 'false' (test failed).
The test command is frequently used as part of a conditionalexpression. For instance, the following statement says, 'If 4 is greater than 5, output yes, otherwise output no.'
The following statement says, 'If 6 is greater than 5, output yes, otherwise output no.'
The test command may also be expressed with single brackets [ .. ], as long as they are separated from all other arguments with whitespace. For example, the following statement checks that the system file /etc/passwd exists, and if not, outputs 'uh-oh.'
Syntax
File tests:
String tests:
Shell options and variables:
Simple logic (test if values are null):
Numerical comparison (for integer values only; bash doesn't do floating point math):
Options
The test builtin command takes the following options.
| -afile | Returns true if file exists. Does the same thing as -e. Both are included for compatibility reasons with legacy versions of Unix. |
| -bfile | Returns true if file is 'block-special'. Block-special files are similar to regular files, but are stored on block devices — special areas on the storage device that are written or read one block (sector) at a time. |
| -cfile | Returns true if file is 'character-special.' Character-special files are written or read byte-by-byte (one character at a time), immediately, to a special device. For example, /dev/urandom is a character-special file. |
| -dfile | Returns true if file is a directory. |
| -efile | Returns true if file exists. Does the same thing as -a. Both are included for compatibility reasons with legacy versions of Unix. |
| -ffile | Returns true if file exists, and is a regular file. |
| -gfile | Returns true if file has the setgid bit set. |
| -hfile | Returns true if file is a symbolic link. Does the same thing as -L. Both are included for compatibility reasons with legacy versions of Unix. |
| -Lfile | Returns true if file is a symbolic link. Does the same thing as -h. Both are included for compatibility reasons with legacy versions of Unix. |
| -kfile | Returns true if file has its sticky bit set. |
| -pfile | Returns true if the file is a named pipe, e.g., as created with the command mkfifo. |
| -rfile | Returns true if file is readable by the user running test. |
| -sfile | Returns true if file exists, and is not empty. |
| -Sfile | Returns true if file is a socket. |
| -tfd | Returns true if file descriptorfd is opened on a terminal. |
| -ufile | Returns true if file has the setuid bit set. |
| -wfile | Returns true if the user running test has writepermission to file, i.e. make changes to it. |
| -xfile | Returns true if file is executable by the user running test. |
| -Ofile | Returns true if file is owned by the user running test. |
| -Gfile | Returns true if file is owned by the group of the user running test. |
| -Nfile | Returns true if file was modified since the last time it was read. |
| file1-ntfile2 | Returns true if file1 is newer (has a newer modification date/time) than file2. |
| file1-otfile2 | Returns true if file1 is older (has an older modification date/time) than file2. |
| file1-effile2 | Returns true if file1 is a hard link to file2. |
| test [-n] string | Returns true if string is not empty. Operates the same with or without -n. For example, if mystr=', then test '$mystr' and test -n '$mystr' would both be false. If mystr='Not empty', then test '$mystr' and test -n '$mystr' would both be true. |
| -zstring | Returns true if string string is empty, i.e. '. |
| string1=string2 | Returns true if string1 and string2 are equal, i.e. contain the same characters. |
| string1!=string2 | Returns true if string1 and string2 are not equal. |
| string1<string2 | Returns true if string1 sorts before string2 lexicographically, according to ASCII numbering, based on the first character of the string. For instance, test 'Apple' < 'Banana' is true, but test 'Apple' < 'banana' is false, because all lowercase letters have a lower ASCII number than their uppercase counterparts. Tip: Enclose any variable names in double quotes to protect whitespace. Also, escape the less than symbol with a backslash to prevent bash from interpreting it as a redirection operator. For instance, use test '$str1' < '$str2' instead of test $str1 < $str2. The latter command will try to read from a file whose name is the value of variable str2. For more information, see redirection in bash. |
| string1>string2 | Returns true if string1 sorts after string2 lexicographically, according to the ASCII numbering. As noted above, use test '$str1' > '$str2' instead of test $str1 > $str2. The latter command creates or overwrites a file whose name is the value of variable str2. |
| -ooption | Returns true if the shell optionopt is enabled. |
| -vvar | Returns true if the shell variablevar is set. |
| -Rvar | Returns true if the shell variable var is set, and is a name reference. (It's possible this refers to an indirect reference, as described in Parameter expansion in bash.) |
| !expr | Returns true if and only if the expression expr is null. |
| expr1-aexpr2 | Returns true if expressions expr1 and expr2 are both not null. |
| expr1-oexpr2 | Returns true if either of the expressions expr1 or expr2 are not null. |
| arg1-eqarg2 | true if argumentarg1 equals arg2. |
| arg1-nearg2 | true if argument arg1 is not equal to arg2. |
| arg1-ltarg2 | true if numeric value arg1 is less than arg2. |
| arg1-learg2 | true if numeric value arg1 is less than or equal to arg2. |
| arg1-gtarg2 | true if numeric value arg1 is greater than arg2. |
| arg1-gearg2 | true if numeric value arg1 is greater than or equal to arg2 |
Notes
All arguments to test must be separated by a space, including all operators.
The < and > operators are lexicographical comparisons, based on ASCII numbering. They are not numerical operators (instead, use -lt, -gt, etc. for comparing numbers).
The precise behavior of test, depending on the number of arguments provided, is as follows:
| # of arguments | test behavior |
|---|---|
| 0 | Always return false. |
| 1 | Return true, if and only if the expression is not null. |
| 2 | If the first argument is !, return true if and only if the expression is null. If the first argument is one of the other unary operators (-a, -b, etc.), return true if and only if the unary test of the second argument is true. If the first argument is not an unary operator, return false. |
| 3 | The following conditions are applied in the order listed. If the second argument is one of the binary conditional operators listed above, the result is the binary test using the first and third arguments as operands. Binary conditional operators are those which take two operands, e.g., -nt, -eq, <, etc. The -a and -o operators are considered binary operators when there are three arguments. If the first argument is !, the value is the negation of the two-argument test using the second and third arguments. If the first argument is exactly ( and the third argument is exactly ), the result is the one-argument test of the second argument. In other words, (expr) returns the value of expr. This special case exists as a way to override the normal precedence of operations. Otherwise, the expression is false. |
| 4 | If the first argument is !, the result is the negation of the three-argument expression composed of the remaining arguments. Otherwise, the expression is parsed and evaluated according to precedence using the rules listed above. |
| 5+ | The expression is parsed and evaluated according to precedence using the rules listed above. |
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Exit status
0 for true, 1 for false. Node js for mac catalina island. Anything greater than 1 indicates an error or malformed command.
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Examples
This command returns 1 for false, because it has no arguments to test. (See notes above for how test behaves with various numbers of arguments.) This is good, because the only time test should ever be run with no arguments is when something has gone wrong in your script or command.
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To confirm that test returned false, we'll check the value of the special shell variable ?, which contains the exit status of the previous command executed.
Here are a few more examples.
Related commands
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df — View used and available disk space.
free — View used and available memory.