You can easily test if a file is writable or noting test command under UNIX, Linux, macOS, *BSD family of operating systems when using bash or any other modern shell. This page shows how to test for write access to a file.
The -w option
The -w option is used to test if a FILE exists and write permission is granted or not. It returns true if a file is writable. The general syntax is as follows:
[ -w filename ] && echo "Writable" || echo "Not Writable"
test -w filename && echo "Writable" || echo "Not Writable"
The test command exits with a status of 0 (true) or 1 (false) depending on the evaluation of EXPR such as [ -w filename ]. Expressions may be unary or binary. Unary eressions are often used to examine the status of a file. There are string operators and numeric comparison operators as well. See below for more information.
Example – Test for write access to a file
Here is a sample shell script (test1.sh):
#!/bin/bash FILE="$1" [ $# -eq 0 ] && exit 1 if [ -w "$FILE" ] then echo "Write permission is granted on $FILE" else echo "Write permission is NOT granted on $FILE" fi |
Run shell script as follows:
chmod +x test1.sh
Execute the shell script:
bash test1.sh file1
### OR ####
./test1.sh file2
How to check for a file and whether it is readable and writable
The bash shell test command has many more options as follows:
- -w FILE : FILE exists and write permission is granted
- -x FILE : FILE exists and execute (or search) permission is granted
- -d FILE : FILE exists and is a directory
- -e FILE : FILE exists
- -f FILE : FILE exists and is a regular file
- -r FILE : FILE exists and read permission is granted
- -s FILE : FILE exists and has a size greater than zero
So one can see if file is readable or not using the -r option. For instance:
[ -r "$filename" ] && echo "File is readable" || echo "Sorry not readable"
Bash shell script example to test writable and readable file
Create a file named test2.sh as follows:
#!/bin/bash FILE="$1" [ $# -eq 0 ] && exit 1 if [[ -r "$FILE" && -w "$FILE" ]] then echo "We can read and write the $FILE" else echo "Access denied" fi |
Next run it as follows:
./test.sh /etc/passwd
sudo ./test.sh /etc/passwd
Conclusion
You learned how to check for a file and whether it is readable and writable under bash shell. For more information see test command help page or bash command man page here:
man bash
help test
Sample help
test: test [expr] Evaluate conditional expression. File operators: -a FILE True if file exists. -b FILE True if file is block special. -c FILE True if file is character special. -d FILE True if file is a directory. -e FILE True if file exists. -f FILE True if file exists and is a regular file. -g FILE True if file is set-group-id. -h FILE True if file is a symbolic link. -L FILE True if file is a symbolic link. -k FILE True if file has its `sticky' bit set. -p FILE True if file is a named pipe. -r FILE True if file is readable by you. -s FILE True if file exists and is not empty. -S FILE True if file is a socket. -t FD True if FD is opened on a terminal. -u FILE True if the file is set-user-id. -w FILE True if the file is writable by you. -x FILE True if the file is executable by you. -O FILE True if the file is effectively owned by you. -G FILE True if the file is effectively owned by your group. -N FILE True if the file has been modified since it was last read. FILE1 -nt FILE2 True if file1 is newer than file2 (according to modification date). FILE1 -ot FILE2 True if file1 is older than file2. FILE1 -ef FILE2 True if file1 is a hard link to file2. All file operators except -h and -L are acting on the target of a symbolic link, not on the symlink itself, if FILE is a symbolic link. String operators: -z STRING True if string is empty. -n STRING STRING True if string is not empty. STRING1 = STRING2 True if the strings are equal. STRING1 != STRING2 True if the strings are not equal. STRING1 < STRING2 True if STRING1 sorts before STRING2 lexicographically. STRING1 > STRING2 True if STRING1 sorts after STRING2 lexicographically. Other operators: -o OPTION True if the shell option OPTION is enabled. -v VAR True if the shell variable VAR is set. -R VAR True if the shell variable VAR is set and is a name reference. ! EXPR True if expr is false. EXPR1 -a EXPR2 True if both expr1 AND expr2 are true. EXPR1 -o EXPR2 True if either expr1 OR expr2 is true. arg1 OP arg2 Arithmetic tests. OP is one of -eq, -ne, -lt, -le, -gt, or -ge. Arithmetic binary operators return true if ARG1 is equal, not-equal, less-than, less-than-or-equal, greater-than, or greater-than-or-equal than ARG2. See the bash manual page bash(1) for the handling of parameters (i.e. missing parameters). Exit Status: Returns success if EXPR evaluates to true; fails if EXPR evaluates to false or an invalid argument is given. |
How To Read CPUID Instruction For Each CPU on Linux With x86info and cpuid Commands
Bash Shell Number Comparison
UNIX / Linux: HowTo Use unison File Synchronizer
Regular expressions in grep ( regex ) with examples
How to determine Linux guest VM virtualization technology
Bash Shell: Check File Exists or Not
cat command in Linux / Unix with Examples
hi
it works good.
so thanks.
but i didnt know how can i check the -r–r–r– permission.
thanks
use -r
if[ -r “$file” ]
for checking files readable permission…
hi,
how to find whether the file is exist or not when the file is given from command line argument in unix os and also how to check the file permission of read and write.
thank you.